pH transmitter electrical interference problem

I use two pH transmitters as inputs to my PLC system for controlling pH in two tanks. The two transmitters are two-wire transmitter with automatic temperature compensation. They share one 24VDC power supply. Although they are great when wired to the system alone. But when the other one is wire up to the PLC system, there is serious interference between the two transmitters: neither gives reasonable reading for standard solutions.

Please help me on removing the interference.

Thanks a lot.

So I presume by two wire you mean 4-20 mA current loop type. PLease confirm. These should be OK together if they are wired correctly. Can you post a sketch showing how you've connected them up? It would also be helpful to know the model of PLC that you are hooking up to and if you could tell us whether you have external load resistors or they are built into the PLC. Oh, and while we are at it, what is the model of transmitter you are using?

probably 4-20mA. Show a diagram, pay particular attention on how the two transmitters are grounded with respect to the loop and each other.

You're not trying to connect two sensors to a single input are you?