IO Tutorials for BEMS

8/6/2019 IO Tutorials for BEMS

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Input/output TutorialPurpose of this Guide:

This portion of Getting Started provides descriptions of input and output devices written forsomeone new to the field or a seasoned engineer.

Chapter 1: Input/output (I/O) Basics

Chapter 2: Input Devices and SensorsChapter 3: Output Devices

Chapter 1:

Input Output (IO) Basics

Terminology

The following terms have been defined to help readers better understand the material covered in

the Input/output document.

Accuracy

The term accuracy describes the total of all deviations between a measured value and the actualvalue. Accuracy is usually expressed as the sum of non-linearity, repeatability and hysteresis.

Accuracy may be expressed as the percent of a full-scale range or output, or in engineering units.

Address

An address is a unique numeric or alphanumeric data (point) identifier.

Analog/Modulating/Continuous

These synonymous terms are used to describe data that has a value that is continuous between set

limits represented by a range or span of voltage, current or resistance. The value is non-integer(real) with a resolution (number of significant digits) limited only by the measurement and

analog-to-digital signal conversion technology. In typical DDC systems, analog data from aninput device is converted into a value for processing within the controller. Likewise, values are

converted into analog output signals for use by a controlled device, such as an actuator.

Controlled Medium


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A controlled medium is a process medium of which one or more properties are made to conformto desired conditions by means of a control loop (see EMS Systems Overview Basic Control

loop).

Digital/Binary/Discrete

These synonymous terms are used to describe data that has a value representing one state oranother. Typical values are "on/off", alarm or normal, 0 or 1, high or low, etc. In the hardware

side of the DDC world, these values most commonly relate to the state of a set of switch or relaycontacts (open or closed).

External Point

Data that is received by a controller from an external source, or sent by a controller to an external

source, is an external point. The terms hardware, input or output may be used to describe anexternal point.

Global Point

Global points originate from a controller within a network that is broadcast via the network toother controllers.

Hysteresis

Hysteresis is the maximum difference in measured value or output when a set value is

approached from above, and then below the value.

Input

The term input is used to define data flow into a controller or control function.

Internal Point

An internal point is one that resides within a digital controller that does not directly originatefrom input or output points. Internal points can be constants such as fixed set points created by a

programmers or operators assignment. Internal points may also be created as defined by theprogrammer/ operator by applying logic and mathematics to other virtual, input or output pointsor combinations of points. The terms virtual, numeric or data may be used to describe an internal

point.

Non-linearity

Non-linearity is the maximum difference in measured value or output from a specified straightline between calibration points.


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Output

Output defines the data flow out of a controller or control function.

Point

Point is a generic term used to describe a single item of information in a control system. Pointsmay be further described as input, output, digital, binary, discrete, analog, modulating, internal,

external, virtual or global. Each unique point used by digital controllers, or in digital controlsystems, is typically identified by an address.

Process Medium

A process medium is a material in any phase (solid, liquid or gas) that is being used in a process.

The most common types of process mediums used in commercial and industrial heatingventilating and air conditioning systems are liquid mediums (i.e., chilled water for cooling) or

gaseous mediums (i.e., airflow in a duct).

Repeatability

Repeatability is the maximum difference in a measured value or output when a set value isapproached multiple times from either above or below the value.

Sensor

A sensor is a device in primary contact with a process medium. It measures particular propertiesof the process medium (i.e., temperature, pressure, etc.) and relates those properties to electrical

signals such as voltage, current, resistance or capacitance.

Transducer

Transducers accept an input of one character and produce an output of a different character.(Examples: voltage to current, voltage to pneumatic (pressure) and resistance to current.)

Transmitter

A transmitter is a transducer that is paired with a sensor to produce a higher-level signal

(typically) than is available directly from the sensor. These sensors may be integral or remoteand may include digital or analog signal processing. (Examples: temperature transmitter

employing a temperature sensor. The temperature sensor varies the resistance with temperaturechange and the transmitter outputs a related 4-20 mA current output for use by a controller.)


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Internal Point

An internal point is one that resides within a digital controller that does not directly originate

from input or output points. Internal points can be constants such as fixed set points created by aprogrammers or operators assignment. Internal points may also be created as defined by the

programmer/ operator by applying logic and mathematics to other virtual, input or output pointsor combinations of points. The terms virtual, numeric or data may be used to describe an internalpoint.

Non-linearity

Non-linearity is the maximum difference in measured value or output from a specified straight

line between calibration points.

Output

Output defines the data flow out of a controller or control function.

Point

Point is a generic term used to describe a single item of information in a control system. Pointsmay be further described as input, output, digital, binary, discrete, analog, modulating, internal,

external, virtual or global. Each unique point used by digital controllers, or in digital controlsystems, is typically identified by an address.

Process Medium

A process medium is a material in any phase (solid, liquid or gas) that is being used in a process.The most common types of process mediums used in commercial and industrial heating

ventilating and air conditioning systems are liquid mediums (i.e., chilled water for cooling) orgaseous mediums (i.e., airflow in a duct).

Repeatability

Repeatability is the maximum difference in a measured value or output when a set value is

approached multiple times from either above or below the value.

Sensor

A sensor is a device in primary contact with a process medium. It measures particular propertiesof the process medium (i.e., temperature, pressure, etc.) and relates those properties to electrical

signals such as voltage, current, resistance or capacitance.

Transducer


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Transducers accept an input of one character and produce an output of a different character.(Examples: voltage to current, voltage to pneumatic (pressure) and resistance to current.)

Transmitter

A transmitter is a transducer that is paired with a sensor to produce a higher-level signal(typically) than is available directly from the sensor. These sensors may be integral or remoteand may include digital or analog signal processing. (Examples: temperature transmitter

employing a temperature sensor. The temperature sensor varies the resistance with temperaturechange and the transmitter outputs a related 4-20 mA current output for use by a controller.)

Chapter 1:Input Output (IO) Basics

Digital Inputs

A digital input typically consists of a power supply (voltage source), a switch and a voltage-sensing device (analog-to-digital converter). Depending on the switchs open/closed status, thesensing device detects a voltage or no voltage condition, which in turn generates a logical 0 or 1,

on or off, alarm or normal or similarly defined state.

Circuit Diagrams

The following circuit diagrams are examples of commonly used digital input configurations.

Digital Outputs


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A digital output typically consists of a switch (either mechanical as in a relay, or electronic as ina transistor or triac) that either opens or closes the circuit between two terminals depending on

the binary state of the output.

Circuit Diagrams

The following circuit diagrams are examples of commonly used digital output configurations.

Figure 2 shows an open collector transistor-type digital output operating a pilot relay, which in

turn energizes the motor starter coil for a fan. Figure 3 shows a triac-type digital output operatinga pilot relay that is used to energize a fan motor starter coil.


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Analog Inputs

An analog input is a measurable electrical signal with a defined range that is generated by asensor and received by a controller. The analog input changes continuously in a definable

manner in relation to the measured property.

The analog signals generated by some types of sensors must be conditioned by converting to ahigher-level standard signal that can be transmitted over wires to the receiving controller. Analog

inputs are converted to digital signals by the analog-to-digital (A/D) converter typically locatedat the controller. Analog-to-digital conversion is limited to a small range of DC voltage, so that

internal or external input circuitry must change the character of non-compatible signal types to aDC voltage range within the limits of the A/D converter.

Common Types

There are basically three types of analog input signals; voltage, current and resistance.

Voltage

Common voltage signals used in the controls industry are 1-5 Volts Direct Current (VDC), 2-10VDC, 3-15 VDC, 0-5 VDC, 0-10 VDC and 0-15 VDC.

Current


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The 4-20 mA signal has become the industrys standard current signal for use with analog anddigital controllers. A variation of the 4-20 mA signal is 0-20 mA.

Resistance

Resistance measurement is most commonly associated with direct inputs from temperaturesensing devices, such as thermistors and RTD's. RTD nominal resistances are typically 100 ;,

500;, 1000 ; or 2000;. Common thermistor nominal resistances are 2252 ;, 3k;, 10k;, 20

k;or 100 k;.

Circuit Diagrams

The following circuit diagrams are examples of commonly used analog input configurations.

Figure 4 shows a voltage input circuit where the sensor output voltage does not match thecontroller.


