I am assuming that the 4-20 ma signal is linearly proportional to PH values. This is usually the case with 2-wire PH transmitters, which it sounds to me like you are using. If this is indeed the case, you simply need to subtract the 4 ma offset, and scale the remaining 16 ma proportionately to 0 to 14 ph. This also assumes that you have an analog input capable of dealing with a 4-20 ma. current input. If you have only voltage inputs, as most A/D cards have, then you have to use a shunt resistor across the input of the A/D convert it to a voltage signal. Hope this helps.
Dave
