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ELET 241
PROCESS INSTRUMENTATION
Temperature Measurement
MODULE#2

DISCLAIMER
•These powerpoint slides act only as a tool in
delivering lectures to the students.
•The materials presented in these slides are not
comprehensive as most of the materials are
explained to the students verbally with the guide of
these PowerPoint slides, smartboard and ELET241
reference book.
•Hence, the students are reminded that the main
reference for ELET241 is Instrumentation and
Process Control by Franklyn W. Kirk text book
2

CONCEPT OF HEAT AND TEMPERATURE
•Heat: it is a the measure of Thermal Energy in material.
•Heat Transfer : it is the exchange of thermal energy from one sample to
another, by way of conduction (direct contact), convection (transfer via a
moving fluid), or radiation (emitted energy).
•Temperature: is the measured heat degree of a material on a definite scale.

• It is normally NOT possible to measure temperature directly.
• Usually effects of temperature are indicated.
• Most commonly used temperature measurements are based on:
1) Thermal Expansion
2) Electrical properties
3) Optical effects

Temperature Scales
The four common temperature scales are:
• Fahrenheit, o
F
• Rankine, o
R
• Celsius o
C, and
• Kelvin o
K

Example 1:- Find the following temperature in Fahrenheit scale:
i) -15°C ; ii) 46°C
Answer 1 : i) -15°C
Example 2:- Find the following temperature in Celsius scale:
i) 14°F ; ii) 104°F
Answer 2 : i) 14°F
ii) 46°C
ii) 104°F

Thermal Expansion
•Materials usually expand when heated and contract when cooled. For
example, a metal rod that is heated or cooled changes in length and
volume.
•The coefficient of linear expansion is the amount a unit length of a
material lengthens or contracts with temperature changes.

LIQUID IN GLASS THERMOMETER
• It is a temperature sensor used to measure temperature based on the thermal expansion of volume
of liquid. When temperature changes occurs, the volume of liquid in thermometer is changed.

Working Principle
• The operation of a liquid-in-glass thermometer depends on volumetric Expansion of the
liquid. The volume of a liquid changes when the temperature changes.
• As the temperature of the liquid in the bulb increases, it expands and drives up the
capillary tube and the temperature is measured with the help of the marked scale of the
thermometer
• The liquid is usually Mercury or Organic fluids such as alcohol.

The structures of typical liquid-in glass thermometer
• The Bulb is usually a thin-walled glass chamber that serves as a tank for
the liquid.
• The Stem is a glass tube that contains the capillary for the liquid.
• Capillary is a narrow path which the liquid can rise and fall.
• The Scale is a series of markings that is used to read the temperature
measurements.
• Immersion Line indicates the proper immersion depth on partial
immersion thermometers.
• The Contraction Chamber it increases the volume of the capillary and
prevents total contraction of the fluid into the bulb at low temperatures.
• The Expansion Chamber it protects the thermometer from rupture at
high temperatures.

liquid in-glass thermometers Types:
•Mercury Thermometers
•Organic Fluids Thermometers
Organic fluids are also used in Inexpensive thermometers or in
applications in which the release of mercury is not safe.

LIQUID IN GLASS THERMOMETERS
Advantages
• Low Cost.
• Good accuracy
• Easy to carry and handle.
• Best to be used in laboratories to monitor baths and to check
calibrations of other temperatures.
Disadvantages:
• Less sensitive.
• Can’t be used for high range of temperature.
• Not safe because of the Mercury that poses a potential toxic
hazard.

BIMETALLIC STRIP THERMOMETERS
• Bimetallic Strip Thermometer is a sensor used to measure temperature based on the
expansion of metals when heated. It is based on a strip consisting of two metal alloys with
different linear expansion coefficients that are fused together and formed to a single strip,
and a pointer or indicating mechanism calibrated for temperature reading.

Working Principle:
• In order to get considerable measurement or actuation from expansion of metal, two
different metal strips of same dimensions are bonded together.
• One metal has a high co-efficient of linear expansion (α) and other has a low
co-efficient of linear expansion.
• This forms the bonded strip when heated, one metal expands much more than the
other.
• Thus, when these two bonded metal have change in temperature, the strip will bend.

https://www.youtube.com/watch?v=2wg5u_90ekQ

•Because the longer the length of a bimetallic strip, the greater the movement,
therefore bimetallic thermometers usually have a strip in the form of helix or
spiral.
•For industrial thermometer helical form of strip is used. One end of the strip is
attached to the inside of a protective sheath and the other end is attached to
a spindle which position the pointer over a scale to give a reading of
temperature.
https://www.youtube.com/watch?v=ha0cF3fuvQE

This bending motion is significant enough to drive a pointer mechanism, activate an
electromechanical switch, or perform any number of other mechanical tasks, making this a
very simple and useful primary sensing element for temperature

Advantages
• Low cost because of simple construction
• Robust and Reliable.
• Stable.
• Good range of temperature (up to 600 ⁰c).
Disadvantages
• Not suitable for low temperature.
• Less Accurate
• Slow response time.