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Figure 5 shows the wiring schematic associated with a typical externally powered 4-20 mAanalog input using a loop power 4-20 mA temperature transmitter. For this circuit type, typical

power supply voltage is nominally 24 VDC. The circuitry in the transmitter regulates currentflow in the loop between 4 and 20 mA in proportion to the temperature sensed by the sensor. A

parallel fixed resistor is used at the controller terminals to complete the circuit. The resistance of

the A/D converter in the circuit is very high in comparison to R, essentially all of the currentflows through the resistor. The value of the resistor is chosen to match the input voltage range ofthe controller.

Figure 6 depicts the circuit for converting a resistance to voltage, in this case, a 10 k;

Thermistor-type sensor

Analog OutputsAn analog output is a measurable electrical signal with a defined range that is generated by a

controller and sent to a controlled device, such as a variable speed drive or actuator. Changes inthe analog output cause changes in the controlled device that result in changes in the controlled

process.

Controller output digital to analog circuitry is typically limited to a single range of voltage or

current, such that output transducers are required to provide an output signal that is compatiblewith controlled devices using something other than the controller's standard signal.

Common Types

There are four common types of analog outputs; voltage, current, resistance and pneumatic.

Voltage

Common output voltage ranges are 0-5 VDC, 0-10 VDC, 0-15 VDC, 1-5 VDC, 2-10 VDC and

3-15 VDC.


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Current

Common output current ranges are 4-20 mA, 0-20 mA.

Resistance

Common output resistance ranges are 0-135 ; , 0-270 ; , 0-500 ; ,0-1000 ; , 0-1500 ; , 0-2

k; , 0-3 k;, 0-4 k;, 0-5 k; , 0-10 k; ,0-20 k; , 0-30 k; , 0-40 k; .

Pneumatic

Common output pneumatic ranges are 0-20 psi and 0-15 psi.

Special IOs

Inputs and outputs can also be used in special configurations. Common special applications areaccumulating points, pulse width modulated (PWM) signals, multiplexed PWM signals and tri-state or floating points.

Accumulating Points

Accumulating points are typically associated with inputs and are special in that during each scanthe controller adds the input point value to the accumulated value. Accumulating points may

have either analog or digital input.

One of the most common applications of accumulating points is with turbine-type flow meters,

which generate a pulse or change of input state with each rotation of the turbine rotor. The totalnumber of pulses is proportional to the volume of fluid passing through the meter. The numberof pulses per unit of time is proportional to the flow rate during that time interval. Accumulating

points are also used to determine energy quantities, such as kilowatt-hours from a power sensorand MBtu from flow and temperature sensors.

Pulse Width Modulated (PWM)

Pulse width modulated signals are based on the amount of time a digital output circuit is closed

over a fixed time base. This amount of time can range from 0 to 100 percent of the time base,providing an analog value for each time period that represents the time base of the signal.

Common time bases are 2.85 seconds, 5.2 seconds, 12.85 seconds and 25.6 seconds.

Multiplexed PWM

A single pulse width modulated digital output is sometimes used to transmit analog values to

multiple analog output devices. Many processes are possible. One scheme is to send an"attention" pulse, which is a pulse of longer duration than the time base. This pulse causes all of

the analog devices to look for a selection signal to follow. A "select" pulse is then transmitted


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with duration less than the time base. Each analog device that is multiplexed looks for a fixedunique range of "select" pulse width. The device that receives the select pulse then looks for

another pulse whose width corresponds to its updated analog value. When the pulse is received,the selected analog device updates its output to the new value and the process is repeated.

The time base of the PWM signal and the number of devices multiplexed on one signal limit theupdating of multiplexed output values. Multiplexed outputs may not be suitable for controlapplications requiring rapid responses to system changes.

Tri-State or Floating Point

A Tri-State signal consists of two digital signals used together to provide three commands. This

type of signal is commonly used to operate a damper or valve actuator in a modulating fashion,but may also be used with a transducer to generate an analog signal. If both digital outputs are

"off", the actuator does not move. Output 1 "on" will cause movement in one direction; output 2"on" will cause movement in the other direction. The fourth possible signal (both outputs "on") is

not used in tri-state operation. The concept was initially developed to allow electric controlsconsisting of single pole, double throw switches with a center-off position to control actuators in

a modulating fashion. Modulating operation is achieved by this action because the actuatorsbeing controlled drive slowly so the change in position is proportional to the amount of time the

output remains energized.

Input Devices and Sensors

Switches Intro

In the world of HVAC control, there is basically one type of device used to complete a digital

input (DI) circuit. A switch, employed in various forms, is this device.

A switch is an electrical device used to enable or disable flow of electrical current in an electrical

circuit. Switches may be actuated in a variety of ways, including movement of two conductingmaterials into direct contact (mechanical), or changing the properties of a semi-conducting

material by the application of voltage (electronic).

Switches are typically rated in terms of voltage, voltage type (AC or DC), current carryingcapacity, current interrupting capacity, configuration, and load characteristic (inductive or

resistive). Also specified are applicable ranges of ambient conditions over which the ratings arevalid. Current carrying capacity (or current rating) is the maximum current that may

continuously flow through the closed switch contacts without exceeding the maximumpermissible temperature.

Process medium property sensing switches are also rated by parameters such as adjustmentrange, accuracy or repeatability, and deadband or differential. The range of a control switch is

specified by upper and lower process values between which the switch has been designed tooperate. The accuracy or repeatability of a control switch is a value typically measured in process

units or percent of range that represents the expected maximum deviation from setpoint at which


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Types of Switches

The following sections outline common switching devices currently used by the industry.

Hand Switches

Hand switches are used as digital input devices and in hardwired electrical control circuits

associated with digital outputs. Hand switches come in numerous sizes, shapes, andconfigurations. Common switch types include rotary, selector type switches, toggle switches, and

pushbuttons. Selector and toggle switches are almost always maintained contact type.Pushbuttons may be momentary or maintained contact type. Selector switches can have key

operators to prevent tampering.


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Figure 2.2- Pushbuttons and Selector Switches (courtesy IDEC)

Limit Switches

Limit switches convert mechanical motion or proximity into a switching action. Limit switchesare most commonly used in DDC control systems for HVAC to provide position status feedback

to the controller for valve and damper positions. A wide variety of configurations are available.Common types include industrial limit switches, mercury, and proximity switches.

Figure 2.3-Industrial Limit Switches

Figure 2.4-Mercury Limit Switches


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Figure 2.5-Proximity Switches

Temperature Switches

Temperature switches (also called thermostats, aquastats or freezestats depending on application)

are commonly used in DDC control systems to provide a digital input when a process mediumtemperature rises or falls to a set temperature. Switches with a number of different operating

principles are manufactured. Some of the common types include bimetallic, fluid thermalexpansion, freezestat and electronic.

Bimetallic temperature switches use a bonded "bimetal" strip consisting of two dissimilar metals

with different thermal coefficients of expansion. When the temperature changes, the metalsexpand or contract at different rates causing the strip to bend. Various configurations such as

coiled elements are used to increase the thermal movement to cause two contacts to cometogether or separate. Some configurations use the bimetallic principle to change the orientation

of a bulb containing liquid mercury so that the mercury flows into contact with two electrodes,completing the circuit.

Fluid thermal expansion temperature switches use the principle of thermal expansion of a fluid tocause displacement of a bellows, diaphragm, bourdon tube, or piston to open or close a set of

contacts. Fluid system based temperature switches can be connected to a remote fluid containingbulb by a capillary tube, allowing the switch element to be remote from the sensing bulb.


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Figure 2.6- Remote Bulb Thermostat

The freezestat is commonly used to prevent water or steam coils in air handling units fromfreezing. Freezestats use a fluid that is a saturated vapor at the switch set point temperature. This

fluid is confined within a long capillary tube. The tube is installed in a serpentine fashion overthe area of the air stream to being monitored. If any point along the tube falls below the

saturation temperature, the vapor begins to condense causing a rapid change in pressure in thesystem and actuating the switch mechanism.

Electronic temperature switches use the same sensing technologies used for analog temperature

sensing to electronically operate a set of output contacts. Refer to the Temperature Measurementportion of the Analog Input Device Section for more details of sensing technology.

