Introduction to transducers

Electrical
Measurement-II
PROF. YOGESH K. KIRANGE M.E. (ELECTRICAL MACHINES & DRIVES)
ASSISTANT PROFESSOR
DEPARTMENT OF ELECTRICAL ENGINEERING
R.C.PATEL INSTITUTE OF TECHNOLOGY, SHIRPUR
1

Unit-II
Introduction to Transducer
 Definition, classification, selection of transducer.
 Measurement of temperature: using R T D, thermocouple,
bimetallic thermometer, Pressure thermometers, pyrometers.
 Pressure Measurement: Bourdon Tubes, bellows, diaphragms
 Vacuum Measurement: McLeod gauge, pirani gauge
2

It is a device that converts energy from one form to another.
Usually a transducer converts a signal in one form of energy to a signal in
another.
Transducers are often employed at the boundaries
of automation, measurement, and control systems, where electrical signals are
converted to and from other physical quantities (energy, force, torque, light,
motion, position, etc.).
The process of converting one form of energy to another is known as
transduction.
Transducer 3

Example:
 Temperature transducers
 Thermocouples
 Resistance-Temperature Detectors (RTD)
 Thermistors
 Resistive position transducers
 Displacement transducers
 Strain gauge Transducer
Excitation
Physical
Quantity
Electrical
Output
4

CLASSIFICATION OF TRANSDUCERS
A] Broadly classified in TWO groups;
1] Active Transducer or Self generating type transducer :- develop their own
voltage or current (e.g. Thermocouple and thrmopile , piezoelectric pick up ,
photovoltaic cell etc.)
2] Passive Transducers or Externally powered transducers :- derive power from
external source (e.g RTD , Thermistor, potentiometric devices etc)
5

B] Based on type of output
1] Analog transducer:- convert i/p physical quantity into an analogous o/p which
is continuous function of time.
(e.g Strain guage , Thermocouple , Thermistor, LVDT etc)
2] Digital transducer :- convert i/p physical quantity into an electrical o/p which
may be in pulse form
6

C] Based on Electrical principle involved
1.Variable-resistance type:
a. Strain and pressure gauges b. Thermistors, resistance thermometers c. Photoconductive cell
2.Variable –inductance type-
a. LVDT b. Reluctance pick up c. Eddy current gauge
3.Variable-capacitance type-
a. capacitor microphone b. Pressure gauge c. Dielectric gauge
4.Voltage generating type-
a. Thermocouple b. Photovoltaic cell c. Rotational motion tachometer d. Piezzoelectric pick up
5. Voltage divider type-
a. Potentiometer position sensor b. Pressure actuated voltage divider
7

SELECTION CRITERIA OF THE TRANSDUCERS
 Operating principle
 Sensitivity
 Operating range
 Accuracy
 Errors
 Environmental capability
 Insensitive to unwanted Signal
 Stability
Significant Parameters which dictate
the transducers capability are ;
Linearity
Repeatability
Resolution
Reliability
8

SPECIFFICATIONS FOR TRANSDUCERS
 Ranges available
 Squaring system
 Sensitivity
 Maximum working temperature
 Method of cooling employed
 Mounting details
While selecting the proper transducer for any applications ,or
ordering the transducers, the following specifications should be
thoroughly considered;
Linearity and hysteresis
Output for zero input
Temperature coefficient of zero drift
Natural frequency
9

Measurement of Temperature
Resistance Temperature Detector
(RTD)
10

 The thermoelectric effect
 Resistance change
 Sensitivity of semiconductor device
 Radiative heat emission
 Thermography
 Thermal expansion
 Resonant frequency change
 Sensitivity of ﬁbre optic devices
 Acoustic thermometry
 Color change
 Change of state of material.
Instruments to measure temperature can be divided into separate
classes according to the physical principle on which they operate.
The main principles used are:
11

Resistance Temperature Detector (RTD)
Resistance thermometers, also called resistance temperature detectors (RTDs),
are sensors used to measure temperature.
Many RTD elements consist of a length of fine wire wrapped around a ceramic or
glass core but other constructions are also used.
The RTD wire is a pure material, typically platinum, nickel, or copper. The material
has an accurate resistance/temperature relationship which is used to provide an
indication of temperature. As RTD elements are fragile, they are often housed in
protective probes.
RTDs, which have higher accuracy and repeatability, are slowly
replacing thermocouples in industrial applications below 600 °C
12

Resistance/temperature relationship of metals
Common RTD sensing elements constructed of platinum, copper or nickel have
a repeatable resistance versus temperature relationship (R vs T) and operating
temperature range.
The R vs T relationship is defined as the amount of resistance change of the
sensor per degree of temperature change.
The relative change in resistance (temperature coefficient of resistance) varies
only slightly over the useful range of the sensor.
13

For example : Platinum
Platinum was proposed by Sir William Siemens as an element for a resistance
temperature detector at the Bakerian lecture in 1871.
It is a noble metal and has the most stable resistance–temperature relationship
over the largest temperature range.
Nickel elements have a limited temperature range because the amount of
change in resistance per degree of change in temperature becomes very non-
linear at temperatures over 572 °F (300 °C).
Copper has a very linear resistance–temperature relationship; however, copper
oxidizes at moderate temperatures and cannot be used over 302 °F (150 °C).
14

Platinum is the best metal for RTDs due to its very linear resistance–temperature
relationship, highly repeatable over a wide temperature range. The unique
properties of platinum make it the material of choice for temperature standards
over the range of −272.5 °C to 961.78 °C. It is used in the sensors that define the
International Temperature Standard, ITS-90. Platinum is chosen also because of
its chemical inertness.
The significant characteristic of metals used as resistive elements is the linear
approximation of the resistance versus temperature relationship between 0 and
100 °C.
15

