Hello, I recently came across an electrical conductivity (EC) circuit schematic, and it mentions that they used probes with small surface area and at a fixed distance apart.

(found on Sparky's Widgets - open sourced)


I tried this circuit out and it works well. It is my understanding that it uses an oscillated signal, rather than a DC signal, through the water for more accurate results. It looks like it then uses a "peak detector" circuit at the end, which is the output. The more conductive the liquid, the higher the DC output. You then read this DC output with a volt meter (or analog-to-digital converter).

It is unclear to me how I might modify this to work with probes with more surface area exposed to the water? When I try this with a probe that has more surface area, it pretty much just maxes out and there is no longer any discernable difference between different liquids.

Do I just need to put an extra resistor right before the "OUT" of the probe? Or modify the gain of the op-amp with different resistor values? I wasn't quite understanding this.

Any help or advice is greatly appreciated, thanks!

 

You could try increasing the value of R7 to reduce the drive to the probe/rectifier section.

 

An alternative would be to reduce the value of R9 as this forms a potential divider with the sample resistance. The op amp adjusts it's output voltage such that the voltage on pin 6 is the same as pin 5. So V out (At pin 7) = OSCVout * (1 +R9/R sample).

Les.

 

Yes, but R9 is already a quite low value so the adjustment range is limited.

 

Hi Albert, That's a good point.

Les.