Figure 2.7-Freezestat

Humidity Switches

Humidity switches, or humidistats, are used in DDC control systems to provide a digital inputwhen a process or space humidity level rises or falls to a set level. Common applications are high


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limit safety interlocks for humidifiers, space or process humidity alarms, and simple on-offhumidity control.

Mechanical humidistats use a hygroscopic material such as animal hair, nylon or other plastic

material that changes dimension with changes in relative humidity. The dimensional change is

amplified via a mechanical link to causing a switch to operate.

Mechanical humidistats are rapidly being replaced by electronic humidistats that use thin film

capacitance or bulk polymer resistance analog humidity sensing technologies combined withelectronic switching circuitry to produce a switching action at an adjustable set point. These

sensing technologies are described in the Humidity Measurement portion of the Analog InputDevice Section.

Flow Switches

Flow switches are used to provide a digital input to DDC controls systems when a fluid flow rate

has risen above or fallen below the set value. Common applications include safety air and waterflow interlocks for electric heaters and humidifiers, chiller safety interlocks, and burner safetyinterlocks. Numerous technologies are available, but the most common types used in DDC

systems for HVAC control are mechanical and differential pressure types.

Mechanical flow switches operate on the principle that the kinetic energy of a flowing fluidcreates a force on an object suspended in the flow stream. The magnitude of the force varies with

(the square of) the velocity of the fluid. Various configurations are used to transfer this force intooperation of a switch. Common configurations include paddles or sails, pistons or discs.

Differential pressure type flow switches (Figure 2.8) operate on the principle that a difference in

pressure is always associated with fluid flow, or the principle that the total pressure of a flowingfluid is always greater than the static pressure. These differences in pressure can be accurately

predicted for a given situation and related to the fluid flow rate. For more information see theFlow Measurement portion of the Analog Input Section.

Level Switches


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Level switches are used in DDC control systems (for HVAC) to provide a digital input when thefluid level in a tank, vessel or sump has reached a predetermined height. Common applications

include cooling tower sump level control and monitoring, steam condensate tank level, stormwater and sewage sump level monitoring and control and thermal storage tank level monitoring.

Numerous mechanical and analog technologies are currently available. Some analog

technologies include capacitance, ultrasonic, and magnetostrictive-based devices in combinationwith solid-state electronics to provide a switching action based on level. More commonly usedtechnologies include devices that employ the use of a float (integral, rod and float, submersible),

conductivity probe, or differential pressure mechanism.

Integral float type level switches typically combine an metal or plastic float attached to the armof a submersible rotary switch mechanism, or a float that encloses a magnet which slides on a

hollow rod enclosing one or more reed switches.

Submersible float switches utilize an encapsulated integral float type switch or mercury switchsuspended on a fluid tight cord in the vessel being monitored. When the level is below the cord

attachment, the float hangs down and the switch is in its normally open or closed position. Whenthe fluid level rises, the float rises above the cord attachment point, changing the float

orientation. When the float has position has inverted sufficiently, the internal switch changesposition.

Conductivity probe-type level switches work for conductive liquids only and use the liquid itselfto conduct low level electrical signals between two or more electrodes to operate higher level

electronic switching devices such as transistors or triacs.

Pressure Switches

Pressure switches are used in DDC systems to provide status indication for fans, filters andpumps, and to provide flow and level status indication by virtue of the predicable relationships

between pressure and these values. Pressure switches may be mechanical or electronic.

Mechanical pressure switches use a piston, bellows, bourdon tube or diaphragm and a magneticor mechanical linkage to convert the forces resulting from the measured pressure into repeatable

motions used to operate one or more switches (Figure 2.3). Low pressure switches commonlyused to measure air pressures in the range of 0.05 inches water column to 1 psig typically use a

flexible diaphragm. Piston, bourdon tube and bellow type switches are available

Vibration Switches

Vibration switches are used to provide a signal when vibration levels in rotating machinery such

as fans, reach unsafe levels. Vibration switches are commonly applied on large cooling towerand air handling unit fans.

Moisture Switches


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Moisture detecting switches are commonly used to detect moisture under raised floors, in pipingand tank containment areas and in the drain pans of air handling units to alert system operators

before damage or flooding occurs. Most moisture detecting switches are instruments of the floattype or conductivity type. Float types are adapted to actuate at very low levels. Conductivity

types may consist of point sensitive probes located very close to the bottom of a low point or

sump where water will collect, or they may be ribbons or strips with wires separated by a non-conductive material, such that when any portion of the ribbon is exposed to liquid moisture, theelectrical circuit is completed and the switch mechanism activates.

Current Switches

Current sensing relays are used in DDC systems to monitor the status of electrical devices. The

devices typically have one or more adjustable current set points. Common applications includefan and pump on/off status feedback. Current switches can detect broken fan belts if properly

adjusted. Current relays can also be used for phase monitoring.


Temperature Measurements

One of the most common properties measured in the HVAC control world is temperature.

Human comfort, computer room requirements, and a host of other considerations maketemperature measurement necessary to HVAC control strategies.

Types of Temperature Measurement Devices

Several temperature measurement technologies exist for use with DDC control systems. The

most common utilize resistance temperature detectors (RTDs) and thermistor based devices.

Resistance Temperature Detectors- RTD

Resistance Temperature Detectors (RTD's) operate on the principle that the electrical resistance

of a metal changes predictably and in an essentially linear and repeatable manner with changes intemperature. The resistance of the element at a base temperature is proportional to the length of

the element and the inverse of the cross sectional area. RTD's are commonly used in sensing airand liquid temperatures in pipes and ducts, and as room temperature sensors. DDC systems may

accept RTD inputs directly, or a transmitter with voltage or current output may be used.

RTDs are typically characterized by their resistance in Ohms () at 0 C and by their temperaturecoefficient of resistance (commonly know as "alpha"). Alpha is expressed in terms of /( C) and is

the slope of the line representing the resistance of the element between 0 C and 100 C. Theresistance of a RTD can be expressed mathematically by the following equation (source i):

R(T) = R0 [1 + A(T - T0)]


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Where:

y R(T) = the resistance at temperature Ty R0 = the resistance at reference temperature T0y A = temperature coefficient of resistance (alpha)y

T0 = a reference temperature (usually 0 C)

RTDs with R0 resistance from 10 to 2000 are readily available. Currently, the most commonly

used RTDs in HVAC applications are sensors with an R0 resistance of 100 , 500 or 1000.

The accuracy of a RTD sensor is typically expressed in percent of nominal resistance at 0 C

(R0). RTDs are relatively accurate when compared to other sensing devices and have goodstability characteristics. RTDs with accuracies of 0.2% to 0.01% are commonly available.

RTDs are constructed in thin film, thick film, totally supported and "bird-cage" configurations.

They can be made from many materials, some of which include platinum, tungsten, silver,

copper, nickel, nickel alloys and iron. Currently, the most common RTDs (used in the HVACfield) are constructed in film type configurations with platinum, nickel or nickel iron.

Since the resistance of the sensor is the property being measured, the resistance of all elements ofthe circuit, including the sensor leads, affects the measurement. With RTD's and particularly

those with lower base resistance values, the resistance of long leads can amount to severalpercent or more of the sensor circuit. This can result in significant error. One option for

correcting this problem is to locate a transmitter at the sensor. The other way is to compensatefor the lead resistance by the method of wiring.

Three different wiring methods are used, involving two, three and four wires. These are applied

based on accuracy requirements for the application. The circuit diagrams in Figure 2.9 show thevarious methods. Two and three wire configurations commonly use a Wheatstone bridge circuitto create an output voltage that is proportional to the RTD resistance. The two-wire method

provides the lowest accuracy, but is adequate for non-critical measurements. The three-wiremethod provides better accuracy because the lead resistances L1 and L3 cancel when the leads

are of identical length. The effect of L2 is small as long as the bridge is balanced or a highimpedance voltage measuring technique is used. The four-wire circuit is the most accurate, and

uses a constant current source to cancel the effect of unequal length leads. A high-impedancevoltage measurement circuit is used so that the current flow in the measurement leads is

negligible.


