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