Thermocouple by Engr. Imran Tanvir

ThermocoupleTransducer

Imran Ahmed TanvirTalha SharifLubna NoorKawish PervaizTooba Yasmin

COMSATS Institute Of Information Technology Attock Campus, Pakistan

Transducer A transducer is a device that converts one form of energy to

other form. It converts the measurand to a usable electrical signal.

In other word it is a device that is capable of converting the physical quantity into a proportional electrical quantity such as voltage or current

What Is Thermocouple ? Thermocouple is a couple of two wires which measures

unknown temperature of the body with reference to the known temperature of the other body.

Thermocouple Basic circuit

Construction Of Thermocouple

A thermocouple can be formed by joining the two dissimilar metals at their ends forming two junctions called the thermojunctions. There are various methods of joining the two dissimilar metals to form the thermojunction. One is to weld the wires together. This produce a brittle joint.

Construction… Another method of forming thermojunction is by soldering

the wires together.

Working Principal Seebeck effect  The Seebeck effect states that when two different or unlike

metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals.

Eemf = - S T

where, S is the Seebeck coefficient

= T1 – T2

Working Principal… Peltier effect

When a current is made to flow through a junction between two conductors, A and B, heat may be generated or removed at the junction. The Peltier heat generated at the junction per unit time is equal to

where is the Peltier coefficient of conductor A (B) and I is the electric current (from A to B)

Working It comprises of two dissimilar metals, A and B. These are

joined together to form two junctions, p and q, which are maintained at the temperatures T1 and T2 respectively. Remember that the thermocouple cannot be formed if there are not two junctions. Since the two junctions are maintained at different temperatures the Peltier emf is generated within the circuit and it is the function of the temperatures of two junctions.

Working…If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net current flowing through the junction is zero. If the junctions are maintained at different temperatures, the emf will not become zero and there will be a net current flowing through the circuit. The total emf flowing through this circuit depends on the metals used within the circuit as well as the temperature of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the suitable device.

Working…The device for measuring the current or emf is connected within the circuit of the thermocouple. It measures the amount of emf flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures. Now, the temperature of the reference junctions is already known, while the temperature of measuring junction is unknown. The output obtained from the thermocouple circuit is calibrated directly against the unknown temperature. Thus the voltage or current output obtained from thermocouple circuit gives the value of unknown temperature directly.

Types Of Thermocouple

i. Type T : Copper - Constantan (270 / 400)ii. Type J : Iron - Constantan (0 / 816)iii. Type E : Chromel - Constantan (0 / 900) iv. Type K : Chromel - Alumel (-270 / 1260) v. Type R : Platinum - Rhodium (0 / 1600)vi. Type S : Platinum - Rhodium (0 / 1750)vii. Type C : Copper – Rhodium (0 / 2200)

Thermocouple Materials Vs EMF

Applications Type B, S, R and K thermocouples are used extensively in the

steel and iron industries to monitor temperatures and chemistry throughout the steel making process

Gas-fed heating appliances such as ovens & water heaters In the testing of prototype electrical and mechanical

apparatus Thermopile radiation sensors Power production Process plants

Comparison with RTDParameter Thermocouple RTD

Typical measurement range

-267°C to 2316°C -240°C to 649°C

Sensitivity Medium MediumSpeed of response High High

Accuracy Moderate HighSize Small LargeCost Low High

Material Used Copper-constantan, Iron-constantan etc.

Nickel, Copper, Platinum etc.

Applications Suitable for applications which

require wide temperature range

Suitable where speed of response and

accuracy are more important

Merits Thermocouple have a better response. They have a higher range of temperature measurements. The sensing element of a thermocouple easily installed. Cheaper than resistance thermometers. Very convenient for measurement of temperature at one

particular point in a piece of apparatus.

Demerits Low accuracy and as such they can’t be use for precision

work. The milli-voltmeters used with thermoelements must be more

sensitive and therefore careful handling. A rise in temperature causes a change in the resistance of

heater.