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Thermistor

Thermistors are commonly used for sensing air and liquid temperatures in pipes and ducts, andas room temperature sensors. The term "thermistor" evolved from the phrase thermally sensitive

resistor. Thermistors are temperature sensitive semiconductors that exhibit a large change inresistance over a relatively small range of temperature. There are two main types of thermistors,

positive temperature coefficient (PTC) and negative temperature coefficient (NTC). NTCthermistors are commonly used for temperature measurement.

Unlike RTD's, the temperature-resistance characteristic of a thermistor is non-linear, and cannot

be characterized by a single coefficient. Manufacturers commonly provide resistance-temperature data in curves, tables or polynomial expressions. Linearizing the resistance-

temperature correlation may be accomplished with analog circuitry, or by the application ofmathematics using digital computation.

The following is a mathematical expression for thermistor resistance (source ii):


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HVAC control applications. The American National Standards Institute has standardizedthermocouple types. Common types are listed in Table 2.1.

Infrared Temperature Sensors that sense the wavelength of radiation emitted from the surface ofan object without being in physical contact with the object are available with voltage or current

outputs that are compatible with DDC systems.

Comparison

Table 2.2 is a comparison of the most common temperature measurement technologiesapplicable to DDC control systems for HVAC. The comparisons made are general in nature and

not intended to be all inclusive for each sensor type.


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Installation

RTD's, thermocouples, thermistors, and solid-state temperature sensors are all small devices withsimilar mounting techniques used for all of the types. Sensors for pipe and duct mounting are

commonly sheathed in a stainless steel sheath of 1/8 to 1/4" diameter (larger and smallerdiameters are available). Wiring may be exposed or contained in various types of enclosures.

Sensors for liquid piping systems may be mounted with direct immersion into the fluid orinstalled in a tubular sheath called a thermowell or well to allow removal without draining the

piping system and to reduce the likelihood of leaks. Sensors installed in wells should be installedwith a heat transfer compound filling the space between the sensor and the well to insure good

thermal contact between the measured fluid and the sensor.

In measuring the temperature of air in large ducts, it is often desirable to use an averaging

element because the air temperature can vary significantly over the cross section of the duct.RTD and thermistor sensors have been developed that accomplish this using multiple sensorsinstalled in a single flexible tubular element. The element is typically arranged in a serpentine

fashion so as to obtain representative measurements over the entire cross sectional area of theduct. Very large ducts or air handling unit casings ften require multiple sensors that are

customarily wired in parallel-series arrangements. Averaging elements are commonly applieddownstream of mixing dampers, and following large or multiple heating or cooling coils.


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Sensors for outdoor air applications should be located in normally shaded areas to prevent theheating effects of solar radiation. These sensors are usually provided with a shield or hood to

reduce the effects if exposed to direct sunlight and prevent direct contact with precipitation.

In adverse or outdoor environments, it is sometimes desirable to enclose sensors in aspirated

cabinets to prolong their life and reduce maintenance. Aspirated cabinets typically include afiltered air intake and an exhaust fan to provide positive airflow through the enclosure. Flushmount wall sensors, wire guards or locking guards are also used to protect sensors in areas

subject to vandalism.


Humidity Measurements

Humidity is the presence of water vapor in air. The amount of water vapor present in air can

affect human comfort and numerous material properties. It is a parameter that HVAC designs

often must take into account and therefore can be a required measurement in HVAC controlschemes. The amount of water vapor in air can be defined by one of several ratios, which includerelative humidity, humidity ratio, specific humidity, and absolute humidity. By far the most

common measurement of humidity in the HVAC industry is relative humidity (RH).

Relative humidity is the ratio of partial water vapor pressure in an air-water mixture, to thesaturation vapor pressure of water at the same temperature. This is analogous to the ratio of the

number of water molecules per unit volume of the mixture to the number of water molecules thatwould exist in a saturated mixture at the same temperature.

Types of Relative Humidity Sensors

Relative Humidity sensors are used in DDC control systems for HVAC to measure relativehumidity in conditioned spaces and ducts. Commonly applied sensor types include thin-film

capacitance, bulk polymer resistance, and integrated circuit type. The integrated circuit typecombines a sensor (commonly of the capacitance type) and some of the signal conditioning

circuitry to form a solid-state device. Relative humidity can also be measured along with dewpoint and other humidity measurements by chilled mirror hygrometers. See the Chilled Mirror

Hygrometers section in the section on Dew Point Measurement.

Thin Film Capacitance

Thin film capacitance sensors operate on the principle that changes in relative humidity cause thecapacitance of a sensor (made by laminating a substrate, electrodes, and a thin film of

hygroscopic polymer material) to change in a detectable and repeatable fashion. Because of thenature of the measurement, capacitance humidity sensors are combined with a transmitter to

produce a higher-level voltage or current signal. Key considerations in selection of transmittersensor combinations include range, temperature limits, end-to-end accuracy, resolution, long-

term stability, and interchangeability.


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Capacitance type relative humidity sensor/transmitters are capable of measurement from 0-100% relative humidity with application temperatures from -40 to 200 F. These systems are

manufactured to various tolerances, with the most common being accurate to 1%, 2%, and 3%.Capacitance sensors are affected by temperature such that accuracy decreases as temperature

deviates from the calibration temperature. Sensors are available that are inter-changeable within

plus or minus 3% without calibration. Sensors with long term stability of


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Calculation from Temperature and Relative Humidity

It is common practice when measuring relative humidity to combine a temperature sensor and

transmitter into the same device as the humidity sensor. Using a microprocessor, it is thenpossible to calculate and transmit dew point. Accuracy is limited by the combined accuracy of

the sensors and the electronics. Typical accuracy is 1.8 F. Typical repeatability is 0.7 F.Commonly, these devices can be configured to output calculated humidity ratio, wet bulbtemperature, and absolute humidity as well as dew point.

Chilled Mirror Hygrometers

Chilled mirror sensing technology has been in use since the 1950's for determination of dew

point temperature. Modern chilled mirror hygrometers use a thermoelectric heat pump (alsocalled a Peltier device) to move heat away from a mirror. A light beam from an LED is directed

to the mirror and back to a photocell. When condensation (above 0 C) or frost (below 0 C) formson the mirrors surface, the light reaching the mirror is scattered and the intensity detected by the

photocell is reduced. The mirror is maintained at the dew point temperature by controlling theoutput of the thermoelectric heat pump. A high accuracy, platinum resistance thermometer

(RTD) senses the temperature of the mirrors surface and therefore reports the dew pointtemperature. Chilled mirror hygrometers require a vacuum pump to draw the sample through the

sensor, and additional filtration elements in dirty environments.

Chilled mirror hygrometers are subject to inaccuracies resulting from soluble and insolublecontaminants depositing on the mirror. Insoluble contaminants affect the optical characteristics

of the mirror. Soluble contaminants affect the vapor pressure of the condensed moisture on themirror. Most sensors have insoluble contaminant compensation cycles that heat the mirror (to dry

it) and then reset the optical parameters of the light sensor to the current mirror optical

parameters. Unless the soluble contaminants are volatile, the insoluble contaminantcompensation does not remove the soluble contaminants. Virtually all chilled mirror sensorsrequire periodic inspection and cleaning.

Many chilled mirror hygrometers have microprocessor control and when combined with a drybulb temperature sensor can calculate and output any humidity parameters desired in addition to

or instead of dew point. Chilled mirror hygrometers are available for sensing dew/frost pointtemperatures from -100 to 185 F. Accuracy of better than 0.5 F is available.


Pressure Measurements

Pressure is measured in DDC controls systems for HVAC in order to control the operation andmonitor the status of fans and pumps. Space pressure is sometimes measured and used for

control. Pressure is also the basis of many flow and level measurements.

Types of Pressure Sensors


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Diverse electrical principles are applied to pressure measurement. Those commonly used withDDC control systems include capacitance and variable resistance (piezoelectric and strain gage).

Capacitance

Capacitance pressure sensors typically use a capacitance cell (Figure 2.11) consisting of adiaphragm exposed to the pressure medium separated from another plate by a fill fluid. When theapplied pressure deflects the diaphragm, the capacitance characteristic of the sensing element

changes. The capacitance cell is excited by a high frequency source. The frequency changes asthe capacitance of the cell changes. This frequency shift is converted to the output signal by the

transmitter electronics. Capacitance transmitters are available configured for either differential orgauge pressure measurement. Usual outputs are voltage or current.

