Control.com - Nerds in Control

We have a pressure transmitter which is at a distance around 300 mtrs from PLC end... recently we had a problem that it is showing less value at local display of PIT and in SCADA... I checked the PIT by directly giving 24VDC and it is showing correct value, but when it is connecting to the SCADA it is showing wrong value in the sense of low reading... some times automatically it is showing correct reading. What I need from you is... will there is be any isolators that can be incorporated in between PLC and PIT??? If yes please let me know the information... and this PIT is of 2 wire system. Please do reply.

What may be the problem actually?

I assume that the transmitter is working on a 4-20mA loop. It seems that you have run into a supply voltage problem. When the transmitter is sending 20mA (current through loop is 20mA) what is the voltage drop in the connecting cable between SCADA and transmitter? When the transmitter is increasing the current passing through, the voltage drop across the connecting cable is lowering the supply across the transmitter thus lowering the output.

In such a situation you have a couple of ways to get around the problem:

1. Try to use a different power supply to this transmitter, and set it to a higher output voltage such that the required minimum voltage on the transmitter is available.

2. Change the supply cable with one having a higher cross-sectional area (to lower voltage drop) This is the more costly option.

Having said the above, you did not indicate whether this is a new installation, and so the transmitter never worked well, or else it was OK but suddenly developed this fault, which would indicate, given your analysis of the problem, that you have a problem with the transmitter power supply of the SCADA.

Since the issue is recentl, check the connections, terminations and cables. Looks like higher loop resistance than it was before.

It sounds like the voltage at the transmitter is too low. Does it read correct at low pressures and low at high pressures? When the pressure is at maximum a typical transmitter needs a minimum of about 12 V across its terminals. Tell us the following and we can be more helpful.

Transmitter model No

Is there more than 1 receiver, e.g. SCADA and chart recorder, controller or anything else in series with the 4-20mA signal?

Please provide a few measurements:
While the pressure is near the upper end of transmitter range
Voltage at the SCADA terminals
Voltage at the transmitter terminals
% of transmitter range.

A loop sketch would be wonderful.

I don't think the 300 meters is the issue, you may just need a slightly higher Voltage or to re-arrange the loop.

Regards,
Roy

It is possible with the length of cable in the loop that you have exceeded the maximum loop resistance to be able to read full scale.