Overview of TC Transmitters

Thermocouple technology is based on Seebeck effect. When two different metals are fused together, the temperature of one node is different from that of the other, resulting in current.

1. Operation of TC transmitter

The TC transmitter has two contacts. The measuring end (sometimes called the hot end) is where two metals are connected. The reference end (also known as the cold end) is connected to the measuring circuit.

When there is a temperature difference between the hot end and the cold end, a mV signal proportional to the temperature difference is generated. mV value increases with the increase of temperature. The relationship between mV and temperature is nonlinear.

In practice, thermocouple measuring circuit can measure any temperature except 0 ℃. The measuring circuit must measure the temperature of the cold end and recover the temperature to 0 ℃. This kind of electrical compensation is called cold end compensation (or reference end compensation). Most thermocouple measuring circuits perform this operation.

2. TC transmitter structure

Thermocouple joints can be constructed by connecting to the outer sheath via hot contacts, either grounded or ungrounded (insulated from the sheath). Grounded thermocouples respond faster, but they are exposed to process voltage. Therefore, it is important to isolate the measurement circuit to prevent the formation of grounding circuit and avoid measurement error.

In temperature modules, thermocouples are usually embedded in magnesium oxide (MgO) and metal sheaths. Then insert it into the thermowell or protective tube. This helps to protect the transmitter from environmental pollution. When MgO is polluted by water and salt, even the ungrounded thermocouple will eventually be grounded.

Suggestion: use isolated measuring circuit to measure thermocouple.

3. TC transmitter accuracy

It is better to use TC transmitter conforming to ASTME230 standard, which specifies the accuracy of E, J, K and T type thermocouples. 

4. Special thermocouple wire of TC transmitter

Thermocouples can be constructed with high-quality or super grade wires to reduce the uncertainty by half. High quality/super grade basically indicates that the wire has a higher purity alloy mixture.