Getting intermittent noise 4-20ma signal

[jeffkinder]

I have a two wire 4-20ma sensor.

At the sensor end, the shield is ungrounded.

I am experiencing periods of good signal quality followed by periods of bad signal quality...During the bad periods, I lose miliamps as the noise increases.

I have tried bringing the panel end of the shield to its own dedicated ground, bonded to the pilings of the machine which go about 30 feet down. This seems to reduce the noise.

I have brought a dedicated earth ground to the AIO card as well.

I think I must be doing something wrong.

I am a bit perplexed as to what is happening when the signal cleans up for a few hours and then gets noisy again. There is a 30A 120V circuit nearby that runs intermittently. I am wondering if this is having an impact on the signal wires or the ADC. Curiously, I have a second instrument brought into the AIO card that is also a 4-20ma 2-wire signal. This one comes in cleanly and doesn't get noisy. Both are shielded cables, wired the same on the panel end. The main difference that may be important is that the sensor that is working well has a plastic housing, whereas the noisy sensor has a metal housing. In the lab, we did not have any of these issues.

I would appreciate any guidance or thoughts about the best practices for wiring the AIO card to avoid noise and if anyone can spot something else I may be missing.

[dirk]

Hello jeffkinder is the sensor supplied via the current interface or does it still have a power supply connection?
Is the sensor electrically connected to the system?