This temperature coefficient of resistance is denoted by α (Alpha) and is usually
given in units of Ω / Ω°C ;
𝛼 =
𝑅100 − 𝑅0
100℃ ∗ 𝑅0
Pure platinum has α = 0.003925 Ω/(Ω·°C) in the 0 to 100 °C range and is used in
the construction of laboratory-grade RTDs.
16

Element Types:
The three main categories of RTD sensors are thin-film, wire-wound, and coiled
elements. While these types are the ones most widely used in industry, other
more exotic shapes are used; for example, carbon resistors are used at ultra-
low temperatures (−173 °C to −273 °C)
17

Element Types…
Carbon resistor elements are cheap and widely used. They have very
reproducible results at low temperatures. They are the most reliable form at
extremely low temperatures. They generally do not suffer from
significant hysteresis or strain gauge effects.
Strain-free elements use a wire coil minimally supported within a sealed
housing filled with an inert gas. These sensors work up to 961.78 °C and are
used in the SPRTs that define ITS-90. They consist of platinum wire loosely coiled
over a support structure, so the element is free to expand and contract with
temperature. They are very susceptible to shock and vibration, as the loops of
platinum can sway back and forth, causing deformation.
18

Element Types…
Thin-film elements have a sensing element that is
formed by depositing a very thin layer of resistive
material, normally platinum, on a ceramic substrate
(plating). This layer is usually just 10 to 100 ångströms
(1 to 10 nanometers) thick. This film is then coated
with an epoxy or glass that helps protect the
deposited film and also acts as a strain relief for the
external lead wires.
19

Element Types…
Disadvantages of this type are that they are not as stable
as their wire-wound or coiled counterparts. They also can
only be used over a limited temperature range due to the
different expansion rates of the substrate and resistive
deposited giving a "strain gauge" effect that can be seen
in the resistive temperature coefficient. These elements
work with temperatures to 300 °C (572 °F) without further
packaging, but can operate up to 600 °C (1,112 °F) when
suitably encapsulated in glass or ceramic. Special high-
temperature RTD elements can be used up to 900 °C
(1,652 °F) with the right encapsulation.
20

Element Types…
Wire-wound elements can have greater accuracy,
especially for wide temperature ranges. The coil
diameter provides a compromise between
mechanical stability and allowing expansion of the
wire to minimize strain and consequential drift. The
sensing wire is wrapped around an insulating
mandrel or core. The winding core can be round or
flat, but must be an electrical insulator. The
coefficient of thermal expansion of the winding core
material is matched to the sensing wire to minimize
any mechanical strain.
21

Element Types…
This strain on the element wire will result in a
thermal measurement error. The sensing wire
is connected to a larger wire, usually referred
to as the element lead or wire. This wire is
selected to be compatible with the sensing
wire, so that the combination does not
generate an emf that would distort the
thermal measurement. These elements work
with temperatures to 660 °C.
22

Element Types…
Coiled elements have largely replaced wire-wound
elements in industry. This design has a wire coil that
can expand freely over temperature, held in place
by some mechanical support, which lets the coil
keep its shape. This “strain free” design allows the
sensing wire to expand and contract free of
influence from other materials; in this respect it is
similar to the SPRT, the primary standard upon
which ITS-90 is based, while providing the durability
necessary for industrial use.
23

Element Types…
The basis of the sensing element is a small coil of
platinum sensing wire. This coil resembles a filament in
an incandescent light bulb. The housing or mandrel is
a hard fired ceramic oxide tube with equally spaced
bores that run transverse to the axes. The coil is
inserted in the bores of the mandrel and then
packed with a very finely ground ceramic powder.
This permits the sensing wire to move, while still
remaining in good thermal contact with the process.
These elements work with temperatures to 850 °C.
24

Properties of RTD
1.Linearity of resistance
2. Resist to corrosion and oxidation under the temp. range
3. To provide reproducible and consistent results
4. Good sensitivity
5.High sensitivity i.e unit can be fabricated in compact and convenient size.
6.No change of phase or state within a reasonable temperature range
Note:- Industrial thermometers are usually made as Platinum , Nickel , Copper
and basically for precise measurement Platinum is preferred because physically
stable , high electrical resistance characteristics .
*Platinum resistance thermometer to its accuracy, stability, sensitivity temp
range from b.p of Oxygen -182.9 deg. C to freezing point of antimony 630.5 deg.
C
25

Advantages of RTD
1.Resist to corrosion and oxidation
2. Physically stable
3. Measurement is very accurate
4. It has a lot flexibility
5. It does not require a reference junction
6. Temp. sensitive resistance element can be replaced and installed easily
7. High Accuracy
8. Low drift
9. Wide operating range
10. Suitability for precision applications
26

Dis-advantages of RTD
1. It is more costly
2. They suffer from time lag
3. Leakage current is possible between resistance element to ground
4. Resistance change due to temperature changes of measuring resistors
27

Limitations of RTD
RTDs in industrial applications are rarely used above 660 °C. At temperatures
above 660 °C it becomes increasingly difficult to prevent the platinum from
becoming contaminated by impurities from the metal sheath of the
thermometer.
This is why laboratory standard thermometers replace the metal sheath with a
glass construction. At very low temperatures, say below −270 °C (3 K), because
there are very few phonons, the resistance of an RTD is mainly determined
by impurities and boundary scattering and thus basically independent of
temperature. As a result, the sensitivity of the RTD is essentially zero and therefore
not useful.
Compared to thermistors, platinum RTDs are less sensitive to small temperature
changes and have a slower response time. However, thermistors have a smaller
temperature range and stability.
28