Capacitance transmitters are available with ranges from a few inches water column (in. w.c.) to

thousands of pounds per square inch (psi). Transmitter accuracy of 1% of full scale is commonfor inexpensive versions. Accuracy to 0.1% of full scale is available with 'smart' transmitters

using microprocessor signal conditioning and compensation. Smart transmitters can be calibratedusing hand-held operator interface devices, or by digital communication over analog signal

wiring using any of several protocols. Varying grades of transmitter packaging (molded plastic toforged stainless steel) are available depending on the application and price.

Variable Resistance

Variable resistance technology includes both strain gage and piezo-resistive or piezoelectric

technologies.

Traditional strain gages are constructed of wire filament bonded to a substrate. The resistance ofthe wire changes in proportion to the strain in the substrate, which is transmitted to the wire

through the bond. Strain gauges are applied to diaphragms or other mechanical pressure elements


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and change resistance in response to strains induced in the element by the applied pressure.When arranged to form a Wheatstone bridge circuit, an analog voltage signal is produced that is

proportional to applied pressure.

Piezo-resistive sensors operate on the principle that certain semiconductor materials, such as

silicon, change resistance with stress or strain. These piezo-resistive elements are implanted on asolid-state chip that is attached to a mechanical sensing element or used as the sensing element.When the piezo-resistive elements are arranged to form a bridge circuit (as with the wire

filament strain gage sensor), an analog voltage signal is produced that is proportional to theapplied pressure.

Piezo-resistive type sensors have a sensitivity of approximately 100 times greater than a wire

strain gage. Also, other strain gages must usually be bonded to a dissimilar force sensingmaterial with different composition and thermal characteristics. The wire strain gage sensor is

subject to degradation from failure of the bond to the force sensing element, thermal effects andplastic deformation of the force-sensing element. In contrast, the silicon based piezo resistors

may be integral with a silicon wafer that serves as the force-sensing element. This eliminatesmany of the inherent problems with thermal effects and bonding. Silicon has very good elasticity

throughout the typical operational range and normally fails only by rupturing.

Strain gage and piezo-resistive transmitters are available with ranges of a few inches water

column (in. w.c.) to thousands of pounds per square inch (psi). Transmitter accuracy of 1% offull scale is common for inexpensive versions. Accuracy better than 0.1% of full scale is

available with 'smart' transmitters using microprocessor signal conditioning and compensation.Smart transmitters can be calibrated using hand-held operator interface devices, or by digital

communication over analog signal wiring using any of several protocols. Available transmitterpackaging ranges from molded plastic to forged stainless steel depending on the application and

price.

Installation

Process connections for pressure instruments are typically made using piping or tubing. Themajority of applications in the HVAC DDC field fall into two categories, the first being

ductwork and plenums, and the second being piping.

Ductwork and Plenums

Special sensing tips are often used when connecting pressure instruments to ductwork for

measurement of static, velocity, or total pressures. This is necessary because improperorientation of an open-ended tube type probe can result in unreliable readings due to thedirectional nature of the pressures being measured (with the exception of very low velocity

flow). Numerous types of pressure probes have been developed for these applications. Many ofthese probes are adaptations of the Pitot tube used in pressure and flow measurement and

discussed in detail in the Differential Pressure Measurement Systems section of this document

Piping


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The major considerations for the installation of a pressure element in a fluid system shouldinclude provisions for the following:

y sensor location (pipe mounted, tank mounted, remote);y isolation of the sensing element from undesirable and potentially damaging transient

pressures, such as those resulting from water hammer and turbulence;y temporary isolation from the pressure source for maintenance and release of trapped

pressure when removing the sensor for maintenance or for setting zero during calibration;

y over-range protection for differential pressure instruments;y protection from process temperature outside of the range of the sensor application;y venting trapped, non-condensable gases in liquid sensing piping;y draining trapped liquids from gas.

Pressure snubbers or dampeners are used to reduce the magnitude of pressure transients. These

can be a sintered metal element with small openings, a small orifice fitting, a high-pressure dropvalve (such as a needle valve), or a pressurized gas filled container mounted on the sensing

piping.

A variety of valving schemes to provide isolation, venting, drain, and pressure relief for pressure

instruments are shown in the Figures 2.12-2.14. One valve (not shown) or two-valve manifoldsare commonly applied to gauge and absolute pressure instruments. Three- and five- valve

manifolds are used with differential pressure instruments. The equalizing valve in the three- andfive- valve manifold insures a proper zero for the transmitter. It also allows the pressure to be

equalized to prevent exposing low differential transmitters to potentially damaging gaugepressures during installation and removal.


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Flow Measurements

Flow measuring devices are widely used in DDC control systems for HVAC to monitor andcontrol various air and liquid flows. Typically, airflow-measuring devices are used to monitor

and control the output of fans, dampers, and associated equipment used to control outsideairflow, VAV box airflow, and building and space pressures. Liquid flow is commonly measured

to maintain required flows in boilers, chillers and heat exchangers, and to control and monitorenergy production and use (requires temperature measurement also).


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Numerous reliable technologies are available for use with DDC systems. Some technologieshave been applied to both air and liquid flow measurements as their principles of operation hold

true in either application. Other technologies lend themselves to being airflow or liquid flowspecific.

Methods for Measuring Flow

Flow rate is typically obtained by measuring a velocity of a fluid in a duct or pipe and

multiplying the by the known cross sectional area (at the point of measurement) of that duct orpipe. Common methods for measuring airflow include hot wire anemometers, differential

pressure measurement systems, and vortex shedding sensors. Common methods used to measureliquid flow include differential pressure measurement systems, vortex shedding sensors, positive

displacement flow sensors, turbine based flow sensors, magnetic flow sensors, ultrasonic flowsensors and target flow sensors.

Hot Wire Anemometers

"Hot Wire" or thermal anemometers operate on the principle that the amount of heat removed

from a heated temperature sensor by a flowing fluid can be related to the velocity of that fluid.Most sensors of this type are constructed with a second, unheated temperature sensor to

compensate the instrument for variations in the temperature of the air. Hot wire sensors areavailable as single point instruments for test purposes, or in multi-point arrays for fixed

installation. Hot wire type sensors are better at low airflow measurements than differentialpressure types, and are commonly applied to air velocities from 50 to 12,000 feet per minute.

Differential Pressure Measurement Systems

Differential pressure measurement technologies can be applied to both airflow and liquid flowmeasurements. Sensor manufacturers offer a wide variety of application specific sensors used for

airflow and pressure measurements, as well as wet-to-wet differential pressure sensors used forliquid measurements. Both lines offer a wide variety of ranges.

For airflow measurements, differential pressure flow devices in common use in HVAC systems

include Pitot tubes (Figure 2.15) and various types of proprietary velocity pressure sensing tubes,grids, and other arrays. All of these sensing elements are combined with a low differential

pressure transmitter to produce a signal that is proportional to the square root of the fluidvelocity. For example, when using a Pitot-static tube, this signal can be related to the flow

according to the following equations (source iii):

Where:


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Velocity = Velocity (ft/min)VP = velocity pressure (in w.c.)

p = density of air (lbm/ft2)gc = gravitational constant (32.174 lbmft/lbfs2)

C = unit conversion factor (136.8)

Figure 2.16 depicts an example of a velocity pressure measurement with a U tube manometer

and Figure 2.17 depicts an example of the relationship between velocity pressure (VP), static

pressure, and total pressure.


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As a permanently mounted sensor, the Pitot tube is limited to small ducts and applications withlow accuracy requirements due to the need to sense the velocity at more than one point to

achieve reliable measurements in larger ducts. The need to sense multiple points in the crosssection of a duct gave rise to averaging type sensors with arrays of pressure sensing points. This

type is most commonly used in HVAC applications.

Some differential pressure based flow stations include transmitters that have the capability toelectronically extract the square root of the measured pressure and provide an analog signal that

is linear with respect to velocity, whereas others provide an analog signal that is proportional tomeasured pressure and depend upon the DDC system to calculate the square root and therefore,

resulting (averaged) velocity. Once the velocity is obtained, flow can be calculated bymultiplying by the cross sectional area of the duct. Velocity range is limited by the range and

resolution of the pressure transmitter used. Most differential pressure type stations are limited toa minimum velocity in the range of 400 to 600 feet per minute. Maximum velocity is only

limited by the durability of the sensor.

