Analog Output Different with Local Indicator

What is the LRV (zero) and the URV (span) for each of the transmitters?

Check the PLC ladder logic and make sure the treatment of each signal is the same.

Er.. sorry Wrenchpuller! Yours was much more in-depth. Octo, read and follow Wrenchpuller's post.

There are a number of possibilities that can cause these problems. To narrow down the possible problems you first need to find out if it is in the field or in the PLC.

Lets assume 200 psig in the tank then both trans should be outputting 12 ma and displaying 200 psig locally. Lets also assume B trans reads 30 psig lower on the PLC.

The simplest way to do this is move the wiring to see if the problem stays with the original channel or moves with the wiring.

If the lower reading moves then your problem is in the field, if the lower reading stays on the same channel then your problem is in the PLC.

You can also check the current signal for each trans. at the PLC terminals and make sure you check to see if you have the same current on each wire.

It is very rare but I have seen a current leakage internally in a transmitter or you may have a shield wire not isolated at the transmitter end causing a difference in ground potential.

Interesting problem let us know what you find.

hi,

we already encountered this problem. if they reads same values in local side, try measure their mA outputs and it must be the same considering they are measuring same process. if they are not the same mA output, check their respective LRV and URV. check also each x'mtr when reading is zero wether it gives 4mA, if not you may try ouput trim.

if their mA outputs are the same, then the problem is in the PLC. one cause of this mismatch reading between local and remote is the range of AI module at the PLC. you can check it out the ranges. Compare your LRV (0=4mA) and URV (400=20mA) at the x'mtr to PLC AI module configuration. And also, the mA configuration if is it using 4~20mA or 0~20mA.

if you find something difference, then re-range the PLC AI module in accordance with the range of x'mtr.

You could also try open loop check by injecting 4~20mA signal at the local side.

Hope it could help.

If after checking the analog signal outputs for the exact calibration range, there is still a problem, then you may have the following issues:

1.- In the wiring between the transmitters and receiver you may have a different cable resistance.

2.- The analog input card might have a measurement shift, software or hardware wise.

Do not use the local indicators since in a 0-4000 range 0.5% difference would be around 20 units so you cannot visually check the differences or equalities in measurement.