RTD V/S Thermocouple
The two most common ways of measuring temperatures for industrial
applications are with resistance temperature detectors (RTDs) and
thermocouples. Choice between them is usually determined by four factors.
1. Temperature :
If process temperatures are between −200 and 500 °C (−328.0 and 932.0 °F), an
industrial RTD is the preferred option. Thermocouples have a range of −180 to
2,320 °C (−292.0 to 4,208.0 °F), so for temperatures above 500 °C (932 °F) they are
the only contact temperature measurement device.
2. Response time
If the process requires a very fast response to temperature changes (fractions of
a second as opposed to seconds), then a thermocouple is the best choice. Time
response is measured by immersing the sensor in water moving at 1 m/s (3 ft/s)
with a 63.2% step
29

RTD V/S Thermocouple
Size
A standard RTD sheath is 3.175 to 6.35 mm (0.1250 to 0.2500 in) in diameter;
sheath diameters for thermocouples can be less than 1.6 mm (0.063 in).
Accuracy and stability requirements
If a tolerance of 2 °C is acceptable and the highest level of repeatability is not
required, a thermocouple will serve. RTDs are capable of higher accuracy and
can maintain stability for many years, while thermocouples can drift within the
first few hours of use.
30

Construction:
These elements nearly always require insulated leads attached. PVC, silicone
rubber or PTFE insulators are used at temperatures below about 250 °C. Above
this, glass fibre or ceramic are used. The measuring point, and usually most of
the leads, require a housing or protective sleeve, often made of a metal alloy
that is chemically inert to the process being monitored. Selecting and
designing protection sheaths can require more care than the actual sensor, as
the sheath must withstand chemical or physical attack and provide convenient
attachment points.
31

Wiring Configuration
Two-wire configuration
The simplest resistance-thermometer
configuration uses two wires. It is only used when
high accuracy is not required, as the resistance
of the connecting wires is added to that of the
sensor, leading to errors of measurement. This
configuration allows use of 100 meters of cable.
This applies equally to balanced bridge and
fixed bridge system.
For a balanced bridge usual setting is with R2 =
R3, and R1 around the middle of the range of
the RTD. So for example, if we are going to
measure between 0 and 100 °C (32 and 212 °F),
RTD resistance will range from 100 Ω to 138.5 Ω.
We would choose R1 = 120 Ω. In that way we get
a small measured voltage in the bridge.
32

Three wire configuration
In order to minimize the effects of the lead resistances, a three-wire configuration
can be used. The suggested setting for the configuration shown, is with R1 = R2,
and R3 around the middle of the range of the RTD.
33

Looking at the Wheatstone bridge circuit shown, the
voltage drop on the lower left hand side is V_rtd +
V_lead, and on the lower right hand size is V_R3 +
V_lead, therefore the bridge voltage (V_b) is the
difference, V_rtd - V_R3. The voltage drop due to the
lead resistance has been cancelled out. This always
applies if R1=R2, and R1, R2 >> RTD, R3. R1 and R2 can
serve the use of limiting the current through the RTD,
for example for a PT100, limiting to 1mA, and 5V,
would suggest a limiting resistance of approximately
R1 = R2 = 5/0.001 = 5,000 Ohms.
34

Four wire configuration
The four-wire resistance configuration increases
the accuracy of measurement of
resistance. Four-terminal sensing eliminates
voltage drop in the measuring leads as a
contribution to error. To increase accuracy
further, any residual thermoelectric voltages
generated by different wire types or screwed
connections are eliminated by reversal of the
direction of the 1 mA current and the leads to the
DVM (digital voltmeter). The thermoelectric
voltages will be produced in one direction only.
By averaging the reversed measurements, the
thermoelectric error voltages are cancelled out
35

Temperature Relationships
(°F) = 9/5*(°C) +32
(°C) = 5/9*[(°F) –32]
(°F) = (°R) – 459.67
(°C) = (K) – 273.15
36

Thermocouple
37

Thermocouple
When two metals having
different work functions are
placed together. A volt1ge
is generated at the junction
which is nearly proportional
to the temperature. This
junction is called a
Thermocouple. This principle
is used to convert heat
energy to electrical energy
at the junction of two
conductors as shown in
figure.
Fig: A Thermocouple Circuit
38

Thermocouple
The heat at the junction is produced by the electric current flowing
in the heater element while the thermocouple produces an emf
at its output terminals which can be measured with the help of a
PMMC instrument.
The emf produced is proportional to the temperature and hence
to the rms value of current.
Therefore the scale of PMMC instrument can be calibrated to
read the current through the heater.
39

Thermal emf
The thermal emf developed in a circuit composed of two
dissimilar metals with junctions kept at absolute
temperatures T1& T2 (with T2 >T1) may be
approximately written as:
𝐸 = 𝑎 𝑇1 − 𝑇2 + 𝑏(𝑇1 − 𝑇2)2
Where a & b are constants whose value depends upon material
used.
Δt = difference of temperatures of hot and cold junction= 𝑇1 − 𝑇2
∴ 𝐸 = 𝑎 Δt + 𝑏(Δt)2
Thus emf of a thermocouple is approximately a parabolic function of the
temperature difference between the junctions. The-approximate values of
constants a and b are :
40

Thermal emf…
a=40 to 50 µV or more per deg-centigrade difference of
temperature,
b=few tenths or hundredths of a microvolt
41