For water flow measurements, differential pressure flow devices in common use in HVAC

systems operate either by measuring velocity pressure (insertion tube type), or by measuring thedrop in pressure across a restriction of known characteristic (orifice, flow nozzle, Venturi).

Insertion tube type flow sensors are usually constructed of a round or proprietary shape tube with

multiple openings across the width of the flow stream to provide an average of the velocitydifferential across the tube and an internal baffle between upstream and downstream openings to

obtain a differential pressure. Insertion tube type meters have a low permanent pressure loss, andwith proper installation and associated pressure instruments are satisfactory for many common

applications. Insertion tube flow sensors are available that can be installed and removed througha full port valve so that installation and service are possible without draining the section of

piping in which they are installed.

A concentric orifice plate is the simplest and least expensive of the differential pressure typemeters. The orifice plate constricts the flow of a fluid to produce a differential pressure across

the plate (see Figure 2.18). The result is a high pressure upstream and a low pressure downstreamthat is proportional to the square of the flow velocity. An orifice plate usually produces a greater


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The turndown (ratio of the full range of the instrument to the minimum measurable flow) ofdifferential pressure devices is generally limited to 4:1. With the use of a low range transmitter in

addition to a high range transmitter or a high turndown transmitter and appropriate signalprocessing, this can sometimes be extended to as great as 16:1 or more. Permanent pressure loss

and associated energy cost is often a major concern in the selection of orifices, flow nozzles, and

venturis. In general, for a given installation, the permanent pressure loss will be highest with anorifice type device, and lowest with a Venturi. Benefits of differential pressure instruments aretheir relatively low cost, simplicity, and proven performance.

Vortex Shedding Sensors

Vortex shedding flow meters operate on the principle (Von Karman) that when a fluid flows

around an obstruction in the flow stream, vortices are shed from alternating sides of the

obstruction in a repeating and continuous fashion. The frequency at which the sheddingalternates is proportional to the velocity of the flowing fluid. Single sensors are applied to smallducts, and arrays of vortex shedding sensors are applied to larger ducts, similar to the other types

of airflow measuring instruments. Vortex shedding airflow sensors are commonly applied to airvelocities in the range of 350 to 6000 feet per minute.

Vortex flow meters provide a highly accurate flow measurement when operated within the

appropriate range of flow. Vortex meters are commonly applied where high quality water, gasand steam flow measurement is desired. Performance of up to 30:1 turndown on liquids and 20:1

on gases and steam with 1-2 percent accuracy is available. Turndowns are based on liquidvelocities through the meter of up to 25 feet per second for liquids, 15,000 feet per minute for

steam and gases. Actual turndown may be less depending on design velocity limitations.

Positive Displacement Flow Sensors

Positive displacement meters are used where high accuracy at high turndown is required andreasonable to high permanent pressure loss will not result in excessive energy consumption.

Applications include water metering such as for potable water service, cooling tower and boilermake-up, and hydronic system make-up. Positive displacement meters are also used for fuel


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metering for both liquid and gaseous fuels. Common types of positive displacement flow metersinclude lobed and gear type meters, nutating disk meters, and oscillating piston type meters.

These meters are typically constructed of metals such as brass, bronze, cast and ductile iron, butmay be constructed of engineered plastic, depending on service.

Due to the close tolerance required between moving parts of positive displacement flow meters,they are sometimes subject to mechanical problems resulting from debris or suspended solids inthe measured flow stream. Positive displacement meters are available with flow indicators and

totalizers that can be read manually. When used with DDC systems, the basic meter output isusually a pulse that occurs at whatever time interval is required for a fixed volume of fluid to

pass through the meter. Pulses may be accepted directly by the DDC controller and converted toflow rate, or total volume points, or a separate pulse to analog transducer may be used. Positive

displacement flow meters are one of the more costly meter types available.

Turbine Based Flow Sensors

Turbine and propeller type meters operate on the principle that fluid flowing through the turbineor propeller will induce a rotational speed that can be related to the fluid velocity. Turbine and

propeller type flow meters are available in full bore, line mounted versions and insertion typeswhere only a portion of the flow being measured passes over the rotating element. Full bore

turbine and propeller meters generally offer medium to high accuracy and turndown capability atreasonable permanent pressure loss. With electronic linearization, turndowns to 100:1 with 0.1%

linearity are available. Insertion types of turbine and propeller meters represent a compromise inperformance to reduce cost. Typical performance is 1 percent accuracy at 30:1 turndown.

Turbine flow meters are commonly used where good accuracy is required for critical flowcontrol or measurement for energy computations. Insertion types are used for less critical

applications. Insertion types are often easier to maintain and inspect because they can be

removed for inspection and repair without disturbing the main piping. Some types can beinstalled through hot tapping equipment and do not require draining of the associated piping forremoval and inspection.

Magnetic Flow Sensors

Magnetic flow meters operate based upon Faraday's Law of electromagnetic induction, whichstates that a voltage will be induced in a conductor moving through a magnetic field.

Faraday's Law: E=kBDV

The magnitude of the induced voltage E is directly proportional to the velocity of the conductorV, conductor width D, and the strength of the magnetic field B. As shown in Figure 2.21,magnetic field coils are placed on opposite sides a pipe to generate a magnetic field. As the

conductive process liquid moves through the field with average velocity V, electrodes sense theinduced voltage. The distance between electrodes represents the width of the conductor. An

insulating liner prevents the signal from shorting to the pipe wall. The only variable in thisapplication of Faraday's law is the velocity of the conductive liquid V because field strength is


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controlled constant and electrode spacing is fixed. Therefore, the output voltage E is directlyproportional to liquid velocity, resulting in the linear output of a magnetic flow meter.

Magnetic flow meters are used to measure the flow rate of conducting liquids (including water)where a high quality low maintenance measurement system is desired. The cost of magnetic flow

meters is high relative to many other meter types. Typical performance is 30:1 turndown at 0.5%accuracy.

Ultrasonic Flow Sensors

Ultrasonic flow sensors measure the velocity of sound waves propagating through a fluid

between to points on the length of a pipe. The velocity of the sound wave is dependant upon thevelocity of the fluid such that a sound wave traveling upstream from one point to the other is

slower than the velocity of the of the same wave in the fluid at rest. The downstream velocity ofthe sound wave between the points is greater than that of the same wave in a fluid at rest. This is

due to the Doppler effect. The flow of the fluid can be measured as a function of the difference intime travel between the upstream wave and the downstream wave.

Ultrasonic flow sensors are non-intrusive and are available at moderate cost. Many models are

designed to clamp on to existing pipe. Ultrasonic Doppler flow meters have accuracies of 1 to5% to the flow rate (source iv).

Target Flow Sensors

A target meter consists of a disc or a "target" which is centered in a pipe (see Figure 2.22). Thetarget surface is positioned at a right angle to the fluid flow. A direct measurement of the fluid


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flow rate results from the force of the fluid acting against the target. Useful for dirty or corrosivefluids, target meters require no external connections, seals, or purge systems.

Target flow meters are commonly used to for liquid flow measurement and less commonly

applied to steam and gas flow. Target Meters offer turndowns up to 20:1 with accuracy around

1%.

Installation

All airflow sensors work best in sections of ducts that have uniform, fully developed flow. All

airflow sensing devices should be installed in accordance with the manufacturers recommendedstraight runs of upstream and downstream duct in order to provide reliable measurement. A

number of manufacturers offer flow straightening elements that can be installed upstream of thesensing array to improve undesirable flow conditions. These should be considered when

conditions do not permit installation with the required straight runs of duct upstream anddownstream from the sensor.

As with airflow, all liquid flow sensors work best when fully developed, uniform flow is

measured. To attain fully developed, uniform flow sensors should be installed in accordance withthe manufacturers recommended straight runs of upstream and downstream pipe in order to

provide the most reliable measurements.

