Answer / carl_ellis

When you need troubleshooting assistance you need to supply
certain critical information:

1) Has the device ever worked correctly before for some
time period and then failed?

Or is this a first time commissioning issue?

2) What is the brand name and model number of the device?

3) Because all devices have an input and an output, exactly
what each is (is your sensor one thermocouple, two
thermocouples, or one RTD?)
- You do tell us the output is 21.5mA.

4) What are the error codes or what the error symptoms?
- You mention the display shows ambient temperature

5) What you've done to troubleshoot the problem, like
substitute a known source (simulator) and the result
was . . . . ?

Evaluation:
ambient temperature: A short circuit (a wire) across the
terminal inputs of a thermocouple input will display near-
to-ambient temperature, specifically the temperature of the
terminal block connectors. The value is the cold junction
compensation (CJC) temperature.

Some high end transmitters will show the CJC temperature.
Is the display stuck on the CJC temperature, rather than
its input? Is there a short across the (+) and (-) input
terminals?

21.5mA is a very common fail-safe alarm level, outside the
normal 4-20mA range, but indicating that there is a serious
fault in the transmitter setup or operation. If the fail-
safe is configured for fail high, an open input (among many
other things) could cause the fault.

Smart transmitters generate an error code and save it. If
the transmitter has HART, the error code is in one of the
diagnostics registers, but a HART communicator is needed to
access the error code.

Carl Ellis
Measure First

Answer / carl_ellis

A short circuit (both thermocouple wires touching each
other) in the thermocouple extension wire leads will create
an ambient or near-ambient temperature reading, but I am
unaware of any temp transmitter that can detect that problem
and go to fault mode at 21.5mA.

Carl Ellis
Measure First

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