A thermocouple is an electrical device consisting of two
dissimilar electrical conductors forming electrical junctions at
differing temperatures.
A thermocouple produces a temperature-dependent voltage as a
result of the thermoelectric effect, and this voltage can be
interpreted to measure temperature.
Thermocouples are a widely used type of temperature sensor
Thermocouple
42

Commercial thermocouples are inexpensive, interchangeable
and are supplied with standard connectors, also it can measure a
wide range of temperatures.
In contrast to most other methods of temperature measurement,
thermocouples are self powered and require no external form of
excitation.
The main limitation with thermocouples is accuracy; system errors
of less than one degree Celsius (°C) can be difficult to achieve.
Thermocouple
43

Thermocouples are widely used in
1.Science and industry
2.Temperature measurement for kilns
3.Gas turbine exhaust
4.Diesel engines
5.Other industrial processes
6.Used in homes, offices and businesses
as the temperature sensors in thermostats
7.As flame sensors in safety devices for gas-powered
major appliances
Thermocouple Applications :
44

In 1821, the German physicist Thomas Johann Seebeck discovered
that when different metals are joined at the ends and there is a
temperature difference between the joints, a magnetic field is
observed.
At the time Seebeck referred to this as thermo-magnetism.
The magnetic field he observed was later shown to be due to
thermo-electric current.
In practical use, the voltage generated at a single junction of two
different types of wire is what is of interest as this can be used to
measure temperature at very high and low temperatures.
Thermocouple: Principle of Operation
45

The magnitude of the voltage depends on the types of wire used.
Thermocouple: Principle of Operation
46

These thermometers use the following two principles:
1. All metals change in dimension, that is expand or contract when
there is a change in temperature.
2. The rate at which this expansion or contraction takes place
depend on the temperature co-efficient of expansion of the metal
and this temperature coefficient of expansion is different for different
metals. Hence the difference in thermal expansion rates is used to
produce deflections which is proportional to temperature changes.
Bimetallic Thermometer: Basic Principle
47

Continued…
Fig: Bimetallic Strip
48

Continued…
3. The bimetallic thermometer consists of a
bimetallic strip. A bimetallic strip is made of
two thin strips of metals which have different
coeffcients of expansion.
4. The two metal strips are joined
together by brazing, welding or
reveting so that the relative
motion between them is arrested.
The bimetallic strip is in the form of
a cantilever beam.
5. An increase in temperature
will result in the deflection of the
free end of the strip as shown i
diagram. This deflection is linear
and can be related to
temperature changes.
49

Continued…
The radius of the curvature of the bimetallic strip which
was initially flat is determined using the following
relationship.
R= t{3(1+m)² + (1+m)[m²+1/m]}/6(άh-άl)(T2-T1)(1+m) ²
where,
R= radius of the curvature at the temperature T2.
t = total thickness of the bimetallic strip = (t1+t2)
m=t1/t2 = Thickness of lower – expansion metal/thickness of higher – expansion
metal.
άl= coefficient of expansion of lower expansion metal.
άh= coefficient of expansion of higher expansion metal.
T1 = Initial temperature ,
T2 =Final temperature.
50

Properties of a Material
The following are the important proporties a material should have
to be selected for bimetallic thermometers.
Coefficient of expansion.
Modulus of elasticity.
Elastic limit after cold rolling.
Electrical conductivity.
Ductility.
Metallurgical ability.
51

Continued…
Different common forms of bimetallic sensors are listed;
Helix type.
Spiral type.
Cantilever type.
Flat type.
52

Applications:
Application of bimetallic strips and thermometers;
The bimetallic strip is used in control devices.
The spiral strip is used in air conditioning thermostats.
The helix strip is used for process application such as refineries, oil
burners, tyre vulcanisers etc.,
53

Advantages of Bimetallic Thermometer
1. They are simple, robust and inexpensive.
2. Their accuracy is between +or- 2% to 5% of the scale
3. They can with stand 50% over range in temperaures
4. They can be used where evr a mecury –in-glass thermometer is
used.
54

Limitations of Bimetallic Thermometer
1. They are not recommended for temperature above 400 0C.
2. When regularly used, the bimetallic may permanently deform,
which in turn will introduce errors.
55

Pressure Spring Thermometer
The liquid expansion thermometer utilizes
the cubical expansion of liquid, generally
mercury to indicate the temperature.
The gas expansion thermometer
operates at substantially constant
volume , the pressure of the gas being
proportional for temperature.
Vapor-actuated thermometer is
operated by vapor pressure of liquid.
A metal bulb contains the thermometer
fluid, a liquid, a gas, or a liquid-vapor,
and is inserted at the point at which
temp is to be measured.
56

The bulb comes to the temp equilibrium with its surroundings,
thereby developing a given pressure of fluid.
A metal capillary is connected to the bulb and transmits the
pressure to the bulb to the receiving element at the instrument.
The receiving element is a form of bourdon tube and is used to
convert pressure of the fluid in the thermometer bulb into a
motion.
This motion is multiplied by a linkage to operate a pen arm over a
moving chart for recording purposes.
The thermal system, consisting of bulb, capillary and receiving
element is a hermitically scaled unit.
Continued…
57

 The construction thermometer bulb and thermal well is
shown in figure.
 the bulb is composed of cylindrical piece of metal
tubing, closed at one end and with the capillary and
extension neck inserted at the other end.
 The size of bulb varies considerably, depending on type
of filling medium, temp span of instrument, and length of
capillary tubing with which it is used.
Continued…
58

