What is a thermocouple sensor? They work according to the thermoelectric phenomenon, in which an electric current is created in the circuit whenever we heat the junction of two heterogeneous metals that form a closed circuit. The following figure shows the phenomenon. (Making a thermocouple)
In fact, a thermocouple is a self -converting transducer that is made by connecting two or more non-homogeneous metals. The copper connectors used to connect to the measuring instrument are called the Reference Junction and are denoted by TC. Thermocouples are measured relative to the cold connection. The copper interface forms a new thermocouple that greatly affects the output voltage of the thermocouple. Perhaps the biggest drawback of this sensor is that the cold connection temperature must be compensated for in the measured value of the sensor.
The output voltage of the thermocouple is calculated according to the following formula.
E = C (TH – TC) + K (TH 2 – TC 2 )
E: Output voltage in mV
C and K: constant coefficients depending on the type of thermocouple
TH: Hot Junction
TC: Cold Junction Temperature
It is very common in industry to measure temperature, due to the many advantages of thermocouples.
Advantages and Disadvantages
The most important advantages of this sensor are the following:
- High temperature range
- Long life
- Simplicity of components
- Self-feeding (no need for an external power supply)
- High diversity
- Low volume temperature measurement point
And in addition to having many advantages, they also have the following disadvantages:
- Low output voltage
- Need to compensate for the cold spot temperature
- Low sensitivity
- Low stability
Cold connection voltage compensation method:
There are many ways to compensate for the cold connection voltage, the simplest way is to put the cold connection in the melting ice container to zero the cold connection temperature; But this method is not possible in practice.
Another method of compensation is by measuring the cold spot temperature by another sensor. Due to the fact that they are always used to show or control the temperature with a controller or temperature indicator. Inside these controllers, the compensation operation can be done automatically by measuring the cold point temperature; In this case, the interface wires between the thermocouple and the controller must be made of thermocouple wires.
Thermocouples are divided into the following hands based on the type of metals used in their construction.
Type j- Type B- Type K -Type R-Type S-Type N-Type E-Type T
One of the most important issues to consider when selecting these sensors is the temperature range required for measurement. This limit is different and as a result different applications in the industry are considered for each. The table below shows the specifications of thermocouples and temperature range of each type.
Rose Calibration Company in Melbourne, Australia with over ten years of experience provides all calibration, maintenance, and repair services throughout Australia. If you live in Sydney, Melbourne, Adelaide, Perth, Geelong, and Brisbane, you can receive your quote in less than two hours by fill-up the form via the “Booking” link.