Thermocouple and its function
Thermocouple and its function: A thermocouple consists of two different metal pieces whose ends are joined together (by screwing, soldering or welding). When heat is applied to the junction, a voltage in the range of millivolts (mV) is generated. Hence it is said that a thermocouple is self-energizing. The following figure shows a complete thermocouple circuit.
The voltage generated at each connection depends on the connection temperature. If the temperature T1 is higher than T2, the voltage generated at junction 1 will be higher than that at junction 2. In the above circuit, the loop current shown on the galvanometer depends on the relative voltage of the two connections.
In order to use a thermocouple to measure the process temperature, one end of the thermocouple must be in contact with the process while the other end is kept at a constant temperature. The end that is in contact with the process is called the hot connection or measurement. It is kept at a constant temperature, called a cold or reference connection. The relationship between the total circuit voltage (emf) and emf in the connections is:
Circuit emf = Measurement emf – Reference emf
If the emf circuit and the EMF reference are known, the emf measurement can be calculated and the relative temperature determined. A transmitter is required to convert the emf generated by the thermocouple to a standard 4-20 mA signal. This type of transmitter is called a temperature transmitter. The figure below shows a simplified temperature transmitter connection.
In the figure above, the temperature measuring circuit consists of a thermocouple connected directly to the temperature transmitter. Hot and cold connections can be placed anywhere to measure the temperature difference between the two connections. In most cases, we need to monitor the temperature rise of the equipment to ensure safe operation. Increasing the temperature of a device is the operating temperature using the ambient or room temperature as a reference. To achieve this, the hot connection is located in the device or its connection point and the cold connection point is in the meter or transmitter as shown in the figure below.
Advantages and disadvantages of thermocouples
- Most transformers use thermocouples. The hot connection point is inside the transformer oil and the cold connection point is inside the meter installed on the outside. With this simple and uneven installation, the meter directly reads the air temperature above the ambient temperature.
- In general, thermocouples are used exclusively around turbine halls due to their rugged construction and low cost.
- A thermocouple is able to measure a wider temperature range than RTD.
- If the thermocouple is a short distance from the measuring device, expensive thermometer wires or compensating cables should be used.
- Thermocouples are not used in areas with high radiation fields (for example, in a reactor room). Radioactive radiation (for example, beta radiation from neutron activation) causes voltage across the thermocouple wires. Since the signal from the thermocouple is also a voltage, the resulting voltage causes an error in the output of the temperature transmitter.
- Thermocouples are slower to react than RTDs
- If the remote control logic is located and temperature transmitters (mV to mA converters) are used, the power supply will definitely cause faulty readings.
- The open circuit in the thermocouple detector means that there is no way to flow, so it will cause low temperature reading (off-scale).
- A short circuit in the thermocouple detector will also cause the temperature to read low because it creates a leakage current path to the ground and a smaller measured voltage.
The process environment in which temperature monitoring is required is often not only hot but also pressurized and possibly chemically corrosive or radioactive. To facilitate the removal of temperature sensors (RTD and TC), to check or replace and provide mechanical protection, the sensors are usually mounted inside thermal thermowells (Figure below)
A thermal thermowell is actually a hollow metal tube with one end sealed. It is usually installed in pipe work. The sensor is inserted into it and makes contact with the sealed end. The problem with thermal thermowells is that they take a long time to respond because heat must be transferred to the sensor through the thermowell. An example of a temperature response for naked and thermal sensors is shown in the figure below. However, minimizing the air space between the sensor and the thermowell can reduce this thermal delay.