Pyrometer
 All matter that has a temperature(T) greater than absolute
zero emits electromagnetic radiation(photon particles) due
to the internal mechanical movement of molecules.
 Radiation thermometers or pyrometers are measurement
instruments which determine the temperature of an object
based on the infrared radiation emitted from that object.
 Types of pyrometer:-
1)Radiation pyrometer
2)Optical pyrometer
59

What is Pyrometer ?
 A pyrometer is a device that is used for the temperature
measurement of an object. The device actually tracks and measures
the amount of heat that is radiated from an object. The thermal heat
radiates from the object to the optical system present inside the
pyrometer. The optical system makes the thermal radiation into a better
focus and passes it to the detector. The output of the detector will be
related to the input thermal radiation. The biggest advantage of this
device is that, unlike a Resistance Temperature Detector
(RTD) and Thermocouple, there is no direct contact between the
pyrometer and the object whose temperature is to be found out.
60

RADIATION PYROMETER
Principle
 Temperature measurement is based on the measurement of
radiation either directly by a sensor or by comparing with the
radiation of a body of known temperature.
 The radiation pyrometer is non-contact type of temperature
measurement.
 The wavelength region having high intensity is between 0.1 to 10
micrometer.
 In this region, 0.1 to 0.4 micrometer is known as ultraviolet region.
61

 0.4 to 0.7 micrometer is known as the visible region. 0.7
micrometer onwards is the infrared region.
 With the increasing temperature, the radiation intensity is
stronger towards shorter wavelengths.
 The temperature measurement by radiation pyrometer is limited
within 0.5 to 8 micrometer wave length region.
 Radiation pyrometer consist of optical component to collect
the radiation energy emitted by object , a radiation detector
that converts the radiant energy in to an electrical signal and
an indicator to read the measurement.
RADIATION PYROMETER 62

OPTICAL PYROMETER
This pyrometer is also known as disappearing filament pyrometer.
Main principle :
In this type of pyrometer, the tungsten filament of an electric bulb
is used as radiator.
The intensity of radiation of filament is compared with the intensity
of the radiation of the hot surface.
When both the intensity match, the filament of bulb is disappears
against the background.
64

1. As shown in the figure above, an optical pyrometer has the
following components.
2. An eye piece at the left side and an optical lens on the right.
3. A reference lamp, which is powered with the help of a battery.
4. A rheostat to change the current and hence the brightness
intensity.
5. So as to increase the temperature range which is to be
measured, an absorption screen is fitted between the optical
lens and the reference bulb.
6. A red filter placed between the eye piece and the reference
bulb helps in narrowing the band of wavelength.
66

Working
The radiation from the source is emitted and the optical objective lens captures
it. The lens helps in focusing the thermal radiation on to the reference bulb. The
observer watches the process through the eye piece and corrects it in such a
manner that the reference lamp filament has a sharp focus and the filament is
super-imposed on the temperature source image. The observer starts changing
the rheostat values and the current in the reference lamp changes. This in turn,
changes its intensity. This change in current can be observed in three different
ways.
1. The filament is dark. That is, cooler than the temperature source.
2. Filamnet is bright. That is, hotter than the temperature source.
3. Filament disappears. Thus, there is equal brightness between the filament and
temperature source. At this time, the current that flows in the reference lamp is
measured, as its value is a measure of the temperature of the radiated light in
the temperature source, when calibrated.
67

 Simple assembling of the device enables easy use of it.
 Provides a very high accuracy with +/-5 degree Celsius.
 There is no need of any direct body contact between the optical
pyrometer and the object. Thus, it can be used in a wide variety of
applications.
 As long as the size of the object, whose temperature is to measured fits with
the size of the optical pyrometer, the distance between both of them is not
at all a problem. Thus, the device can be used for remote sensing.
68
Advantages

 This device can not only be used to measure the temperature, but can
also be used to see the heat produced by the object/source. Thus,
optical pyrometers can be used to measure and view wavelengths
less than or equal to 0.65 microns. But, a Radiation Pyrometer can be
used for high heat applications and can measure wavelengths
between 0.70 microns to 20 microns.
69
…continued

Disadvantages
1. As the measurement is based on the light intensity, the device
can be used only in applications with a minimum temperature
of 700 degree Celsius.
2. The device is not useful for obtaining continuous values of
temperatures at small intervals.
Applications
1. Used to measure temperatures of liquid metals or highly heated
materials.
2. Can be used to measure furnace temperatures.
70

 The intensity of filament can be controlled by current
flowing through it.
 The maximum temperature of the filament is 2800 to
3000 °C at the rated voltage.
 The minimum visible radiation is at 600°C . Hence we can
measure the temperature in between 600°C.
71

Pressure Measurement:
Pressure is defined as a force per unit area.
Pressure are exerted by gases, vapours and liquids.
The instruments use record pressure as the difference between
the two pressures.
Thus, it is the difference between the pressure exerted by a
fluid of interest and the ambient atmospheric pressure.
Such devices indicate the pressure either above or below that
of the atmosphere.
72
What is Pressure ?

The atmospheric air exerts a normal pressure upon all
surfaces with which it is in contact and it is known as
atmospheric pressure. It is also known as Barometric
pressure.
73
Atmospheric Pressure:
Note: The local atmospheric pressure lower, if place is
higher than sea level and higher if place is lower than sea
level.

