Thermocouple operation

Thermocouple is an instrument used for measuring temperature in the form of voltage. It consists of two pieces of dissimilar metals with their ends joined together by methods such as twisting, soldering or welding. 

When heat is applied to the junction, a voltage, in the range of milli-volts (mV), is generated. A thermocouple is therefore said to be self-powered. This can be seen the figure above.

The voltage generated at each junction depends on the junction temperature. If temperature T1 is greater than T2, then the voltage generated at junction 1 will be higher than that of junction 2. In the circuit shown in the above figure, the loop current shown on the galvanometer depends on the relative magnitude of the voltages at the two junctions. 

In order to use a thermocouple to measure process temperature, one end of the thermocouple has to be kept in contact with the process while the other end has to be kept at a constant temperature. The end that is in contact with the process is called the hot or measurement junction. The one that is kept at constant temperature is called cold or reference junction. The relationship between total circuit voltage (emf) and the emf at the junctions can be represented as:

                               Circuit emf = Measurement emf - Reference emf

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

To convert the emf generated by a thermocouple to the standard 4-20 mA signal, a transmitter is needed. This kind of transmitter is called a temperature transmitter. The below figure shows the simplified temperature transmitter connection.

The temperature measurement circuit consists of a thermocouple connected directly to the temperature transmitter. The hot and cold junctions can be located wherever required to measure the temperature difference between the two junctions.

There are certain advantages and disadvantages to using thermocouple for temperature measurement:

1) Advantages:

a) Thermocouples are used on most transformers. The hot junction is inside the transformer oil and the cold junction at the meter mounted on the outside. With this simple and rugged installation, the meter directly reads the temperature rise of oil above the ambient temperature of the location.

b) Thermocouples are used exclusively around the turbine hall because of their rugged construction and low cost.

c) A thermocouple is capable of measuring a wider temperature range than an RTD.

2) Disadvantages:

a) If the thermocouple is located some distance away from the measuring device, expensive extension grade thermocouple wires or compensating cables have to be used.

b) Thermocouples are not used in areas where high radiation fields are present (for example, in the reactor vault). Radioactive radiation (e.g., Beta radiation from neutron activation), will induce a voltage in the thermocouple wires. Since the signal from the thermocouple is also a voltage, the induced voltage will cause an error in the temperature transmitter output.

c) Thermocouples are slower in response than RTDs.

d) If the control logic is remotely located and temperature transmitters (milli-volt to milli-amp transducers) are used, a power supply failure will of course cause faulty readings.