With most liquid flows measured for HVAC applications, density changes with pressure andtemperature are relatively small and most often ignored due to their insignificant effect on flow

measurements. When measuring the flow of steam or fuel gases, unless temperature and pressureare constant, ignoring the effect density changes with varying temperature and pressure will

often result in significant or gross errors. For this reason, it is common to measure thetemperature and pressure, in addition to the flow, and electronically correct the result for the

fluid density. This correction may be done using an integral or remote microprocessor based"flow computer" or it may be made in the DDC controller with suitable programming.


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Liquid Level Measurements

Liquid level measurements are typically used in DDC control systems for HVAC applications to

monitor and control levels in thermal storage tanks, cooling tower sumps, water system tanks,pressurized tanks, etc.

Types ofLiquid Level Sensors

Numerous sensing technologies are available. Common technologies applicable to HVAC

system requirements are based on hydrostatic pressure, ultrasonic, capacitance andmagnetostrictive-based measurement systems.

Hydrostatic

Level measurement by hydrostatic pressure is based on the principle that the hydrostatic pressure

difference between the top and bottom of a column of liquid is related to the density of the liquidand the height of the column. For open tanks and sumps, it is only necessary to measure the

gauge pressure at the lowest monitored level. For pressurized tanks it is necessary to take thereference pressure above the highest monitored liquid level. Pressure transmitters are available

that are configured for level monitoring applications. Pressure instruments may also be remotelylocated, however this makes it necessary to field calibrate the transmitter to compensate for

elevation difference between the sensor and the level being measured.

Bubbler type hydrostatic level instruments have been developed for use with atmosphericpressure underground tanks, sewage sumps and tanks, and other applications that cannot have a

transmitter mounted below the level being sensed or are prone to plugging. Bubbler systemsbleed a small amount of compressed air (or other gas) through a tube that is immersed in the

liquid, with an outlet at or below the lowest monitored liquid level. The flow rate of the air isregulated so that the pressure loss of the air in the tube is negligible and the resulting pressure at

any point in the tube is approximately equal to the hydrostatic head of the liquid in the tank.

The accuracy of hydrostatic level instruments is related to the accuracy of the pressure sensor

used.

Ultrasonic

Ultrasonic level sensors emit sound waves and operate on the principle that liquid surfacesreflect the sound waves back to the source and that the transit time is proportional to the distance

between the liquid surface and the transmitter. One advantage of the ultrasonic technology is thatit is non-contact and does not require immersion of any element into the sensed liquid. Sensors

are available that can detect levels up to 200 feet from the sensor. Accuracy from 1% to 0.25% ofdistance and resolution of 1/8" is commonly available.


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Capacitance

Capacitance level transmitters operate on the principle that a capacitive circuit can be formed

between a probe and a vessel wall. The capacitance of the circuit will change with a change influid level because all common liquids have dielectric constant higher than that of air. This

change is then related proportionally to an analog signal suitable for DDC analog inputs.Resolution of 1/8" and accuracy of 1% to 0.25% of span are available.

Magnetostrictive

Magnetostrictive level transmitters (Figure 2.23) operate on the principle that an externalmagnetic field can be used to cause the reflection of an electromagnetic wave in a waveguide

constructed of magnetostrictive material. The probe is composed of three concentric members.The outermost member is a protective, product-compatible outer pipe. Inside the outer pipe is a

waveguide, which is a formed element constructed of a proprietary magnetostrictive material. Alow-current interrogation pulse is generated in the transmitter electronics and transmitted down

the waveguide creating an electromagnetic field along the length of the waveguide. When thismagnetic field interacts with the permanent magnetic field of a magnet mounted inside the float,

a torsional strain pulse, or waveguide twist, results. This waveguide twist is detected as a returnpulse. The time between the initiation of the interrogation pulse and the detection of the return

pulse is used to determine the level measurement with a high degree of accuracy and reliability.Accuracy and resolution of 1/16" or better are available from some manufacturers.


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Light Measurements

Light level sensors are used by DDC control systems for lighting control. They are typically usedto turn on night lighting when light level drops below a set level and are also used to turn off

indoor and outdoor lighting when ambient levels are sufficient. Light level sensors can be used tocontrol the output of dimmable fluorescent lighting to set levels. Accuracy of 1% of reading is

common.


Electrical Measurements

Monitoring of electrical system attributes is performed by DDC control systems to protect

system components, determine power and energy consumption of various components, andimplement usage and demand control strategies to conserve energy. A variety of hardware and

techniques are applied to these measurements.


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Types of Electrical Measurement Devices

There are many devices that measure electrical attributes on the market today. The two most

common electrical measuring devices used for DDC are current transducers and powermeasuring devices.

Current Transducers

Current transducers are used in DDC control systems to monitor current flow to motors, heaters,or electrical distribution systems. Their input may be used for demand limiting purposes, control,

or energy accounting. The sensing element of a current transducer is typically a currenttransformer. It transforms the current being monitored into a higher voltage, lower current.

Additional circuitry reduces this voltage to the desired level. Current transducers may have lineand load terminals for the monitored current, or they may be arranged as a coil that the current

carrying conductor passes through. With this arrangement, the load conductor induces the currentin the transformer via the electromagnetic field surrounding the conductor. Current transformers

and transducers are available with solid or split cores. The split core device may be installedwithout disconnecting the power conductor provided that there is sufficient slack in the

conductor and room in the enclosure. Accuracy of 0.5 % of full scale is readily available.

Power Monitoring Devices

Commonly monitored characteristics of a power system include:

y Power Demand (typically measured in kW)y Power Consumption (typically measured kW per hour)y Voltage (typically measured in Volts)y Current (typically measured in Amps)y Frequency (typically measured in Hertz)y Power Factory Reactive Power - (typically measured in kVAR)

Many panel level monitoring devices measure all or most of these characteristics and can

communicate to the DDC system through a gateway. These are typically used to monitor wholebuilding power systems. Other devices measure power and power consumption only and provide

both analog and pulse signals for input to the DDC system. These sensors are typically installedat the terminal use point of power systems, such as variable speed drive controlled pump and fan

motors. Accuracy 0.2% of reading and 0.04% of full scale are available.

There are other methods of monitoring demand and consumption. One of the simplest methods isto obtain a pulse signal output from the utility company's metering equipment. This can be input

directly to a controller with pulse input capability, or a pulse to analog signal transducer may beused. The pulse represents a set number of kilowatt-hours. Average demand is calculated using a

rolling time average of the number of pulses over the stipulated time period. Average demand istypically calculated for billing purposes over a 5, 15, or 30 minute period. Power consumption

and demand may also be calculated using current transformers to measure current flow and


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voltage transducers to measure voltage on the selected load or system. The DDC controllercalculates the demand from these values, and integrates this value over time to determine power

use.

Other Electrical Measurement Devices

Transducers are available to provide a standard voltage or current input to a controller based onmeasured frequency, reactive power, or power factor. Available devices for load protection are

available that monitor three phase voltages and provide a relay signal to disconnect loads if thepower supply becomes unsuitable for continued operation due to conditions such as phase loss,

phase imbalance, low or high voltage, or phase reversal.

Load protection for motors may be incorporated into the motor starter through the use of a solidstate overload device. These devices provide the required time-current protection to protect the

motor from overload conditions, as well as power monitoring to protect the motor fromunsatisfactory power supply.


Energy Measurements

The measurement of energy is a very important aspect of the DDC system. Savings due tooperational procedures and equipment performance can be directly determined through this

measurement. A variety of devices and methods are currently available.

Types of Energy Measurement Devices

The three most common energy measurements used for DDC systems are airside, waterside and

electrical energy measurements. Airside energy measurements are typically calculated in theDDC system using air temperature and flow rate measurements. Waterside energy can be

calculated in the DDC system or with energy measuring devices called BTU meters. Electricalenergy measurements can be calculated in the DDC system or with Power Monitoring Devices.

BTUMetering Devices

BTU meters are used to determine energy flows in hydronic systems within a facility for

accounting or control purposes. Determination of heat flow requires measuring the heat transfer

medium flow and the difference in temperature between the supply and return to the meteredload or producer.

With suitable software, this can be accomplished using the DDC system. This may also beaccomplished external to the DDC system using a microprocessor-based computer with flow and

temperature inputs, and analog output to the DDC system representing totalized energyconsumption in BTU or ton-hours, or energy flow in BTU per hour, tons, or similar units. Many

manufacturers of flow measurement devices offer this type of system.


