Tony.Morgan
Member
Has anyone any experience with grounding problems with pH loops.
I have problems with 2 similar loops comprising of:
Prominent Combination Glass Electrode with integral 4-20mA loop powered transmitter - screened 1pair unarmoured field cable (50m) - Prominent panel mounted pH Controller. Also in series with the transmitter is a 4-20/4-20mA signal isolator (230V ac). The output of the signal isolator goes directly into an anolgue input channel on a Mitsubishi PLC. The 4-20mA output from the pH controller goes to a remote variable speed drive (100m).
My problem lies with the PLC input and associated SCADA/HMI/Telemetry displays. I am able to calibrate the pH system no problem. The controller display is stable and correct. On my Fluke DMM the 4-20mA loops are stable and correct. However the PLC indications are very erratic - swinging by +/- 3pH. If I take the pH probe out of the process and put it in a buffer solution in a plastic beaker the indications become stable.
I spent many hours last night trying to narrow down the cause of the problem - adding/removing screens/earths etc. with little joy. Apart from these two loops all the other channels are ok. I have tried putting these signals onto another channel but have the same problem.
Any advice will be gratefully received
I have problems with 2 similar loops comprising of:
Prominent Combination Glass Electrode with integral 4-20mA loop powered transmitter - screened 1pair unarmoured field cable (50m) - Prominent panel mounted pH Controller. Also in series with the transmitter is a 4-20/4-20mA signal isolator (230V ac). The output of the signal isolator goes directly into an anolgue input channel on a Mitsubishi PLC. The 4-20mA output from the pH controller goes to a remote variable speed drive (100m).
My problem lies with the PLC input and associated SCADA/HMI/Telemetry displays. I am able to calibrate the pH system no problem. The controller display is stable and correct. On my Fluke DMM the 4-20mA loops are stable and correct. However the PLC indications are very erratic - swinging by +/- 3pH. If I take the pH probe out of the process and put it in a buffer solution in a plastic beaker the indications become stable.
I spent many hours last night trying to narrow down the cause of the problem - adding/removing screens/earths etc. with little joy. Apart from these two loops all the other channels are ok. I have tried putting these signals onto another channel but have the same problem.
Any advice will be gratefully received