# Thermocouple and its Principle

A thermocouple consists of two pieces of dissimilar metals with their endsÂ joined together (by twisting, soldering or welding). When heat is applied toÂ the junction, a voltage, in the range of milli-volts (mV), is generated. AÂ thermocouple is therefore said to be self-powered. Shown in Below Figure is aÂ completed thermocouple circuit.

The voltage generated at each junction depends on junction temperature. IfÂ temperature T1 is higher than T2, then the voltage generated at Junction 1Â will be higher than that at Junction 2. In the above circuit, the loop currentÂ shown on the galvanometer depends on the relative magnitude of theÂ voltages at the two junctions.

In order to use a thermocouple to measure process temperature, one end ofÂ the thermocouple has to be kept in contact with the process while the otherÂ end has to be kept at a constant temperature. The end that is in contact withÂ the process is called the hot or measurement junction. The one that is keptÂ at constant temperature is called cold or reference junction. The relationshipÂ between total circuit voltage (emf) and the emf at the junctions is:

Circuit emf = Measurement emf â€“ Reference emf

If circuit emf and reference emf are known, measurement emf can beÂ calculated and the relative temperature determined.

To convert the emf generated by a thermocouple to the standard 4-20 mAÂ signal, a transmitter is needed. This kind of transmitter is called aÂ temperature transmitter. Below Figure shows a simplified temperatureÂ transmitter connection.

In Figure above, the temperature measurement circuit consists of aÂ thermocouple connected directly to the temperature transmitter. The hot and
cold junctions can be located wherever required to measure the temperatureÂ difference between the two junctions.

In most situations, we need monitor the temperature rise of equipment toÂ ensure the safe operation. Temperature rise of a device is the operatingÂ temperature using ambient or room temperature as a reference. ToÂ accomplish this the hot junction is located in or on the device and the coldÂ junction at the meter or transmitter as illustrated in Below figure.

• Thermocouples are used on most transformers. The hot junction isÂ inside the transformer oil and the cold junction at the meter mountedÂ on the outside. With this simple and rugged installation, the meterÂ directly reads the temperature rise of oil above the ambientÂ temperature of the location.
• In general, thermocouples are used exclusively around the turbineÂ hall because of their rugged construction and low cost.
• A thermocouple is capable of measuring a wider temperature rangeÂ than an RTD.

• If the thermocouple is located some distance away from theÂ measuring device, expensive extension grade thermocouple wires orÂ compensating cables have to be used.
• Thermocouples are not used in areas where high radiation fields areÂ present (for example, in the reactor vault). Radioactive radiationÂ (e.g., Beta radiation from neutron activation), will induce a voltageÂ in the thermocouple wires. Since the signal from thermocouple isÂ also a voltage, the induced voltage will cause an error in theÂ temperature transmitter output.
• Thermocouples are slower in response than RTDs
• If the control logic is remotely located and temperature transmittersÂ (milli-volt to milli- amp transducers) are used, a power supplyÂ failure will of course cause faulty readings.

Failure Modes:

• An open circuit in the thermocouple detector means that there is no path forÂ current flow, thus it will cause a low (off-scale) temperature reading.
• A short circuit in the thermocouple detector will also cause a lowÂ temperature reading because it creates a leakage current path to the groundÂ and a smaller measured voltage.

Thermal Wells / Thermo wells

The process environment where temperature monitoring is required, is oftenÂ not only hot, but also pressurized and possibly chemically corrosive orÂ radioactive. To facilitate removal of the temperature sensors (RTD and TC),Â for examination or replacement and to provide mechanical protection, theÂ sensors are usually mounted inside thermal wells (Below Figure).

A thermal well is basically a hollow metal tube with one end sealed. It isÂ usually mounted permanently in the pipe work. The sensor is inserted into itÂ and makes contact with the sealed end.

A drawback to thermal wells is their long response time because heat mustÂ be transferred through the well to the sensor. An example of theÂ temperature response for bare and thermal well installed sensors is shown in Below Figure. Minimizing the air space between the sensor and the well,Â however, can decrease this thermal lag.

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