Thermocouple
Working Principle:
• A thermocouple is electrical thermometer consists of two dissimilar metal wires joined at one
end to form measuring junction (Hot Junction J1). And other end wires are connected to
voltmeter to form a (reference Junction J2).
• When the two junctions are at different temperature, current will flow through the circuit and
the small voltage that results from the current flow is measured to determine the temperature
of the measuring junction(Hot Junction).
• This voltage is known as Seebeck voltage(Vab). the Seebeck voltage is linearly proportional to
temperature:
• Where ∝ is Seebeck linear coefficient

Thermocouples
Example
Find the Seebeck voltage for a thermocouple with a α = 40 μV/°C , if the
junction temperatures are 40°C and 80°C.
Vab
= α (T1
– T2
)
Vab
= (40 μV/°C)×(80°C - 40°C) = 1.6 mV
Solution

Thermocouple Types
• Thermocouples exist in many different types, each with its own color codes for the dissimilar-metal wires. Here is
a table showing the more common thermocouple types and their standardized colors

Thermocouple
Reference Junction Compensation
• The cold junction, or reference junction, is the end of a thermocouple used to provide a reference
point. Thus, in order to measure the hot junction directly it is essential that the temperature at the
cold junction must be accurately known. This method called reference junction compensation. There
are two main methods to achieve the compensation of reference junction temperature:
1. Constant reference junction temperature
• The reference junction is held at a constant or known fixed temperature so it is always stable. This
may be accomplished by immersing the reference junction in a bath of ice (AT 0 ⁰C) which allows
measuring temperature to be read directly from an indicator without the need of calculating a
correction.
• In laboratory this method is very practical. However, In Industry refrigeration units are often used.

Thermocouple
2. Software Compensation
• Instead of canceling the effect of the reference junction manually by the ice bath, we cancel the effect
arithmetically inside the microprocessor-based transmitter. In other words, Another electrical-based
sensor(such as RTD) is connected to microprocessor along with reference junction connected to the same
Microprocessor. The receiving analog-digital converter circuit see the difference in voltage between the
measurement and reference junctions (Vinput = VJ1 − VJ2), but then after digitizing this voltage measurement
we have the microprocessor add the equivalent voltage value corresponding to the ambient temperature
sensed by the RTD (Vrtd):
Compensated total = Vinput + Vrjc

Thermocouple Sheath
•In industrial practice, it is essential to package thermocouples in a more
rugged form than a bare metal junction. For instance, most industrial
thermocouples are manufactured in such a way that the dissimilar-metal
wires are protected from physical damage by a stainless steel or ceramic
sheath.

Thermocouple Connection
•Thermocouples in Parallel
Averaging thermocouple: An averaging thermocouple is an electrical thermometer consisting of a set of
parallel-connected thermocouples that is commonly used to measure an average temperature.
Swamping Resistance: Large resistor is used to maintain equal values of resistance between the wires, so the resultant
voltage will be more accurate.
The different thermocouples are positioned at different depths in the tube and the circuit averages the voltage readings.

Thermocouple Connection
•Thermocouples in Series
• A thermopile is an electrical thermometer consisting of several
thermocouples connected in series to provide a higher voltage output.
• In a thermopile, the individual voltages of each thermocouple are
added together. A thermopile can be used to measure extremely small
temperature differences.
Thermopile

Thermocouple Extension Wires
•Thermocouple are normally installed some distance away from the
voltmeter or computer that measures the small voltage generated by
the thermocouple.
•For this purpose, cheaper and lower grade thermocouple wires,
called extension wire are used to connect the thermocouple to the
measuring device at the reference junction.
•Compensating leads must be matched to the thermocouple and for
each type of thermocouple, corresponding extension leads are
available.

Thermocouples
Advantages of Thermocouples
• The thermocouple is inexpensive
• It has wide temperature ranges measurements
• It has rugged construction.
• It is self powered and does not require any bridge circuit.
• It has high speed of response.
• Stable.
• Good Precision.

Thermocouples
Disadvantages of Thermocouples
•As output voltage is very small, it needs amplification.
•The cold junction and lead compensation is essential.
•Fairly linear( shows some non-linearity).
•It has low sensitivity.
•Susceptible to corrosion.

Resistance Temperature Detector
•Working Principle:
•A resistance temperature detector (RTD) is an electrical thermometer that its
resistance increases when it is exposed to heat. A protective sheath material
covers the RTD wires, which are coiled around an insulator that serves as a
support.

• RTDs are identified by there material and resistance at 0 °C. For example, PT-100 means an RTD made of
platinum having resistance of 100 ohms at 0 °C.
• In a resistance temperature detector the variation of resistance with temperature is given by
RT2
= RT1
(1 + α[T2
− T1
])
• where RT2
is the resistance at temperature T2
and RT1
is the resistance at temperature T1
.
• α is called Temperature Coefficient of Resistance and depends on the material used.
Example : What is the resistance of a platinum resistor at 250°C, if its
resistance at 20 °C is 170 Ω?
Resistance at 250 °C
= 170(1 + 0.00385 [250 − 20])
= 170(1 + 0.8855)
= 320.45 Ω

• In order to measure the temperature by using RTD, a temperature transmitter in the form of a
Wheatstone bridge is generally used. A Wheatstone bridge is electrical circuit consisting of an
RTD, three fixed resistors, and a power supply .
• In this circuit, An electrical excitation current is passed through the bridge. When all the
resistors are at equal values, the voltage at M(Vab) is zero thus, the bridge is said to be in null
balance. As temperature increases, the RT resistance increases, causing the bridge to become
unbalanced, and the voltage (Vab) read by the voltmeter increases.

