Thermocouple – Types, Working principle & Properties

Thermocouple

Thermocouple

Working Principle

Seeback Effect

When heat is applied to a junction of two dissimilar metals, an e.m.f. (thermo e.m.f.) is generated which causes current to flow from hot junction to cold junction. The thermo E.M.F. generated is proportional to the temperature difference between two junctions.

The thermo e.m.f. generated is,

Where,

  • E = Thermo E.M.F. between junction
  • T = Temperature  at measuring junction
  • L = Temperature  at reference  junction
  • C1, C2 = Constant depending on metals (usually C1 in microvolts and C2 in milivolts)

Note: Larger the value of C2 larger is the nonlinearity in the thermocouple characteristics.

There is an opposite principle of seeing back also exists, commonly known as “Peltier Effect”

Peltier Effect

When a current flow across the junction of two metals heat is absorbed or evolved at the junction when the current flows from one metal to another

  • dQ = Heat absorbed or evolved
  • a = Peltier constant
  • I = Current passing through the junction
  • dt = Time of passing the current

Types of Thermocouples

Sr. No. Type Material Range (0C) Tolerance Sensitivity (µV/0C) Comment Applications
1 B Strip 1: Platinum (70%) + Rhodium (30%)

Strip 2: Platinum (94%) + Rhodium (06%)

600 to

1500

 ± 0.0025 0C 5 to 12 More stable than R, ,S

types at higher temperatures

1)     Temperature measurement of flammable materials/ fluids.

2)     Since, they are chemically less responsive, used to measure temperatures of Acids and strong Bases.

2 R Strip 1: Platinum (87%) + Rhodium (13%)

Strip 2: Platinum (100%)

0 to 1400 ± 1 0C 5 to 12 Most stable in all hazardous zones 1)     Temperature measurement of flammable materials/ fluids.

Since, they are chemically less responsive, used to measure temperatures of Acids and strong Bases.

3 S Strip 1: Platinum (90%) + Rhodium (10%)

Strip 2: Platinum (100%)

 

0 to 1400 ± 1 0C 5 to 12 Stable in all hazardous zones 1)     Temperature measurement of flammable materials/ fluids.

Since, they are chemically less responsive, used to measure temperatures of Acids and strong Bases.

4 J Strip 1: Ferrous / Iron (100%) Strip2: Constantan -200 to

1200

± 1.5 0C 45 to 51 Best for temperature under 600 0C Specially used to measure fluid temperatures which require good accuracy and sensitivity e.g. water temperature measurement
5 K Strip 1: Chromel

Strip 2: Alumel

-200 to

1200

± 1.5 0C 40 to 55 Better in oxidizing

atmosphere.

Mostly used in open atmospheric situations
6 T Strip 1: Copper

Strip 2 : Constantan

-200 to 350 ± 0.5 0C 15 to 60 Oxidation occurs beyond

stipulated range

Specially used when measurement of fluid temperature is made remotely
7 E Strip 1: Chromel

Strip 2 : Constantan

-40 to 800 ± 1.50C  

15 to 60

Works in oxidizing

atmosphere

Mostly used in open atmospheric situations

How to select thermocouple?

Before selecting thermocouple’s material for our desired application, we must consider following guidelines:-

  1. The material of thermocouple must have high thermo E.M.F. per unit temperature change.
  2. Low electrical resistance at the couplings/junctions,
  3. Temperature and thermo emf should be linearly proportional in the given range
  4. The high melting point of the materials of the coupling materials for a wider range.
  5. The material should be pure, homogenous and workable in any shape.
  6. The material of thermocouple must be resistant to corrosion and must be usable over a long time without getting brittle.

Thermal properties of materials

Specific Heat

it is the heat required to increase the temperature of 1 Kg mass by 10

Q=MCpΔT

Where,

  • Q = Heat requires (J)
  • M = Mass (Kg)
  • ∆T = Temperature difference (0K)
  • Cp= Specific heat  (J/Kg0K)

Note: Calorimeters is used to calculate specific heat of material

Thermal expansion coefficient

When a solid is heated, it increases in volume. The increase in the length of solid depends upon original length, temperature and thermal expansion of coefficient.

Where,

  • L0 , Lt= Length at 00C and  t 0C respectively (Meter)
  • α = Thermal expansion coefficient of solid (µm/m)

Thermal Conductivity

It is the rate of heat flow per unit time in a homogeneous material under steady conditions per unit area per unit temperature gradient.

