Analog Circuit Troubleshootin

[eight_bools]

Been struggling to wrap my head around this all day, maybe someone else has an idea.

I've got a pressure sensor connected to a 1771-IFE analog input card. The signal is bouncing around, both in the PLC displayed value and measured in series with a fluke 773.

When I check the sensor on its own, its giving a stable output, around 5.51mA. When I simulate a current directly to the input card, it reads a stable value matching the Fluke output. When I connect the sensor to the PLC it starts to bounce around.

I'm currently trying to find another analog sensor I can test this with, I checked both ends of the cable and have the same result. I drew up some photos, hopefully they help.

Forgot to add that there are two inputs, the first two channels on the card that are behaving like this. All the other 6 inputs are working normally. I checked the input by moving the wire to a known good input and it is doing the same thing. Both of these are connected to devices that are wired out of the same remote panel.

 

[chelton]

A few things to check..
Are the internal jumpers configured correctly?
If using differential inputs do all unused channels have their +/- tied together?
If single ended are unused channels tied to common?
Differential inputs are not isolated. Are you supplying inputs from the same power supply?

 

[eight_bools]

A few things to check..
Are the internal jumpers configured correctly?
If using differential inputs do all unused channels have their +/- tied together?
If single ended are unused channels tied to common?
Differential inputs are not isolated. Are you supplying inputs from the same power supply?

I’m guessing the internal jumpers are configured correctly, this worked at one point. I was troubleshooting over the phone with the plant and was in person today. They did say they replaced the input card, but it was running so I couldn’t pull it out to check.

All the channels are used, and it’s set for differential inputs. Sharing a common power supply for what looks like 3 cards total. Of coarse there isn’t any wiring diagrams. I spent the first three hours of the day locating the correct wire numbers, someone added labels in the main panel and the remote panel but got the labels on the wrong cables in the remote panel. That was fun.

It’s weird that the other 6 inputs are working fine, but these two which run to the same remote panel for tank level sensors are not.

We didn’t have a new probe to try, so I wired in a 4-20ma 0-100psi pressure transducer and an airline and it’s also giving a stable output. At this point I am leaning towards bad level sensors. Getting prepared for the barrage of people asking why they both failed at the same time.

 

[danw]

Your test with a pressure transmitter on the same channel says the input card is OK.

Is the level transmitter a smart unit, meaning, can you configure it with HART? If so, most HART transmitters have filter or damping setting that will smooth the output. It could be that the level sensor's sensitivity is so high that every vibration is creating a ripple that is being recorded as a change in level.

If it is hydrostatic (pressure), then most 2-wire, loop-powered field transmitters are isolated from ground. But a ground connection that creates a ground loop usually appears as an offset, although it can also be noisy.

Even if the level transmitter is isolated, a transmitter can be 'flakey'.