Everyday pressure measurements, such as for vehicle tire pressure, are
usually made relative to ambient air pressure. In other cases
measurements are made relative to a vacuum or to some other specific
reference. When distinguishing between these zero references, the
following terms are used:
74
Gauge pressure is zero-referenced
against ambient air pressure, so it is equal
to absolute pressure minus atmospheric
pressure. Negative signs are usually
omitted. Differential pressure is the
difference in pressure between two
points.
Absolute pressure is zero-referenced against a
perfect vacuum, using an absolute scale, so it
is equal to gauge pressure plus atmospheric
pressure.
Note: An absolute scale is a system of measurement that begins at a minimum, or zero point,
and progresses in only one direction
Differential pressure is the
difference in pressure between two
points

Pressure Measurement:
Pressure measurement is the analysis of an applied force by
a fluid (liquid or gas) on a surface.
Pressure is typically measured in units of force per unit of surface
area.
Many techniques have been developed for the measurement of
pressure and vacuum. Instruments used to measure and display
pressure in an integral unit are called pressure gauges or vacuum
gauges.
A manometer is a good example as it uses a column of liquid to both
measure and indicate pressure. Likewise the widely used Bourdon
gauge is a mechanical device which both measures and indicates,
and is probably the best known type of gauge.
75

…continued
A vacuum gauge is a pressure gauge used to measure pressures
lower than the ambient atmospheric pressure, which is set as the
zero point, in negative values (e.g.: -15 psi or -760 mmHg equals total
vacuum).
Most gauges measure pressure relative to atmospheric pressure as
the zero point, so this form of reading is simply referred to as "gauge
pressure".
76

Pressure Measuring Instruments:
3. Instruments for measuring low
vacuum and ultra high
vacuum ( 760 Torr to 10-9 Torr
and beyond ;
a. McLeod
b. Thermal Conductivity
c. Ionization Gauges
771.Instruments for measuring low
pressure ( below 1 mm of Hg)
a. Manometer
b. Low pressure gauges
2. Instruments for medium and
high pressure (between 1 mm of
Hg to 1000 atmospheres)
a. Bourdon tube
b. Diaphragm Gauges ,
c. Bellow Pressure Gauges
d. Dead Weight Pressure Gauge

Pressure Measuring Instruments:
5. Instruments for varying
pressure
Engine Indicator
CRO
784. Instruments for very high
pressure (1000 atmosphere and
above )
a. Bourdon tube
b. Diaphragm Gauges
c. Electrical Resistance Pressure
Gauges

Relation between Pressure:
79
1. Absolute Pressure= Atmospheric
Pressure + Gauge Pressure
2. Vacuum Pressure= Atmospheric
Pressure – Absolute Pressure

Bourdon Tube:
Basic Principle:
When an elastic transducer ( bourdon tube in this case ) is subjected
to a pressure, it defects. This deflection is proportional to the applied
pressure when
81
Description
:Bourdon Tubes are known for its very high range of differential pressure
measurement in the range of almost 100,000 psi (700 MPa). It is an elastic type
pressure transducer.
The device was invented by Eugene Bourdon in the year 1849. The basic idea
behind the device is that, cross-sectional tubing when deformed in any way will
tend to regain its circular form under the action of pressure. The bourdon
pressure gauges used today have a slight elliptical cross-section and the tube is
generally bent into a C-shape or arc length of about 27 degrees. The detailed
diagram of the bourdon tube is shown below.

83
As seen in the figure, the pressure input is given to a socket
which is soldered to the tube at the base. The other end or
free end of the device is sealed by a tip. This tip is
connected to a segmental lever through an adjustable
length link. The lever length may also be adjustable. The
segmental lever is suitably pivoted and the spindle holds
the pointer as shown in the figure. A hair spring is sometimes
used to fasten the spindle of the frame of the instrument to
provide necessary tension for proper meshing of the gear
teeth and thereby freeing the system from the backlash.
Any error due to friction in the spindle bearings is known as
lost motion. The mechanical construction has to be highly
accurate in the case of a Bourdon Tube Gauge. If we
consider a cross-section of the tube, its outer edge will have
a larger surface than the inner portion. The tube walls will
have a thickness between 0.01 and 0.05 inches.

84Working
As the fluid pressure enters the bourdon tube, it tries to be reformed
and because of a free tip available, this action causes the tip to
travel in free space and the tube unwinds. The simultaneous actions
of bending and tension due to the internal pressure make a non-
linear movement of the free tip. This travel is suitable guided and
amplified for the measurement of the internal pressure. But the main
requirement of the device is that whenever the same pressure is
applied, the movement of the tip should be the same and on
withdrawal of the pressure the tip should return to the initial point.

85
A lot of compound stresses originate in the tube as soon as the
pressure is applied. This makes the travel of the tip to be non-linear in
nature. If the tip travel is considerably small, the stresses can be
considered to produce a linear motion that is parallel to the axis of
the link. The small linear tip movement is matched with a rotational
pointer movement. This is known as multiplication, which can be
adjusted by adjusting the length of the lever. For the same amount
of tip travel, a shorter lever gives larger rotation. The approximately
linear motion of the tip when converted to a circular motion with the
link-lever and pinion attachment, a one-to-one correspondence
between them may not occur and distortion results. This is known as
angularity which can be minimized by adjusting the length of the
link.

86
Other than C-type, bourdon gauges can also be constructed in the
form of a helix or a spiral. The types are varied for specific uses and
space accommodations, for better linearity and larger sensitivity. For
thorough repeatability, the bourdon tubes materials must have
good elastic or spring characteristics. The surrounding in which the
process is carried out is also important as corrosive atmosphere or
fluid would require a material which is corrosion proof. The
commonly used materials are phosphor-bronze, silicon-bronze,
beryllium-copper, inconel, and other C-Cr-Ni-Mo alloys, and so on.

