Why can’t an MV signal be applied to the thermocouple circuit?
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Today, we need to test the thermocouple as well as the secondary instruments connected to it on site. First, we measured the voltage of the thermocouple and found it to be 4.59 mv. We removed the two wires connected to the thermocouple and applied an mv signal using a signal generator; however, the secondary instrument showed no reaction and continued to display the same temperature value. After reconnecting the thermocouple, the temperature reading on the secondary instrument became normal. At this point, we removed the wiring again and applied a signal directly using the signal generator When measured with a multimeter, the MV output is normal. When a signal is applied again, the secondary instrument still shows the maximum value as before. At this point, when the multimeter is used to measure the two wires connected to the secondary instrument, a signal of nearly 320 MV is detected. Why is this? I need to test this temperature loop; what signal generator should I use? And where does the MV signal of the secondary meter come from?
It can be verified using a multi-functional signal tester.
I use a signal generator – active or passive types are fine, as well as those that can generate current or temperature signals. But why can’t I apply a signal? Some people say that Fluke devices work, but the impedance of these domestic devices is incorrect; I don’t understand it
Connect it directly to the display panel to see if it works; if it does, it’s likely a problem with the signal generator’s load capacity. The resistance of the thermocouple compensation wire is high.
In theory, it’s possible. Thermocouples have positive and negative poles; similar to process calibrators that have thermocouple outputs, you don’t need to set a voltage value – you just need to specify the temperature, and the corresponding voltage value will be generated automatically
The input signal is too large, which is why the maximum value is displayed. Reduce the input signal to the appropriate mV value for the thermocouple. It’s not possible to reduce the signal level; this simply means that the range of the mV generator you’re using is too large – use a mV generator with a smaller range.