I currently have a situation where there are two PH probes in a waste water tank. The first probe measures the PH of the liquid within the tank, while the second is used to record the PH of the liquid leaving the tank. When both of the probes are removed from the tank it is easy to calibrate them using buffer solutions of PH 4, 7, and 10, but when both probes are placed back into the tank, thay will both read the same PH, even though it is way off. They will read a PH of 11, but the PH will actually be 5 on a hand held calibrated PH meter. At first I thought this was a simple problem, the PH probes are connected to a device that converts the signal to 4-20ma so the PLC can read it. I thought for sure that this device was wired incorrectly, or not properly grounded. Well here I am three days later, still looking for answers. Any suggestions you can give me will be greatly appreciated. The tank is all plastic, with all plastic piping. I have heard theories of transient voltage within the liquid, how would I eliminate this? Is there a specific probe I should be using?