Measuring devices Measurands Measuring instruments

Force, load Analytical balance

Platform balance

Proving ring

Torque Prony brake

Hydraulic dynamometer

Pressure Bridgeman gauge

Mcload gauge

Pirani gauge

Introduction

Force: It is defined as the reaction between the two bodies or

components.

The reaction can be either tensile force (Pull) or it can be

Compressive force (Push).

Measurement of force can be done by any two methods:

◦ Direct Method: This involves a direct comparison with a known

gravitational force on a standard mass. Example: Physical Balance.

◦ Indirect Method: This involves the measurement of effect of force

on a body. E.g. Force is calculated from acceleration due to gravity and

the mass of the component.

Direct Method: Analytical Balance

(Equal arm balance)

Unequal arm balance:

Unequal arm balance:• For balance of

moments,

Ft * a = Fg * b

or test force,

Ft = Fg * (b / a)

Therefore, the test force is

proportional to the

distance ‘b’ of the mass

from the pivot.

Proving ring Standard for calibrating material testing machine.

Capacity 1000 N to 1000 kN.

Deflection is used as the measure of applied load.

This deflection is measured by a precision micrometer.

Micrometer is set with a help of vibrating reed.

P = force or load

M = Bending moment

R = Radius of proving ring

Proving Ring: A ring used for calibrating tensile

testing machines. It works on theprinciple of LVDT which sensesthe displacement caused by theforce resulting in a proportionalvoltage.

It is provided with the projectionlugs for loading. An LVDT isattached with the integral internalbosses C and D for sensing thedisplacement caused byapplication of force.

When the forces are appliedthrough the integral externalbosses A and B, the diameter ofring changes depending upon theapplication which is known as ringdeflection.

Torque Measurement: Torque: Force that causes twisting or turning moment.

E.g. the force generated by an internal-combustion engine to

turn a vehicle's drive or shaft.

Torque measuring devices are called as dynamometers.

The torque may be computed by measuring the force ‘F’ at a

known radius ‘r’, given by the formula

in N - m

Types of Dynamometers: Absorption dynamometers:◦ They are useful for measuring power or torque developed by

power source such as engines or electric motors.

Driving dynamometers:◦ These dynamometers measure power or torque and as well

provide energy to operate the device to be tested.

◦ These are useful in determining performance characteristicsof devices such as pumps and compression.

Transmission dynamometers:◦ These are the passive devices placed at an appropriate

location within a machine or in between the machine tosense the torque at that location.

Mechanical Dynamometer (Prony Brake):

Rope

This dynamometer may be used in larger capacities than the

simple Prony brake dynamometer because heat generated

can be can be easily removed by circulating the water in and

out of the housing.

The force acting on the shaft is then measured by using the

force measuring device or strain gauges.

Then by using the relation, T = F . r, we can find the torque

acting on it.

Hydraulic Dynamometer:

Hydraulic Dynamometer

Characters Small in size. Easy installation

Simple dynamometer structure and easy for maintenance

High brake torque

High measurement accuracy

Reliable and stable working condition

High real-time speed measurement accuracy with EM sensors

Fast loading control by electronic-control butterfly valve

High reaction speed which is suitable on dynamic testing Tuning of in-use engines, typically at service centers or for racing applications

PRESSURE SENSOR

TECHNOLOGY

Pressure Definition

Static Pressure. Pressure, P, is defined as

force, F, per unit area, A:

P = F/A

Pressure in open tankA container filled with a liquid has a pressure (due to the

weight of the liquid) at a point in the liquid of:

P = F/A

P = W/A

P = ρgV/A

P = ρghA/A

P = ρgh

P = pressure

F = force

A = Area

W = weight of the liquid

V = volume above the

Area

g = gravitation

ρ = mass density

h = distance from the

surface

A

h

Types of Pressure measurements

Absolute pressure is measured

relative to a perfect vacuum (psia)

Gauge pressure is measured relative

to ambient pressure (psig)

Differential pressure is the difference

in pressure between two points of

measurement. (psid).

Note that the same sensor may be

used for all three types; only the

reference is different.

Dynamic Effects. Static pressure is measured under steady-state or equilibrium

conditions, but most real-life applications deal with dynamic or changing pressure.

For example, the measurement of blood pressure usually gives the two steady-state values of systolic and diastolic pressure.

There is much additional information in the shape of the blood pressure signal, however, which is the reason for the monitors used in critical-care situations.

To measure changing pressures, the frequency response of the sensor must be considered. As a rough approximation, the sensor frequency response should be 5-10 × the highest frequency component in the pressure signal.

Another issue is the remote measurement of pressure where a liquid coupling medium is used. Care must be taken to purge all air because its compressibility will corrupt the waveform.

Pressure in Moving FluidsBernoulli's theorem states that for horizontal flow

the following relation holds:

PS = PO + PI

PS = stagnation (or total) pressure

PO = static pressure

PI = Impact pressure due to moving fluid

PI = ρVo ²/2

Where V0 = the velocity of the fluid

Hence we can measure the velocity if we know the pressure

Pressure Sensing

Pressure is sensed by mechanical elements such as plates, shells, and tubes that are designed and constructed to deflect when pressure is applied.

