Temperature control of thermocouples

Thermocouple is a type of temperature sensing element. It directly measures temperature and converts the temperature signal into a thermoelectric potential signal, which is then converted into the temperature of the measured medium through electrical instruments (secondary instruments). The basic principle of temperature measurement is that two conductors with different compositions form a closed circuit. When there is a temperature gradient at both ends, current will flow through the circuit, and there will be an electromotive force - thermoelectric force - between the two ends,
This is the so-called Seebeck effect. When a third metal material is connected to a thermocouple circuit, as long as the temperature of the two contacts of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged and will not be affected by the connection of the third metal to the circuit. Therefore, when using thermocouples for temperature measurement, a measuring instrument can be connected to measure the thermoelectric potential, and the temperature of the measured medium can be determined.
Two homogeneous conductors with different compositions are thermoelectric electrodes, with the higher temperature end as the working end and the lower temperature end as the free end, which is usually at a constant temperature. Create a thermocouple calibration table based on the functional relationship between thermoelectric potential and temperature; The calibration table is obtained under the condition of a free end temperature of 0 ℃, and different thermocouples have different calibration tables.

Working principle of thermocouple temperature sensor

Two conductors with different compositions (called thermocouple wires or thermoelectric electrodes) are connected at both ends to form a circuit. When the temperature of the junction is different, an electromotive force is generated in the circuit, which is called thermoelectric effect, and this electromotive force is called thermoelectric potential. Thermocouples use this principle for temperature measurement, where the end directly used to measure the temperature of the medium is called the working end (also known as the measuring end), and the other end is called the cold end (also known as the compensating end); Connect the cold end to the display instrument or matching instrument, and the display instrument will indicate the thermoelectric potential generated by the thermocouple.

Thermocouples are actually energy converters that convert thermal energy into electrical energy and measure temperature using the generated thermoelectric potential. Regarding the thermoelectric potential of thermocouples, the following issues should be noted:
1. The magnitude of the thermoelectric potential generated by a thermocouple is not related to the length and diameter of the thermocouple when the material is uniform, but only to the composition of the thermocouple material and the temperature difference at both ends;

2. The thermoelectric potential of a thermocouple is the difference in temperature function between the two ends of the working end of the thermocouple, rather than the temperature difference between the cold end and the working end of the thermocouple;

3. After the composition of the two thermocouple wires is determined, the magnitude of the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; If the temperature of the cold end of the thermocouple remains constant, the thermoelectric potential entering the thermocouple is only a single value function of the working end temperature.

Selection of thermocouple measurement accuracy and temperature measurement range

Two different materials of conductors or semiconductors A and B are welded together to form a closed circuit, as shown in the figure. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between them, resulting in a current of a certain magnitude in the circuit. This phenomenon is called thermoelectric effect. Thermocouples work by utilizing this effect.

The selection of thermocouples should be based on a comprehensive consideration of the operating temperature range, required accuracy, operating atmosphere, performance of the measured object, response time, and economic benefits.

1. The selection of atmosphere for S-type and B-type thermocouples is suitable for strong oxidizing and weak reducing atmospheres, while J-type and T-type thermocouples are suitable for weak oxidizing and reducing atmospheres. If a protective tube with good airtightness is used, the requirements for atmosphere are not very strict.

2. The selection of measurement accuracy and temperature measurement range is generally based on the use of B-type thermocouples when the temperature is between 1300~1800 ℃ and the required accuracy is relatively high; The requirement for accuracy is not high, and the atmosphere allows for the use of tungsten rhenium thermocouples. Generally, tungsten rhenium thermocouples are selected for temperatures above 1800 ℃; S-type thermocouples and N-type thermocouples are available for use at temperatures ranging from 1000 to 1300 ℃, which require high accuracy; K-type and N-type thermocouples are generally used below 1000 ℃, while E-type thermocouples are generally used below 400 ℃; At 250 ℃ and negative temperature measurements, T-type thermocouples are generally used. At low temperatures, T-type thermocouples are stable and have high accuracy.

3. The selection of durability and thermal responsiveness: Thermocouples with larger wire diameters have better durability, but slower response. For thermocouples with larger heat capacity, the response is slower. When measuring temperatures with large gradients, temperature control is poor under temperature control. It is required to have a fast response time and a certain degree of durability, so choosing an armored puppet is more suitable.

4. The selection of thermocouples based on the properties and states of the measured object

The temperature measurement of vibrating objects, vibrating objects, and high-pressure vessels requires high mechanical strength, protective tubes are required for atmospheres with chemical pollution, and high insulation is required in the presence of electrical interference.

5. Precautions

The insertion depth of the thermocouple is very small: it should not be less than 8-10 times the outer diameter of its protective sleeve (except for special products)

◆ Thermocouple [1] nominal pressure: generally refers to the static external pressure that the protective tube can withstand and rupture at the working temperature.

◆ Insulation resistance: When the ambient air temperature is 15-35 ℃ and the relative humidity is less than 80%, the insulation resistance is ≥ 5 megohms (voltage 100V). Thermocouples with splash proof junction boxes have an insulation resistance of ≥ 0.5 megohms (voltage 100V) when the relative temperature is 93 ± 3 ℃

◆ Insulation resistance at high temperatures: For thermocouples, the insulation resistance (per meter) between the hot electrode (including the double branch type) and the protective tube, as well as between the double branch hot electrodes, should be greater than the values specified in the table below.