A thermocouple consists of two dissimilar material wire joined together at one forming a hot junction. The two wires are terminated at other end. This junction called reference or cold junction. The cold junction is maintained at a known constant temperature called reference temperature of thermocouple.
The scientist see back discovered that when a temperature difference exists between the hot and cold junction, an EMF is produced. This emf (Electro Motive Force) causes a current in the circuit. The meter indication will be proportional to the temperature difference between the hot junction and cold junction. This thermo electric effect caused by the constant potential at the junctions. It is known as the SEE BACK EFFECT.
The magnitude of EMF depends on the wire material used and also on the temperature difference between the junctions. A sensitive millivoltmeter is directly connected across the cold junction. Now the deflection of the meter is directly proportional to the difference in temperature between the hot junction and the cold junction of . The DMM Digital multimeter (voltmeter) reading is converted into temperature.
Advantage:
- Rugged design
- Wide range
- No bridge circuit is required
- Good Accuracy.
Disadvantage:
- Temperature range from 270 c to 2800 c.
- Output voltage is less than 10mV so a very sensitive meter is required.
Applications: Mainly used in industrial.
HOW TO TEST YOUR THERMOCOUPLE:
Before testing below components are needed.
- Thermocouple.
- Digital Multimeter (Auto Ranging). DMM (Digital Multimeter)
- 100W soldering Iron.
- Connect DMM + ve, -ve probe to J1 and J2 of thermocouple (Called cold Junction).
- Select DMM selector switch to DC mV range (or) Dc auto range.
- Keep the soldering iron near the hot junction of thermocouple.
- Read the DMM value ( This is normal room temperature).
- Switch on the soldering iron with AC 220 V.
- You can see the DMM reading raise( in mV range) in proportion to the temperature.
Verification:- From the above simple test, we can know whether the thermocouple GOOD or NOT.
