Instrument knowledge

1、Some people think that because the compensation wire is an extension of the thermocouple, the thermocouple potential is only related to the hot end and the cold end, and compensation wire is independent of the temperature at the junction of the thermocouple. Is this statement right?

Answer: This statement is wrong. Generally, the performance of the compensation wire is only the same in a certain temperature range(0-100℃)and the thermocouple. If the temperature exceeds this temperature, an additional potential is generated at the junction of the compensation wire and the thermocouple. Therefore, the thermocouple potential and the compensation wire cannot be said in general terms. It is independent of the temperature at the junction of the thermocouple.

2、In the DDZ-Ⅲ type meter, why is the 4~20mA current signal used between the field and the control room, and the 1~5V voltage signal is used inside the control room?

Answer: The 4~20mA current signal is suitable for long-distance transmission between the field and the contral room. It can avoid the error caused by the voltage drop loss on the transmission line. The 1~5V voltage signal is suitable for controlling the short-distance contact inside the room. The instruments can be connected in parallel. Connection for easy design, installation and maintenance.

3、What effect dose the polarity of the compensation wire have on the temperature measurement? How to judge the index number and the polarity of the compensation wire?

Answer: The thermoelectric performance of the compensation wire within a certain range is the same as that of the thermocouple, so If the polarity is wrong, the cold junction error of the thermocouple will increase. If the indexing and polarity of the compensation wire are unknown, the ends of the two cores can be screwed together into the boiling water, and the other end of the display instrument connected to the indexing should display about 1000℃, indicating that the indexing number is correct. If the difference is far away, the index number is wrong. When the multimeter mv file is used, the “+” indicates that the red pen is connected to the “positive” of the compensation line. If the “-” is displayed, the red pen is connected to the negative of the compensation wire.

4、A  thermocouple temperature measurement system, the operator reflects the measurement table indicating instability, sometimes no, high and low, know that the measurement instrument itself is no problem, try to analyze the cause.

Answer:There may be several reasons:

①The thermocouple electrode is in poor contact with the terminal block.

②The thermocouple has intermitter short circuit or intermittent grounding phenomenon.

③ The thermocouple has been broken or will not be broken, and there is a phenomenon of intermittent connection.

④The thermocouple installation is not reliable, swinging occurs.

⑤The compensation wire has grounding, intermittent short circuit or open circuit.

5、The compensation wire of a temperature measuring instrument is reveraed to the polarity of the thermocouple, and at the same time it is connected to the input end of the meter. Can it generate additional measurement error? What is the additional measurement error?

Answer: Additional measurement errors can be generated. The error value is related to the temperature difference across the compensation wire. If the temperature difference across the compensation wire. If the temperature difference is zero, the instrucment indicates that there is no additional error in the value. If the cold junction temperature of the thermocouple is higher than the temperature at the input of the meter, the instrument indicates that the value will be twice the temperature difference from the actual value. If the cold junction temperature of the thermocouple is lower than the temperature at the input of the meter, the instrument indicates that the value will be twice the temperature difference from the actual temperature.

 

 

 

 

 


Post time: Sep-03-2019