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Attention to the field use of thermocouple and installation requirements
Dec 17, 2018

Attention to the field use of thermocouple and installation requirements


Thermocouple is a common temperature measuring element in temperature measuring instruments. It directly measures the temperature and converts the temperature signal into thermoelectric electromotive force signal, which is converted into the temperature of the medium under test by electrical instruments (secondary instruments).Thermocouple installation: the main fixed way has thread connection, flange connection and welding, its applicable occasions and requirements are different, mainly according to the pressure, temperature and other parameters to determine.So what should we pay attention to when installing and using?Today I will tell you one by one.

 Attention to the field use of thermocouple and installation requirements

1, Must pay attention to in using thermocouple compensating conductor model match, cannot be wrong polarity, compensation conductor should be connected to the thermocouple temperature should not exceed 100 ℃.

2. When temperature is measured or controlled by thermocouples with large time constants, the temperature displayed by the instrument may fluctuate very little, but the actual furnace temperature may fluctuate very much.In order to accurately measure temperature, thermocouples with small time constants should be selected.

3, According to the meter wiring diagram for the correct wiring and power, the meter first shows the thermocouple index of the meter, then shows the range of the meter, and then the digital tube under the meter shows the set temperature, the digital tube on the meter shows the measurement temperature.

4, The thermocouple can not be installed in the area of little flow of the medium under test, when the thermocouple is used to measure the gas temperature in the pipe, the thermocouple must be installed against the direction of flow velocity, and fully in contact with the gas.

5. In order to ensure sufficient heat exchange between the measuring end of thermocouple and thermal resistance and the medium under test, the position of measuring point should be selected reasonably and the valve should be avoided as far as possible.

6. According to the law of intermediate conductor, in the actual temperature measurement application of thermocouple, the hot end is usually welded and the cold end is open circuit, and the cold end is connected with the display instrument through the connecting wire to form the temperature measurement system.

7. When the insertion depth of the measurement original exceeds 1m, it shall be installed vertically as far as possible, or support frame and protective sleeve shall be installed.

8. Shallow insertion of protection pipe or hot-sleeve thermocouple shall be adopted. The depth of shallow insertion thermocouple protection sleeve inserted into the main steam pipe shall not be less than 75mm.

9. Use the naked wire thermocouple without protective sleeve, but the thermocouple is easy to damage, and should be timely corrected and replaced.

10. Keep the outside of the thermocouple protection tube clean to reduce errors.

11. When the insertion depth of the measuring element exceeds 1m, it shall be installed vertically as far as possible, or support frame and protective sleeve shall be installed.Thermocouples are formed when two different conductors are connected together, and when the measurement and reference points are at different temperatures, the so-called thermoelectric magnetic force (EMF) is generated.Purpose of connection point measurement connection point is the part of thermocouple connection point at the measured temperature.The reference connection point is the part of the thermocouple connection point that remains at a known temperature or that can be automatically compensated for temperature changes.

In order to meet the above requirements, the following points should be paid attention to when selecting the installation location and insertion depth of thermocouple and thermal resistance:

1. In order to ensure sufficient heat exchange between the measuring end of thermocouple and thermal resistance and the medium under test, the measuring point location should be reasonably selected to avoid installing thermocouple or thermal resistance near the dead Angle of valve, elbow, pipe and equipment.

2. Thermocouple and thermal resistance with protective sleeve have heat transfer and heat loss. 

 Attention to the field use of thermocouple and installation requirements

In order to reduce measurement error, thermocouple and thermal resistance should have enough insertion depth:

1. For the thermocouple measuring the fluid temperature in the center of the pipe, the measuring end shall be inserted into the center of the pipe (vertically or obtuse). If the pipe diameter of the flow body being measured is 200mm, the insertion depth of thermocouple or thermal resistance shall be 100mm;

2, for high temperature and high pressure and high velocity of temperature measurement (such as main steam temperature), in order to reduce cases of fluid resistance and prevent cases under the effect of fluid fracturing, the way protecting tube shallow inserted or use hot thermocouple, shallow inserted type thermocouple protection tube, its depth into the steam pipe should be no less than 75 mm;The standard insertion depth of hot-sleeve thermocouple is 100mm;

3, if it is necessary to measure the flue gas temperature in the flue duct, although the flue duct diameter is 4m, thermocouple or thermal resistance insertion depth of 1 m can be;

4. When the insertion depth of the measurement original exceeds 1m, it shall be installed vertically as far as possible, or support frame and protective sleeve shall be installed.