87
In the case of forming processes, empirical relations are known to
choose the tube size, shape and thickness and the radius of the C-
tube. Because of the internal pressure, the near elliptic or rather the
flattened section of the tube tries to expand as shown by the dotted
line in the figure below (a). The same expansion lengthwise is shown
in figure (b). The arrangement of the tube, however forces an
expansion on the outer surface and a compression on the inner
surface, thus allowing the tube to unwind. This is shown in figure (c).

88
Applications :
They are used to measure medium to very high pressures.
Advantages :
1. It gives accurate results.
2. Its cost low.
3. It is simple in construction.
4. They can be modified to give electrical outputs.
5. They are safe even for high pressure measurement.
Accuracy is high especially at high pressures.

89
Disadvantages:
1.Susceptible to shock and vibration
2.Gauges are subjected to hysteresis
Limitations :
 They respond slowly to changes in pressure
 They are subjected to hysteresis.
 They are sensitive to shocks and vibrations.
 Amplification is a must as the displacement of the free
end of the bourdon tube is low.
 It cannot be used for precision measurement

90
Diaphragm Pressure Gauge:
A diaphragm pressure transducer is used for low pressure measurement.
They are commercially available in two types – metallic and non-metallic.
Metallic diaphragms are known to have good spring characteristics and
non-metallic types have no elastic characteristics. Thus, non-metallic types
are used rarely, and are usually opposed by a calibrated coil spring or any
other elastic type gauge. The non-metallic types are also called slack
diaphragm.

91
Working:
When a force acts against a thin stretched diaphragm, it causes a
deflection of the diaphragm with its center deflecting the most.

92
Since the elastic limit has to be maintained,
the deflection of the diaphragm must be kept
in a restricted manner.
This can be done by cascading many
diaphragm capsules as shown in the figure.
A main capsule is designed by joining two
diaphragms at the periphery.
A pressure inlet line is provided at the central
position.
When the pressure enters the capsule, the
deflection will be the sum of deflections of all
the individual capsules.
As shown in figure (3), corrugated diaphragms
are also used instead of the conventional
ones.

93
Corrugated designs help in providing a linear deflection and also
increase the member strength. The total amount of deflection for a
given pressure differential is known by the following factors:
Number and depth of corrugation
Number of capsules
Capsule diameter
Shell thickness
Material characteristics

94
Non-metallic or slack diaphragms
are used for measuring very small
pressures. The commonly used
materials for making the
diaphragm are polythene,
neoprene, animal membrane, silk,
and synthetic materials. Due to
their non-elastic characteristics,
the device will have to be
opposed with external springs for
calibration and precise operation.
The common range for pressure
measurement varies between 50
Pa to 0.1 MPa.
The best example for a slack
diaphragm is the draft gauge. They are
used in boilers for indication of the
boiler draft. The device can control
both combustion and flue. With the
draft, usually of pressure less than the
atmosphere, connected, the power
diaphragm moves to the left and its
motion is transmitted through the
sealing diaphragm, sealed link and
pointer drive to the pointer.

95The power diaphragm is balanced with the help of a
calibrated leaf spring. The effective length of the spring and
hence the range is determined by the range adjusting screw.
By adjusting the zero adjustment screw, the right hand end of
the power diaphragm support link as also the free end of the
leaf spring, is adjusted for zero adjustment through the cradle.

96
What is Diaphragm Pressure Gauge ?
The movement of a diaphragm is a convenient way of sensing a pressure.
The unknown pressure is applied to one side of the diaphragm.
The edge of the diaphragm is rigidly fixed and this causes a deflection.
The displacement of the center of the diaphragm may be measured in order
to know the value of the pressure, because the deflection is directly
proportional to the pressure.
The diaphragms are of two types :
1. Flat (Metallic) 2. Corrugated (Non Metallic)
but diaphragms may be membranes.

97
…continued
It is usual to employ thin circular plates
which may either be clamped around
their circumference between two solid
rings or are machined from a solid piece
of metal.
A flat diaphragm is shown in Figure.
Pressure P =
𝟐𝟓𝟔𝑬𝒕 𝟑 𝒅 𝒎
𝟑 𝟏−𝒗 𝟐 𝑫 𝟒
Where E = Young's modulus in N/m2 , t = thickness of diaphragm in m,
D = diameter of diaphragm in m, v = Poisson's ratio,
dm=deflection at the centre of the diaphragm in m.

98
…continued
The given relationship between pressure and the deflection at the
centre dm is linear. But linearity holds good as long as dm ≤ 0.5 t
and not otherwise.
The maximum stress at the circumference of diaphragm is
𝒔 𝒎=
𝟑𝑫 𝟐 𝒑
𝟏𝟔𝒕 𝟐 𝑵/𝒎 𝟐
Where p =density of diaphragm material in kg/m3.
The lowest natural frequency for air or gas medium is :
ω 𝒏=
𝟐𝟎𝒕
𝑫 𝟐
𝑬
𝟑𝝆 𝟏−𝒗 𝟐 rad/sec

99
…continued
Corrugated diaphragms give a larger displacement which may
be about 2 % of diaphragm diameter.
In order to obtain larger deflections, two corrugated diaphragms
may be welded, brazed or soldered to form a Capsule.
The diaphragms are usually made of mild steel.

100
Diaphragm Pressure Gauge: Actual Look

101
What is Bellows Pressure Gauge ?
Bellows is a thin walled tube approximately 0.1 mm thick having a
corrugated shape.
It is made from a single piece of metal, usually special brass or stainless steel.
Bellows is essentially a pressure activated spring.
The displacement of the Bellows for a particular pressure depends upon
the type and the thickness of the material used.