This is the basic mechanism converting pressure to physical movement.

Next, this movement must be transduced to obtain an electrical or other output.

Finally, signal conditioning may be needed, depending on the type of sensor and the application. Figure 8 illustrates the three functional blocks.

Pressur

e

Signal

Conditioner

Sensing

Element

Transduction

element

displacemen

t

electric

V or I output

Sensing Elements

The main types of sensing elements are Bourdon tubes, diaphragms, capsules, and bellows

All except diaphragms provide a fairly large displacement that is useful in mechanical gauges and for electrical sensors that require a significant movement

Inductive Pressure Sensors

Several configurations based on varying inductance or inductive coupling are used in pressure sensors. They all require AC excitation of the coil(s) and, if a DC output is desired, subsequent demodulation and filtering. The LVDT types have a fairly low frequency response due to the necessity of driving the moving core of the differential transformer

The LVDT uses the moving core to vary the inductive coupling between the transformer primary and secondary.

Capacitive Pressure

Sensors. Capacitive pressure sensors typically use a thin diaphragm as

one plate of a capacitor.

Applied pressure causes the diaphragm to deflect and the capacitance to change.

This change may or may not be linear and is typically on the order of several picofarads out of a total capacitance of 50-100 pF.

The change in capacitance may be used to control the frequency of an oscillator or to vary the coupling of an AC signal through a network.

The electronics for signal conditioning should be located close to the sensing element to prevent errors due to stray capacitance.

Capacitive Pressure

Sensors

Capacitive Pressure Sensors

Piezoelectric Pressure Sensors.

Piezoelectric elements are bi-directional transducers capable of converting stress into an electric potential and vice versa.

One important factor to remember is that this is a dynamic effect, providing an output only when the input is changing.

This means that these sensors can be used only for varying pressures.

The piezoelectric element has a high-impedance output and care must be taken to avoid loading the output by the interface electronics. Some piezoelectric pressure sensors include an internal amplifier to provide an easy electrical interface.

Piezoelectric Pressure Sensors.

Piezoelectric sensors convert stress into an electric potential and vice versa.

Sensors based on this technology are used to measure varying pressures.

Strain Gauge Pressure Sensors

Strain gauge sensors originally used a metal diaphragm with strain gauges bonded to it.

the signal due to deformation of the material is small, on the order of 0.1% of the base resistance

Semiconductor strain gauges are widely used, both bonded and integrated into a silicon diaphragm, because the response to applied stress is an order of magnitude larger than for a metallic strain gauge.

Strain Gauge Pressure Sensors

When the crystal lattice structure of silicon is deformed by applied stress, the resistance changes. This is called the piezoresistive effect. Following are some of the types of strain gauges used in pressure sensors.

Deposited strain gauge. Metallic strain gauges can be formed on a diaphragm by means of thin film deposition. This construction minimizes the effects of repeatability and hysteresis that bonded strain gauges exhibit. These sensors exhibit the relatively low output of metallic strain gauges.

Strain Gauge Pressure Sensors

Bonded semiconductor strain gauge. A silicon bar may be

bonded to a diaphragm to form a sensor with relatively

high output. Making the diaphragm from a chemically inert

material allows this sensor to interface with a wide variety

of media

Pressure Switches

Pressure switches, combining a diaphragm or other pressure measuring means with a precision snap switch, can provide precise single-point pressure sensing.

Alternatively, simple electronic switches may be combined with electrical sensors to construct a pressure switch with an adjustable set point and hysteresis.

Manometer

A mercury manometer is a

simple pressure standard

and may be used for gauge,

differential, and absolute

measurements with a

suitable reference. It is

useful mainly for lower

pressure work because the

height

Selection Considerations

Selection of a pressure sensor involves consideration of the medium for compatibility with the materials used in the sensor, the type (gauge, absolute, differential) of measurement, the range, the type of electrical output, and the accuracy required.

Manufacturer's specifications usually apply to a particular temperature range. If the range of operation in a given application is smaller, for example, the errors should ratio down.

Total error can be computed by adding the individual errors (worst-case) or by computing the geometric sum or root sum of the squares (RSS). The latter is more realistic since it treats them as independent errors that typically vary randomly.

Industrial ApplicationsFluid level in a tank: A gauge pressure sensor located to

measure the pressure at the bottom of a tank can be used for a remote indication of fluid level using the relation:

h = P/ρg

Fluid flow: An orifice plate placed in a pipe section creates a pressure drop. This approach is widely used to measure flow because the pressure drop may be kept small in comparison to some other types of flowmeters and because it is impervious to clogging, which may otherwise be a problem when measuring flow of a viscous medium or one containing particulate matter. The relation is:

Light Emitting Diode: LED

What is an LED?

