Thermocouple selection

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

1. Selection of measurement accuracy and temperature measurement range

When the operating temperature is between 1300 and 1800℃ and the accuracy is relatively high, B-type thermocouples are generally selected; when the accuracy is not high and the atmosphere allows, tungsten-rhenium thermocouples can be used. When the temperature is above 1800℃, tungsten-rhenium thermocouples are generally selected; when the operating temperature is between 1000 and 1300℃ and the accuracy is relatively high, S-type thermocouples and N-type thermocouples can be used; below 1000℃, K-type thermocouples and N-type thermocouples are generally used, and below 400℃, E-type thermocouples are generally used; below 250℃ and negative temperature measurements, T-type thermocouples are generally used. At low temperatures, T-type thermocouples are stable and have high accuracy.

2. Selection of atmosphere

S-type, B-type, and K-type thermocouples are suitable for use in 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 the atmosphere are not too strict.

3. Selection of durability and thermal responsiveness

Thermocouples with large wire diameters have good durability, but slower response. For thermocouples with large heat capacity, the response is slow. When measuring temperatures with large gradients, the temperature control is poor under temperature control. If both fast response time and certain durability are required, it is more appropriate to choose an armored thermocouple.

4. Selection of thermocouples based on the nature and state of the measured object

The temperature measurement of moving objects, vibrating objects, and high-pressure containers requires high mechanical strength. Atmospheres with chemical pollution require protective tubes. In the case of electrical interference, insulation is required to be relatively high.

5. Precautions

◆Nominal pressure of thermocouple: generally refers to the static external pressure that the protective tube can withstand at the working temperature and break.

◆Minimum insertion depth of thermocouple: should not be less than 8-10 times the outer diameter of its protective sleeve (except for special products)

◆Insulation resistance: when the ambient air temperature is 15-35℃ and the relative humidity is <80%, the insulation resistance is ≥5 megohms (voltage 100V). For thermocouples with splash-proof junction boxes, when the relative temperature is 93±3℃, the insulation resistance is ≥0.5 megohms (voltage 100V)

◆Insulation resistance at high temperature: At high temperature, the insulation resistance (per meter) between the thermocouple's hot electrode (including double-branch type) and the protective tube and the double-branch hot electrode should be greater than the value specified in the following table.