What is a Thermocouple ?
Thermocouple is a active transducer which generate e.m.f. by its own.
Thermocouple is the simple electrical temperature sensitive device. Therefore, it provides a reliable method of temperature measurement. It is widely used in industrial applications to monitor temperatures of liquids and gaseous in storage and flowing in pipes and dust. They are used in industrial furnaces as well as for measurement of temperature in cryogenic range.
Thermocouple Principle
It is related to different effects.
1. Seebeck effect
In 1821, the scientists Seebeck discovered that if a closed circuit is formed of two dissimilar metals and two junctions of the metals are at different temperatures. An electric current flows from Cu to Iron at hot junction and it flows from iron to Cu at cold junction. This produce voltage proportional to temperature at the junction.
2. Peltier effect
In 1834, he discovered that when a loop was formed of two dissimilar metals and if externally current was forced to flow through the circuit, one of the junction of heated while the other was cooled.
Amount of heat absorbed when unit current passes from a unit time is called Peltier effect.
3. Thomson Joule Effect
When current of electricity flows along a copper wire whose temperature varies from point to point, heat is liberated at any point P when current at p is in same direction as heat flow. While heat is absorbed at P if current is in opposite direction to heat flow.
Seebeck effect is a combination of Peltier and Thomson Joule effect. Thus for a thermocouple e.m.f. equation can be written as,
e = ∆t + b (∆t)²
where,
∆t = Difference in temperature of hot and cold junction
a and b = Constants.
Usually a >> b.
Thermocouple Working Principle
An e.m.f. is generated across the junction of two dissimilar metals when that junction is heated.
Thermocouple Construction
• Fig. shows the construction of thermocouple.
• Thermocouple basically consists of 2 dissimilar metals wires connected together so as to form two junctions.
• One junction is kept at constant temperature (cold junction) and other is heated (hot junction).
• Hot junction is called measuring junction (sensing junction) and cold junction is called reference junction.
• Thermocouples may be prepared by twisting the two wires together and brazing or welding.
The thermocouple conductors must be insulated from the hot junction to the indicating instrument. The two wires also should not touch each other except hot junction. Therefore, suitable insulating materials must be used according to temperature ranges.
Temperature ranges for insulating material.
Enamel and cotton – 250°F
Glass and asbestos – 900°F
Ceramic – 2600°F
A thermocouple thus consists of pair of dissimilar metal wires joined together at one end (sensing or hot junction) and terminated at other end (reference or cold junction, which is maintained at known constant temperature (reference temperature).
Materials Used and Types of Thermocouple depending on material used –
The following metal pairs are used for Thermocouples.
[Note: First material is more positive when measuring junction is at higher temperature compared to reference junction temperature].
1. Type T i.e. Copper-Constantan thermocouple is expensive, has high output.
2. Type K i.e. Chromel-Alumel thermocouple: Very resistant to oxidation within specified temperature limit.
• It should not be used in reducing atmosphere.
3. Type E i.e. Chrome-Constantan thermocouple:
• Highest output, good stability.
4. Type S i.e. Platinum-Rhodium thermocouple:
• Expensive, low output, most accurate, very stable and resistant to oxidation.
• It is used only for high temperature application.
• This thermocouple is used as a standard from which all e.m.f. temperature tables are derived.
When a temperature difference exists between the sensing junction and the reference junction, an e.m.f. is produced which causes a current in the circuit.
When the reference end is terminated by a meter or recording device, the meter indication is proportional to the temperature differentiate between two junctions.
Measurement of e.m.f. with Thermocouple
Fig. shows the circuit of an iron-constantan thermocouple for e.m.f. measurement.
• The voltmeter is placed between the terminating end.
• The terminating end of reference junction temperature kept constant.
• The temperature at hot junction is varied.
• Then as per the principle of thermocouple the temperature difference between the hot junction and cold junction is proportional to the indication on meter reading which gives voltage as output.
• Output voltage produced by thermocouple is usually small, and that is less than 50 mV.
• Therefore, amplification is necessary for practical applications.
• Due to small signal level, device is susceptible to noise. Hence, usually a high gain differential amplifier which has high input impedance.
• Measuring instrument (voltmeter) should not load thermocouple. To use thermocouples in industry noise reduction technique is used. For this purpose,
(i) Lead wires are twisted and then wrapped with a grounded foil sheath.
(ii) Measurement junction itself is grounded at point of measurement.
(iii) Instrumentation amplifier with excellent CMRR is used.