102
…continued
Like a diaphragm, bellows are also used for pressure measurement and can
be made of cascaded capsules.
The basic way of manufacturing bellows is by fastening together many
individual diaphragms.
The bellows element basically, is a one piece expansible, collapsible and
axially flexible member.
It has many convolutions or fold. It can be manufactured form a single piece
of thin metal.

103
…continued
For industrial purposes, the commonly used bellow elements are:
1. By turning from a solid stock of metal
2. By soldering or welding stamped annular rings
3. Rolling a tube
4. By hydraulically forming a drawn tubing

104
Working of Bellows Pressure Gauge
The action of bending and tension operates the elastic members. For proper
working, the tension should be least. The design ideas given for a diaphragm is
applied to bowels as well. The manufacturer describes the bellows with two
characters – maximum stroke and maximum allowable pressure. The force
obtained can be increased by increasing the diameter. The stroke length can
be increased by increasing the folds or convolutions.

105
…continued
For selecting a specific material for an elastic member like bellows, the
parameters to be checked are:
 Range of pressure
 Hysteresis
 Fatigue on dynamic operation
 Corrosion
 Fabrication ease
 Sensitivity to fluctuating pressures

106
…continued
Out of these hysteresis and sensitivity to fluctuating pressures are the most
important ones. Hysteresis can be minimized by following a proper
manufacturing technique. For instance, a diaphragm when machined from a
solid stock shows less hysteresis compared to the one produced by stamping.
The same technique could be adopted for bellows as well. In the latter case,
the dynamic nature of the variable is likely to induce resonance quickly
depending on the natural frequency of the system. The natural frequency is
calculable from the dimensions of the system and the gauge.

107
…continued
For strong bellows, the carbon steel is selected as the main element. But the
material gets easily corroded and is difficult to machine. For better hysteresis
properties you can use trumpet bass, phosphor bronze, or silicon bronze. Better
dynamic performance can be achieved by using beryllium copper. Stainless
steel is corrosion resistive, but does not have good elastic properties. For easy
fabrication soft materials are sought after.

108
…continued
All bellow elements are used with separate calibrating springs. The springs can
be aligned in two ways – in compression or in expansion when in use. Both
these types, with internal compression springs or external tension springs, are
commercially known as receiver elements and are used universally in
pneumatic control loops. The figures below show the compressed and
expanded type. Spring opposed bellows are also shown below. The open side
of a bellows element is usually rigidly held to the instrument casing and
because of the rigid fixing, the effective or active length of the bellows
element is smaller than its actual length. This device is used in cases where the
control pressure range is between 0.2 to 1 kg/cm2.

109
…continued
Expanded TypeCompressed Type Spring Opposed Bellows

110
…continued
Because of the device’s dynamic operation, the life of a bellow is an
important consideration.
In terms of choice of elastic material for the sensors, the corrosive medium
requires special precaution. Besides this, there are other factors showing that
the medium should not come in direct contact with the measuring element.
They are ;
1. The direct impact of static head on the measuring element may cause error
in response.

111
…continued
2. Direct touch of the medium may cause corrosion, high viscosity fluids may
cause response error and entrailed materials in the medium may clog in the
element.
3. In some critical processes in food processing and pharmaceutical industries,
cleaning of the measuring system is necessitated.
4. Removal of the measuring element for servicing should be convenient.

112
McLeod Gauge is a vacuum gauge that uses
the same principle as that of a manometer.
By using the pressure dividing technique, its
range can be extended from a value of
10-4 Torr.
The basic principle is called the multiple
compression technique. It is shown in the figure.
McLeod Vacuum Gauge

113
If there are two bulbs A and B connected with the McLeod and test gauges
through capillary tubing's, the pressure on the right hand side of the test gauge
is very small and the capillary connection between T and bulb B very long, then
the flow law can be written as
McLeod Vacuum Gauge
Flow Law
𝑽𝒅𝒑 𝟐
𝒅𝒕
= k (𝒑 𝟏 − 𝒑 𝟐)
V- Volume of the bulb ; dp2/dt – Pressure Gradient in time between the two
elements ; K – Flow conductance in the capillary.
As 𝒑 𝟐 is very small when compared to 𝒑 𝟏, the flow rate remains practically
constant and is proportional to the pressure. This forms the basis of the
calibration.

114
In this gauge the temperature of the wire is
determine by measuring the change of resistance.
The pirani gauge employs a single filament (in the
form of four coiled wires of tungsten or platinum are
connected in parallel) enclosed in the glass tube /
chamber whose pressure is to be measured.
As a surrounding pressure changes, the filament
temperature and hence its resistance also
changes.
Pirani Vacuum Gauge

115
A compensating cell is also employed to minimize variation caused by ambient
temperature changes ( its filament material is same as that of the measuring
cell). The resistance change of the filament in the measuring cell is measured by
the use of a resistance bridge which is calibrated in terms of pressure. This
gauges cover from about 10-5 Torr to 1 Torr.
Pirani Vacuum Gauge

116
Advantages
1.This gauge is rugged, inexpensive usually more accurate than
thermocouple gauges
2. The pressure reading range is wider
3.Fast response to changes in pressure
4. Possibility of process control and remote reading
Pirani Vacuum Gauge

117Disadvantages
1. Calibration is non linear and varies from one gas to another
2. Require calibration against some pressure standards
3. Poor transient response
4. Operation requires electrical power

Introduction to transducers

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