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2-W P transmitter/ PLC
Need help diagnosing issue with differential pressure transmitter. Could it be the analog input card?

Hi everyone here,

We have a 2-wire differential pressure ABB transmitter (0-2 bar), and supplied by (29 VDC).

The problem is that: the operators said there was a different readings between the DCS and local screen of the transmitter. I've checked it on the site, and found there was some water leaking inside the transmitter. Then I dried it up after removing the supply wires, and then I get the wires back, but unfortunately it doesn't power up again. While I've tested it with an external 24 v source, it does work ok. I guess the problem lies somewhere either in the PLC's analog input card (S7-300) or the supply cable.

Would anyone lend me a hand, and help me on how to test the Analog Input terminals assigned to this transmitter?

Note: After taking the supply wires back in the transmitter , I measured the voltage. it has been about 3.4 v (rather than 29 v). when I take them off the transmitter, it reads (29 v)again. What is the reason do you think behind such voltage drop?

Thank you so much in advance..
My best regards.

1. Use a 4-20mA source/calibrator to test the analog input.

If the DC power wire got shorted directly to the conductor leading to the analog input, the resistor across the input could have been damaged by the overload. They're typically 1ow wattage resistors and can not handle 24V for any period of time.

2. Water in a transmitter can do all sorts of damage. Residue or moisture can be conductive and cause a ground loop, which can be the cause of a difference between the transmitter's output value and the loop current.

3. Some facilities fuse the DC power line. Does yours? Is the fuse good?

4. Out of curiosity, is the DC power supply voltage actually 29V? At the power supply?

5. The voltage measured across the transmitter terminals is the sum of the analog input resistor drop, wire resistance and the IR drop the transmitter creates to modulate the loop current. For most purposes, the wire resistance is negligible.

In a 24Vdc system, one gets these kinds of voltage drops across the transmitter's (+) and (-) in a loop with a 250 ohm input resistor:

4.0mA drops 1.0V, drop across the transmitter is 23V
8.0mA drops 2.0V, drop across the transmitter is 22V
12.0mA drops 3.0V, drop across the transmitter is 21V
16.0mA drops 4.0V, drop across the transmitter is 20V
20.0mA drops 5.0V, drop across the transmitter is 19V