•The basic circuit described uses a two wire RTD.
•A problem arises when the two wire RTD is installed some distance away
from the transmitter.
•Since the connecting wires are long, resistance of the wires changes as
ambient temperature changes.
•The variations in wire resistance would introduce an error in the
transmitter.
•To overcome this problem Three wire RTD of Four Wire RTD can be used.

Three-wire RTD
• The connecting wires (w1, w2, w3) are made the same length and therefore the same resistance.
• It can be seen that the resistance of the right leg of the Wheatstone bridge is R1 + R2 + RW2. The
resistance of the left leg of the bridge is R3 + RW3 + RTD. Since RW1 = RW2, the result is that the
resistances of the wires cancel and therefore the effect of the connecting wires is eliminated.
• Three wire RTD is the RTD of choice in Industrial applications.

Four Wire RTD
• The four wire RTD, is not suitable with Wheatstone bridge. Instead of using a Wheatstone bridge configuration, a
current source is employed to supply a constant current I, to the thermometer, through wires a and d. A voltmeter
measures the voltage developed across the thermometer, via wires b and c. The measured voltage is directly
proportional to the resistance of the thermometer, so only the conversion from resistance to temperature is
necessary. Wires b and c only act as voltage sense leads, and with M a high impedance meter, virtually no current
flows in wires b and c, and therefore no voltage drop in these leads and thus no lead resistance error in the
measurement.

Advantages:
•Good sensitivity
•It is accurate
•Good for low temperatures
•Suitable for small ranges.

Disadvantages:
•More expensive than thermocouples.
•Limited measuring temperature range.
•Requires Power Supply.
•Self Heating
•Drift Problems.

Thermistor
• A thermistor is similar to a resistance thermometer, but a semiconductor material is
used instead of a metal.
• The electrical resistance of most thermistors decreases with an increase in
temperature.
• Therefore, most thermistors have a negative temperature coefficient (NTC).
• the resistance of a thermistor decreases with increasing temperature.

Thermistor
Advantages
• Very Sensitive. Small changes in temperature can make large changes in
resistance.
• Good stability.
• Low Cost
• Best used for small area sensing.
Disadvantages
• Non Linear
• Limited operating range (-80 to 150 °C)
• Power supply required
• Not Suitable for Industrial Applications.

PYROMETERS
• Pyrometer consists of optical components that used to collect the radiant energy
emitted by target hot object.
• The primary advantage of pyrometer that it does not need to place a sensor in direct
contact with the process.

Pyrometers
•Pyrometers are used to measure extremely high temperatures.
•Pyrometers measure temperature without contact with the object.
•There are two types of Pyrometers
• Total Radiation Pyrometers
• Optical Pyrometers

Pyrometers
Total Radiation Pyrometer
•Radiation from hot source are reflected from a mirror and focused onto the
hot junction of a small gauge thermocouple or a thermopile.
• The output of thermocouple or thermopile is indicated or recorded on an
instrument having a scale of temperature.

Pyrometers
Optical Pyrometer
• Optical pyrometer use visible light emitted by hot bodies to measure their temperature.
• A commonly used optical pyrometer is Disappearing Filament Pyrometer
• It has an electrically heated, calibrated tungsten filament contained within a telescope tube.
• The telescope tube also contains a red glass filter that restricts the brightness of the hot source
to one specific wavelength of red light.
• The current to the filament is manually adjusted until the apparent brightness matches that of
the target source.
• When the brightness of the filament matches the brightness of the hot source, the filament
disappears from view.
• Measuring circuitry in the pyrometer converts the filament current value to a temperature
reading on a temperature indicator.

Thermal Imaging
•A very useful application of non-contact sensor technology is thermal
imaging, where a dense array of infrared radiation sensors provides a
graphic display of objects in its view according to their temperatures.
•Each object shown on the digital display of a thermal imager is
artificially colored in the display on a chromatic scale that varies with
temperature, hot objects typically registering as red tones and cold
objects typically registering as blue tones.
•Thermal imaging is very useful in the electric power distribution
industry, where technicians may inspect power line insulators and
other objects at elevated potential for “hot spots” without having to
make physical contact with those objects.

THERMOWELLS
•temperature instruments cannot be used without protection from the
environment in which they are used.
• A thermowell is a closed tube used to protect a temperature instrument
from process conditions and to allow instrument maintenance to be
performed without draining the process fluid.

References
•Instrumentation and Process Control by Franklyn W. Kirk
•Lessons In Industrial Instrumentation by Kuphaldt

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