Where,

  • Q = Heat flow per unit time (Watts)
  • K = Thermal conductivity
  • A = Area of material
  • X = Thickness
  • T = Temperature difference (K)

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Signal Conditioning for Thermistors

In the last article we have seen some signal conditioning circuits for RTD. Today we are going to learn signal conditioning for thermistors. We know that thermistor is temperature measuring sensor made up of semiconducting material. The resistance of thermistor normally decreases as the temperature increases hence it is has negative temperature coefficient (NTC).

Change in resistance of thermistor due to change in temperature is given by the following equation,

Where

RT is the temperature at T(K),

R0 is the resistance at T0 normally at 298K,

? is the characteristic temperature constant of thermistor,

The characteristics of thermistor resistance vs. temperature is non linear therefore linearization circuit is also included along with amplifier in the signal conditioning circuits for thermistor.

Signal Conditioning for Thermistors

Signal conditioning of thermistor includes bridge amplifier and linearization circuit. These circuits are explained as follows.

Bridge amplifier

Following circuit shows a bridge amplifier used for amplifying the output of thermistor. As the output range of thermistor is quite low and it not good to use such short range of output for getting good accuracy for operating any field devices.

Bridge amplifier for thermistor

Bridge amplifier for thermistor

Bridge amplifier consist of wheatstone bridge in which inverting amplifier with thermistor as feedback resistor is used in one of the arm as shown in the diagram. This operational amplifier produces output voltage proportional to the change in the resistance of the thermistor.

Linearization of thermistor

For linearization of thermistor characteristics there are several methods available.

  • Using parallel resistor:

In this method a parallel resistor is connected with thermistor. This method increases linearity but also decreases the sensitivity of the circuit.

Parallel Resistor with thermistor

Parallel Resistor with thermistor

the value of the equivalent reistance is given by,

parallel resistor output

parallel resistor output

 

where Rp is value of parallel resistor,

Rtm is thermistor temperature at mid scale temperature,

Tm is mid scale of temperature variation,

? is characteristic temperature constant.

  • Using serial resistor:

In this method a series resitance is used with thermistor. It reduces nonlinearity of conductance vs. temperature characteristics of thermistor.

Series resistor with thermistor

Series resistor with thermistor

The conductance Gs is given as,

series output

series output

Where Gtm is the conductance of thermistor at mid scale temperature Tm

  • Using op amp:

The third method for linearising the thermistor output is by using op amp.

Following circuit shows the linearization circuit for thermistor. Here we have used a thermistor along with series resistor connected to the inverting terminal of the op amp. An adjustable supply voltage is used to adjust the gain of the amplifier.

Thermistor linearization using op amp

Thermistor linearization using op amp

 

In this way we have seen different signal conditioning circuits for thermistor.

See also: Signal Conditioning for RTD

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How to Linearize RTD Output

Hi friends, today we will see how to linearize output of RTD. We know that Resistance Temprature Detector (RTD) is widely used temperature sensor. Sometimes we need a linearly changing sensor output for building any digital control system. For example if we have to control a fuel supply to the burner using the temperature reading of the RTD. We can not apply its output (i.e. non linear resistance) to a digital system for controlling the physical parameter. So the question arises how to linearize RTD output.

We know that the output of normally used platinum (Pt100) RTD is non linear. The non linear change in resistance of RTD with respect to temperature is given by the quadratic equation which includes a non linear term. Following circuit gives the output in the linear form. In the circuit, we have used RTD resistance as a feedback resistance of op amp.

Linearization of RTD

Linearization of RTD

In this way we can easily convert nonlinear output of RTD in the linear form. Above circuit is best suited for the temperature range of 0 to 500 degree Celsius.

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Signal Conditioning Circuits

We know that signal conditioning is a process in which signals from different sensors are transferred into a form necessary to interface with other modules of system.

For example, we know that thermocouple produces very low output voltage and this voltage is not sufficient to operate the other controlling modules. Therefore there is need to amplify such signals. For this purpose we use different signal conditioning circuits. In case of thermocouple, we have to use amplifier, linearization circuits, etc. the purpose of using linearization circuits is that, thermocouple has non linear characteristics but in most of the cases we need linear controlling action.

Signal conditioning circuit

Measurement System Block Diagram

Signal Conditioning Circuits:

There are different types of signal conditioning operations such as amplification, filtering, isolation, linearization, excitation, etc. we will discuss all these operation one by one.

