current loop reading bad on one device, good on another

This worked for a year, then failed. tag = Cmpdng Room Temp

3 wire 100 ohm DIN RTD feeds into 2 wire loop powered transmitter.

4-20mA Current signal from transmitter runs to two devices in series.
- one device AI has 250 ohms input resistance
- the other device has 10 ohms input resistance.

The AI device with 250 ohms input resistance reads the correct temperature

The AI device with 10 ohms input resistance, henceforth known as the 'bad' AI, shows a diagnostic reading of - 260mV, the below the extreme low end of the -20mA to +20mA input

The 2A DC power supply is feeding a dozen other loop powered transmitters that work OK.

A known good current signal was fed into the 'bad' AI and it read the known good current signal OK.

The bad current signal was fed into a known good AI and the readings were 'bad'.

That says that the AI appears to be good, by substitution check with known good signal.

Given that this is a series circuit, how could one device read a 'bad' signal as a correct value and another device read the 'bad' signal as a bad value, when the current is the same everywhere in the circuit?

Dan

SWAG (Scientific Wild A$$ Guess)
The transmitter is going bad, it can no longer handle impedance above a certain value.
If you remove the "good" AI, from the circuit, does the "bad" AI read okay?

Another WAQ:
The "bad" AI is grounding the signal.

The "bad" AI is grounding the signal.

Click to expand...

Maybe, but that would not explain this (unless the bad AI was left in the circuit while testing the signal):

The bad current signal was fed into a known good AI and the readings were 'bad'.

Click to expand...

but Ken's guess would:

The transmitter is going bad...

Click to expand...

There is a high(er) resistance in the loop power going to/from the transmitter. A loose screw on a terminal.

The 10 ohm unit needs higher voltage to get enough POWER to operate. It is loop powered, right ?

You have a ground loop, both your input devices are grounded, one must be floating. It might have been working fine but at some point the power supply has grounded out, possibly not even in the loop in question.
Your 10 Ohm device works OK when you test it because your test source is floating relative to ground.
That's my best guess
Roy

Mystery solved, only it isn't a mystery.

I wrote out what they told me they had seen and done (this is remote troubleshooting), emailed them and asked them to confirm. Then I posted here. They just emailed back and said, "No, we got it backwards. When we moved a known good current source to the bad AI channel, the bad AI channel gave bad readings. The current loop in question works OK on a known good AI."

Oh. That's simple. Then it's bad AI. No mystery there.

Sorry for taking you all from your cherished duties for a false alarm. It does pay to confirm, in writing, what people say over the phone.

Everyone, thanks for your input on what was bad data.

Dan

Dan