Light-emitting diode

Semiconductor

Has polarity

LED: How It Works

When current flows

across a diode

Negative electrons move one way and

positive holes move the other way

Inside a Light Emitting Diode

1. Transparent Plastic

Case

2. Terminal Pins

3. Diode

Kinds of LEDs

L V D Ts

What is an LVDT?

An LVDT is a Linear Position Sensor

With a Proportional Analog Output

An LVDT has 2 Elements, a Moving

Core and a Stationary Coil Assembly

L V D TsLinear Variable Differential

Transformer

Transformer: AC Input / AC Output

Differential: Natural Null Point in Middle

Variable: Movable Core, Fixed Coil

Linear: Measures Linear Position

How LVDT’s Work

Working principle of LVDT

LVDT

Characteristics

Photograph of LVDT

Summary LVDT’s are robust equipment for measuring

deflection.

AC LVDT’s require separate signal conditioning equipment, while DC LVDT’s include signal conditioning equipment on the device.

There are three types of LVDT: unguided armature, captive armature, and spring-extended armature.

AC LVDT’s cost less than DC, but the entire measurement system must be considered.

Thermocouples

Most frequently used method tomeasure temperatures with anelectrical output signal.

What are thermocouples? Thermocouples operate under the principle that a

circuit made by connecting two dissimilar metals produces a measurable voltage (emf-electromotive force) when a temperature gradient is imposed between one end and the other.

They are inexpensive, small, rugged and accurate when used with an understanding of their peculiarities.

Thermocouples Principle of

Operation In, 1821 T. J. Seebeck observed the existence of an electromotive

force (EMF) at the junction formed between two dissimilar metals

(Seebeck effect).

◦ Seebeck effect is actually the combined result of two other

phenomena, Thomson and Peltier effects.

Thomson observed the existence of an EMF

due to the contact of two dissimilar metals at

the junction temperature.

Peltier discovered that temperature gradients

along conductors in a circuit generate an EMF.

The Thomson effect is normally much smaller

than the Peltier effect.

Let’s take a look at this circuit

How thermocouples work It is generally

reasonable to assume that the emf is generated in the wires, not in the junction. The signal is generated when dT/dx is not zero.

When the materials are homogeneous, e, the thermoelectric power, is a function of temperature only.

Two wires begin and end at the same two temperatures.

E (T To) (T To )2

Generally, a second order Eqn. is used.

Material EMF versus Temperature

With reference to

the characteristics

of pure Platinum

emf

Temperature

Chromel

Iron

Copper

Platinum-Rhodium

Alumel

Constantan

Thermocouple Effect Any time a pair of dissimilar wires is joined to

make a circuit and a thermal gradient is imposed, an emf voltage will be generated.

◦ Twisted, soldered or welded junctions are acceptable. Welding is most common.

◦ Keep weld bead or solder bead diameter within 10-15% of wire diameter

◦ Welding is generally quicker than soldering but both are equally acceptable

◦ Voltage or EMF produced depends on:

Types of materials used

Temperature difference between the measuring junction and the reference junction

Thermocouple Tables

(EMF-Temperature) Thermocouple tables correlate temperature to

emf voltage.

◦ Need to keep in mind that the thermocouple tables provide a voltage value with respect to a reference temperature. Usually the reference temperature is 0°C. If your reference junction is not at 0°C, a correction must be applied using the law of intermediate temperatures.

What thermocouple materials should be

used?

Depends on requirements:

◦ Temperature range?

◦ Required accuracy

◦ Chemical resistance issues

◦ Abrasion or vibration resistance

◦ Installation requirements (size of wire)

◦ Thermal conduction requirements

Law of Intermediate

TemperaturesIf a thermocouple circuit develops a net

emf1-2 for measuring junction

temperatures T1 and T2, and a net emf2-3

for temperatures T2 and T3, then it will

develop a net voltage of emf1-3 = emf1-2

+ emf2-3 when the junctions are at

temperatures T1 and T3.

emf1-2+ emf2-3= emf1-3

T2

T3 T1

T3 T2

T1

A Demonstration of the Law of

Intermediate Temperaturesem

f

T 1 T 2

Fe

C

T 3

emf23

emf1-2+ emf2-3= emf1-3

emf13

emf12

emf

T ref T hot

Measure

d Emf

Fe

C

1

2

T measured

3

5

4

Hot Zone

12

3

4

A Demonstration of the Law of Intermediate

Temperatures

If a thermocouple circuit of materials A and C generates a net emfA-C

when exposed to temperatures T1 and T2, and a thermocouple of

materials C and B generates a net emfC-B for the same two

temperatures T1 and T2, then a thermocouple made from materials A

and B will develop a net voltage of

emfA-B = emfA-C + emfC-B

between temperatures T1 and T2.

Sometimes useful in the calibration of different thermocouple wires.

Volume Flow

Measurements

Turbine Flow Meters

Magnetic flowmeter

Based upon Faraday’s Law

The fluid is the conductor, must be electrically conductive.

E=BDVx10-8

E=voltage, volts

B=magnetic flux density, gauss

D= length of the conductor, cm

V=velocity of the conductor, cm/sec

Magnetic flowmeter