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Power Monitoring Devices

Power monitoring devices can be used to monitor electrical energy usage. They can either

directly measure the energy usage by providing pulses that represent kW per hour, or can providean analog signal that measures power which can be used in an energy calculation (over time) in

the DDC system. For more details please refer to the previous section on Power MonitoringDevices.


Occupancy Measurements

Occupancy sensors are commonly used in building control systems to operate lighting and room

air conditioning equipment. Sensors turn lights and air conditioning equipment off (or to reducedlevels) when no occupants are detected. This is done to minimize energy consumption.

Occupancy sensors may be designed to detect motion or differences in background infraredradiation and the radiation emitted from a human occupant. Many occupancy sensors used for

lighting also incorporate photocells or other light sensitive devices to reduce lighting whenambient light is sufficient.


Position Measurements

Position sensors and transmitters are used in HVAC system controls where the feedback ofposition is necessary for precise control of system components, such as valves and dampers, or

where monitoring of position is necessary or desired. Position transmitters commonly operateusing a slidewire or rotary potentiometer to provide a variable resistance that changes with linear

or rotary position.

Getting Started | Chapter 2: Occupancy | Top | Chapter 2: Gas Concentration


Gas Concentration Measurements

With the increased interest in indoor air quality and the need to monitor potentially dangerousgases, gas concentration measurements have become increasing more prevalent in DDC systemdesign. Many devices are currently available for use in HVAC applications.

Types of Gas Concentration Measuring Devices


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There are many types of gas measuring devices available for use with DDC systems. Currently,the three most common gases measured in HVAC applications are carbon monoxide, carbon

dioxide, and refrigerant gases.

Carbon Monoxide

Carbon monoxide is a poisonous gas that is most commonly generated as the byproduct of theincomplete combustion of carbon based fuels. Carbon monoxide is generated by all fuel burning

equipment, including internal combustion engines. Carbon Monoxide detectors are used tooperate ventilation equipment to prevent carbon monoxide levels from becoming unsafe. They

are also used to warn facility owners and occupants of unsafe levels in garages, loading docks,tunnels, and other areas where vehicles are operated. Solid state sensing technology is most

commonly used. Single or multiple sensing point versions are available that can provide contactclosures at one or more set levels and/or analog signals that are proportional to carbon monoxide

concentration.

Carbon Dioxide

Carbon dioxide is a non-toxic gas produced by the respiration of living organisms, by thecomplete combustion of carbon, and by photosynthesis in green plants. Carbon dioxide exists in

the air in the amount of 320-350 parts per million. Carbon dioxide concentration inside ofbuildings has been related to general ventilation adequacy and is commonly monitored by DDC

control systems as a measure of indoor air quality and ventilation adequacy. It is also measuredby DDC systems and used to control outdoor air fans and dampers to keep the concentration

below set levels.

The most commonly used sensing technology is Non-Dispersive Infra-Red (NDIR). This is

based on the principle that carbon dioxide gas absorbs infrared radiation at the 4.2 m wavelength.Attenuation of an infrared source can be related to the gas concentration in air in the range of 0-

5000 parts per million with a general accuracy of plus or minus 150 ppm or 50 ppm overnarrower ranges.

Refrigerant Gas

Refrigerant gas detectors have been in widespread use since safety codes for mechanical

refrigeration required their use in the operation of emergency ventilation systems to evacuatehazardous concentrations of refrigerant gas in machinery rooms and other applicable enclosed

areas.

Detectors broadly sensitive to families of CFC and HCFC gases commonly used, as refrigerantsare available. Gas specific detectors are also available to detect individual refrigerant gases

including CFC, HFC, HCFC and ammonia specific to the equipment in use. The most commonlyused are infrared (IR), photo-acoustic, and solid state sensing technologies. Single or multiple

sensing point versions are available that can provide contact closures at one or more set levelsand/or analog signals that are proportional to refrigerant concentration.


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the inverter. The addition of the DC chopper allows regulation of the voltage to the inverter.Silicon controlled rectifiers also allow regulation of the voltage to the inverter.

The inverter section of the drive consists of solid state switching devices that reconstruct an AC

power signal with controlled frequency. The three most common types of inverters are variable

voltage source (also called six step), current source and pulse width modulated (PWM). The sixstep inverter uses six solid state switching devices in combination with six diodes. The solid stateswitches are controlled to produce a six step voltage wave form for each phase. Changing the

conducting time for each of the six switches results in a change in frequency of the output wave.The current source inverter operates much the same as the six step variable voltage source except

that solid state switching devices construct a six step current wave for each phase instead of avoltage wave. Pulse width modulated inverters use solid state switching devices to produce a

series of constant voltage pulses of various widths to produce an AC output. The timing andnumber of pulses are varied to produce the varying frequency.

Application Considerations For Motors and Drives. The following items should be considered

for any variable speed drive application:

1. Normally, NEMA Design B squirrel cage induction motors with continuous duty ratingare used.

2. Multiple motor loads can be controlled from a single AC variable speed drive, howeverthe manufacturer's guidelines must be followed regarding operation if some or all motorsare not connected. This applies in particular to drives with current source-type inverters.

3. With current source and PWM-type inverters there is some additional stress on the motorinsulation. These stresses are usually not significant.

4. PWM inverters usually cause motors to produce more noise than normal.5. Any type of inverter produces a current waveform that contains harmonics that do not

produce any additional torque, but do cause additional heating in the motor windings.This will typically produce 5% - 15% additional heating load and must be considered

when operating motors controlled by drives near full load conditions.6. With current source inverters, an open circuit (such as a disconnected load) will cause an

excessive voltage rise in the inverter. Unless appropriate protection is provided, thiscondition may cause inverter failure.

7. Jerky shaft motion can result with any inverter type at low speed (typically below about10 hertz) due to badly distorted waveforms at these frequencies. Some PWM drives are

available that are optimized for operation at low speed and can reduce this effect.8. It is important to consider the torque - speed characteristic of the load to be imposed on

the drive. Most HVAC applications are for centrifugal machines (pumps, fans andcompressors) and are described as "variable torque" because the torque is low at low

speed and rises according to the cube of the motor speed. Infrequent applications forHVAC, such as positive displacement pumps, may have constant torque characteristics.

Silicon Controlled Rectifiers (SCRs)


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SCRs are used to regulate an AC power supply to a typically resistive electrical load, such as anelectric heater, to provide continuously variable output. SCRs accept standard analog control

signals (usually voltage or current) and regulate the output of their load proportionally.

With microprocessor-based controls, SCRs can be used in combination with sequenced

contactors to provide vernier control that is continuous in proportion to the input signal, but doesnot require control of the entire load by a SCR and thus reduces the cost.

Actuators

Analog signal controlled actuators are one of the most important components of DDC systemstoday. Air temperature control is commonly accomplished with actuators of various types

through the control of damper position and valve position. The majority of modern HVACdesigns include actuators of one type or another.

Types of Actuators

With the invention and continual refinement of DDC systems, electric motor controlled actuatorsare steadily replacing pneumatic controlled actuators as the application allows. There are still a

large number of both types available and in service today.

Pneumatic Actuators

The pneumatic actuator has been widely used for HVAC control for decades. With theinventions of the electric-to-pneumatic signal transducers and EP relays, DDC systems can

readily integrate pneumatic actuators into the control scheme for steam valves, dampers, etc.Diaphragm- and piston-type actuators are the two most common pneumatic actuators.

Diaphragm-type actuators are most commonly used with low pressure pneumatic control signals

in the range of 0 to 30 psig, but are available for industrial application at higher pressures.Diaphragm actuators typically have an opposing spring, with air supply to only the side of the

diaphragm opposing the spring. The spring constant sets the range of air pressure over which thevalve will operate and also provides for failure in an open or closed position, depending on

orientation. The action of diaphragm actuators is normally linear, but may be converted to rotarymotion approaching 180 degrees through the use of suitable links.

Piston-type actuators are most commonly used with higher air pressures in the range of 80 to 100psig. Piston actuators are generally more compact than diaphragm-type actuators, particularly for

larger valve sizes. Pistons may be single acting (air applied to piston on one side, spring pressureon opposite side of piston provides return pressure) or double acting (

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