I'm troubleshooting a temperature transmitter loop set for a 4 to 20 mA signal over 0 to 100°C. The transmitter is configured right and connections are secure, but the loop current is stuck at 20 mA regardless of the temperature. What could be the cause?

Field Instrumentation Temperature Measurement
1

I’m working on troubleshooting a temperature transmitter loop that’s supposed to output a 4 to 20 mA signal for a temperature range of 0 to 100°C. The transmitter is configured correctly, and all the connections are secure. But the loop current is stuck at 20 mA no matter what the actual temperature is. What do you think could be causing this issue?

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It sounds like you’re facing a frustrating issue with your temperature transmitter loop. Here are a few things you can check to troubleshoot the problem:

  1. Sensor Connection and Condition: Make sure the temperature sensor (RTD or thermocouple) is properly connected and functioning. Sometimes a disconnection or a faulty sensor can cause the transmitter to output a maximum current (20 mA) as a fault indication.
  2. Transmitter Configuration: Double-check that your transmitter is configured correctly for the 0 to 100°C range. Refer to the manual to ensure all settings are correct. Sometimes a misconfiguration can cause the transmitter to behave unexpectedly.
  3. Loop Power Supply: Verify that the loop power supply voltage is within the required range for your transmitter. An incorrect supply voltage can cause various issues. Usually, it should be around 24V DC, but check your transmitter’s specifications to be sure.
  4. Wiring Issues: Inspect all wiring for shorts, opens, and proper connections. Ensure that everything is wired according to the transmitter’s wiring diagram. Incorrect wiring can definitely cause the transmitter to output a fixed current.
  5. Transmitter Hardware: It’s possible that the transmitter itself is faulty. If you have a spare transmitter, try swapping it out to see if the problem persists. This can help determine if the issue is with the transmitter or something else in the loop.
  6. Grounding and Noise: Check for proper grounding and try to minimize any electrical noise that might be affecting the loop. Shielded cables and proper grounding can help reduce interference.

Hope this helps! Let us know how it goes or if you need further assistance.

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Hi, I’ve already had that kind of problems. If PT100

  • Check the setup of your transmitter. 2 wires, 3 wires or 4 wires. Depending on your PT100, the conections are different. If you use a TMTxx from EH, as i see on your pic, and 3 wires. 3 is for white, 5 6 for red wires. Look carefully the manuel BA or PA of your transmitter for the correct wiring.

  • Check in the setup using Readwin, e.g., if there is no Offset added. Put error signal to > 20 mA or <3.8 mA. To help differentiate High or low values and Real Errors. Because 20mA doesn’t mean a problem. It can tell you’re in maximum temp already.