Amplification

We know that most of the sensors produce output in the form of change in resistance, voltage or current. All these parameters are having very low strength i.e. very small voltage in case of thermocouple, small change in resistance in case of RTD, etc. Therefore we have use current or voltage amplifiers in case of sensors which produces output in the form of current or voltage.

If the sensor produces output in the form of change in resistance (such as resistance thermometer) we have to use bridge amplifiers. We can make use of operational amplifiers to amplify the signal.

Filtering

Another important signal conditioning circuit is filter. As mentioned earlier most of the sensor produces very low output and therefore electromagnetic noise may get added in the original output. To remove the electromagnetic noise from sensor output we have to use different filter circuits. Filter circuits eliminates noise i.e. undesired frequency components from original signal without affecting it.

Active filters, passive filters, bypass filters are the common types of filter circuits.

Isolation

Isolation circuits are required to differentiate signals from unwanted common mode voltages. Another advantage of isolation circuit is that, it protects measuring devices (sensors) if high voltage is applied to other circuit. It also breaks ground loops.

Linearization

There are many sensors which produces non linear output such as thermocouple, thermistor, etc. linearization circuits are used to convert non linear signal into linear one. It can be achieved by varying the gain of an amplifier as a function of input signal.

Excitation

Another signal conditioning operation is current or voltage excitation. Signal conditioning circuits provide the required voltage or current excitation to some passive sensors such as strain gauge, RTD, etc.

In the upcoming posts we will see signal conditioning of RTD, thermistor and thermocouple.

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Experiment to determine characteristics of Thermistor

Experiment: Characteristics of Thermistor

This section provides you a detailed steps to determine the characteristics of thermistor.

Aim: To determine the characteristics of thermistor (resistance temperature characteristics).

Apparatus required: Bead type thermistor, multimeter, thermometer, water bath, heater connecting wires, etc.

bead type thermistor
bead type thermistor

Introduction:

Thermistors are the temperature sensitive resistors that exhibit a negative temperature coefficient of resistance. In other words electrical resistance of a thermometer will be reduced when it is placed in an environment of higher temperature likewise its temperature decreases. thus the characteristics of thermistor provides an information about how its resistance changes with the changes in temperature.

It is very essential in temperature measurement, thermistors are manufactured and formed into rods, discs, and washes, beads for special applications, they can be directly or indirectly heated. Temperature determines the resistance of those that are directly heated in environment. The resistance of indirectly heated thermistor is determined by temperature of self-contained heater.

Diagram:

types of thermistor
types of thermistor

Procedure:

Following are the steps to determine the characteristics of thermistor.

1)    Take water in container and place a heater to heat water.

2)    Immerse thermistor and thermometer in water bath.

3)    Switch on the power supply.

4)    Measure the temperature on the thermometer from room temperature (30 C) to 98 C and corresponding resistance of thermistor at that temperature.

5)    Switch off the power supply, and then take reading in decreasing order of temperature in an interval of 10 C.

6)    Plot a graph of temperature on X-axis and Resistance on Y-axis. This graph shows the characteristics of thermistor.

Observation Table:

By taking following readings we can plot the characteristics of thermistor on a graph paper.

Sr. No.: Temp in degree Celsius Resistance(Uploading) Resistance(Downloading)

When we plot characteristics of thermistor it will look like as follows:

Characteristics of thermistor
Characteristics of thermistor

Calculations:

We have To= 30 C, Ro=980?, ß=4000.

R(t) = Ro*exp [  ß* (1/T  –  1/To) ]

Conclusion:

1)    Input and output relationship is non-linear for thermistor (i.e. characteristics of thermistor are non-linear).

2)    In comparison with RTD change in resistance for a given change in temperature is very large.

Analysis:

1)    What is negative temperature coefficient of resistance ?

Answer: The property of a material in which resistance of a material decreases with increase in temperature that material is said to have negative temperature coefficient of resistance.

2)    What are common shapes of most commercially available thermistors.

Answer: The common shapes are bead type, disc type, rod type ans IC chips.

3)    What the difference between directly heated and indirectly heated thermistor?

Answer: In directly heated thermistor, there is direct contact between source and thermistor, but in indirectly heated thermistor there is no direct between source and thermistor.

4)    What is the relationship between thermistor resistance and temperature?

Answer: In thermistor, resistance is inversely proportional to the temperature.

5)    What is the range of temperature for thermistor?

Answer: The temperature range of thermistor is -50 C to 15C

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