Isolating a Conductivity Probe

Discussion in 'General Electronics Chat' started by cdr, Jun 22, 2004.

  1. cdr

    Thread Starter New Member

    Jun 22, 2004
    I have a two-pole conductivity probe which operates on the principle of measuring the resistance of the liquid. Basically I use a 555 timer to generate an osillation and pass that current through the two poles of the conductivity probe. I then measure the frequency of the osillation and calculate the conductivity with a simple equation.

    My problem is that the two pole probe is greatly effected by stray current and and a ground probe in the tank. Also, the conductivity probe effects the accuracy of a pH probe in the same tank.

    Can anyone suggest how to isolate the conductivity probe from external effects? Shielding, etc...
  2. beenthere

    Retired Moderator

    Apr 20, 2004

    You've already seen that all such sensitive measurements are going to affect each other.

    The only wat you're going to be able to measure conductivity by your present means is to do it in a separate container. Otherwise, the ground rod will do you in, and the pH will never be accurate,

    If the measurement has to be continuous, consider using an acquarium pump to run a stream of the liquid into an isolated container. Make sure there is an air gap between the delivery tubing and the liquid surface. The container can have an overflow back into the other container. The air gap prevents the conductivity probe from sending stray ions into the main tank. as it is doing now. Use plastic, and the little external container should let you get accurate results.
  3. cdr

    Thread Starter New Member

    Jun 22, 2004
    I disagree, there are many instruments on the market today that are capable of accurately measuring pH and Conductivity simultaneously without interfering with one another.

    I just need some help designing the signal driver circuit and shielding for the conductivity probe. Right now I am using a 555 CMOS timer to generate the osillation for the conductivity probe, then counting the frequency and relating this to the conductivity with a simple equation. There has to be a way to effectively shield and isolate the conductivity probe current and prevent it from leaking into the tank.

    I realize that ideally you would want to separate the two probes, but I have also seen it possible to have them both in the same tank.
  4. bipin

    Well-Known Member

    Mar 21, 2004
    Hi cdr,
    what about tying out multiplexing of the two measurements?
    if you have good opto-isolators or analog switch before your both probes then you can have deactivated one of the probes and measure the value, hence no interfearence from one to other.
    you will have to compensate for the lost measuting time, by repeatation or hold on.
    bipin :)