Conductivity measurement


Thread Starter

Tommy Johansson

I'm trying to measure conductivity by using National instruments data acquisition components. I'm looking for a probe that can be connected to it and measure conductivity accurately. If known what kind of signal do you get out of the probe and is any special equipment necessary to be able to measure it? What power supply is necessary to use? And is it possible to measure continously?

Steve Myres, PE

When measuring conductivity, pH, ORP, or other process parameter, you cannot connect the probe directly to the control or SCADA system. You must use a transmitter which connects to the probe, and outputs 4-20ma or other common process signal. This transmitter usually has a separate input for 24VDC or 120VAC power. The output from the transmitter can be connected to your monitoring or controlling instrumentation. Instruments are available from Georg Fisher/Signet, GLI (Great Lakes Instruments), Rosemount, and Burkert, among many other companies. One piece packages are available which mount the transmitter in a connection head similar to one for a thermocouple directly on the back end of the probe. Steve
There are several different devices that will read conductivity on a continuous basis. We use them on boiler blowdown streams and do closed loop control of the blowdown flow with them. We also monitor the quality of the return condensate with this device. I believe that ours are either Rosemount or Foxboro meters.
Do you know the range you may be working with? Will it be in the ms or us range? What is the liquid you are measuring? If it is very low, like 10us an inductive probe would be a problem and I have had to use a contact type probe to get low readings accurately. Steam condensate is very clean and is something we measure with a contact type probe. We have tried both. We use Yokogawa and get very good results. And we pay for it. Cheers Mark Ray _______________