【Daily Question 20090313】When using a thermocouple for temperature measurement, the reading on the instrument is unstable; the instrument itself is fine. Analyze the reasons for this

1. The compensation wire is subject to interference； Signal transmission at the millivolt level is very sensitive to interference. 2. Loose connections: compensation wires–the terminals in the junction box, thermocouples–the terminals in the junction box, indicator instrument terminals–compensation wires. Sometimes, improper tightening of screws in the junction box can cause such problems ； Furthermore, factors such as vibration can cause screws to loosen. 3. Moisture/water intrusion, inside the junction box or within the thermocouple protection sleeve. 4. The thermocouple wire may be broken. 5. A chemical reaction has occurred in the material of the thermocouple/complementary wire. 6. Less common causes: issues with the indicator instrument/DCS input card, or incorrect selection of the thermocouple/complementary wire type. 7. Reverse polarity – this shouldn’t be something that professionals would do. 8. Purely software-related problems; in other words, some kind of processing is applied to the signal after it arrives, such as setting excessively long delays/filters, or applying certain calculations before outputting the result. 9. It’s not clear what is meant by “the instrument itself is fine” as stated by the questioner – does it refer only to the indicator instrument? Is it a fault of the AI component itself, which converts millivolts into current signals – or simply a millivolt input?

1: Is the selection of the compensation wire correct?； 2 ； Are the positive and negative connections correct? ； 3: Long duration of the thermocouple wire ； 4: Internal DCS issues ； That’s all I can think of for now; I’m a beginner, haha ；

Unstable measuring medium, poor installation location, interference generated by the operation of high-power equipment, etc

There may be the following reasons: 1) Poor contact between the thermocouple electrode and the terminal； 2) The thermocouple exhibits intermittent short circuits or intermittent grounding ； 3) The thermocouple is disconnected or on the verge of being disconnected, showing intermittent connection ； 4) The thermocouple is not securely installed, causing it to swing ； 5) The compensation wire has grounding, intermittent short circuits, or open circuits.

Since there is nothing wrong with the instrument itself, it must be the measuring medium that is unstable

1. The protective sleeve of the thermocouple is damaged or water has entered the sleeve. 2. The wiring terminals of the thermocouple are not in good contact. 3. Thermocouple bends and touches the wall (grounded).

1 Poor contact 2 Interference present 3 Process issue

1. The shielding is not properly connected. 2. There is interference in the workshop. 3. The connector is unstable. 4. Grounding is not done properly; two-way grounding is present. 5. Short circuit within the same sleeve. 6. The thermocouple wire is broken, but there is still contact. This can also happen with cables

Is the wiring loose? Is the compensation wire selected correctly? Are the positive and negative terminals of the compensation wire connected in reverse? Is the shielding layer grounded in one direction only? Has water entered the sleeve? Is the thermocouple wire about to break?

1. The wires at the terminals are not properly pressed, resulting in a loose connection. 2. There is a break in the compensation wire. 3. The shield wire is not connected properly, causing interference in the vicinity. 4. The secondary meter is damaged