'POT' issues with long cable

[mackrackit]

I think you need another solution. The long wire will have capacitance of it's own.

But, the same setup may work using the ADC on your chip.

Capacitor as a sensor? What exactly are you doing?

[ardhuru]

I think you need another solution. The long wire will have capacitance of it's own.

Yes, I thought of that. But this shouldnt be a problem, I can calibrate the unit once to compensate for the cable capacitance.

Its actually a string of 10 capacitors in series immersed in a water tank. The water shunts out each capacitor as the level rises.

I did have an earlier version with the resistance being a variable, using the ADC, but this tends to get corroded within a few weeks.

The capacitive version seems to work well, but for this problem; and seems to have no corrosion issues either. Would be a pity to have to abandon this design.

Any other ideas?

Anand

[aratti]

I did have an earlier version with the resistance being a variable, using the ADC, but this tends to get corroded within a few weeks.

You can tray with stainless still electrodes if you will use AISI 316 type you will not see any corrosion whatever liquid you will use.

The sensor could be designed as per the attached example.

Al.

[ardhuru]

Al, how did you manage to connect the resistor(s) to the stainless rods? I couldnt figure that out, short of using specially tooled terminals, which was the only reason I couldnt continue with my DC model.

BTW, since you seem to have worked on a similar technique, did you find the readings highly non linear over the range? At times it is difficult to resolve the levels, especially at the lower end.

Regards,

Anand

[aratti]

how did you manage to connect the resistor(s) to the stainless rods

You thread one end of the rod and use 2 nuts and two washers to hold the resistors wires.

did you find the readings highly non linear over the range? At times it is difficult to resolve the levels, especially at the lower end

Linearity depends on the liquid in use (how many ions you have in suspention) but don't expect very high linearity from it.

You can always correct the reading via software knowing the real values taken on the bench shorting the electrodes one by one.

Al.

[Archangel]

Hi ardhuru,
All that stuff you mentioned in Post 1 sounds pretty expensive, and Al's SS Rods do too. So using Al's Idea a little differently, suspend 4 copper stiff wires or rods vertically at different lengths, which you can adjust easily to fit your needs. You can Solder the resistors or Potentiometers to those wires, and install a PIC circuit there, powered by a couple of solar cells easily removed from 2 cheapie Yard Lights, and yes use the nicads that come in those too, then send some form of slow serial data using relatively inexpensive wire, TV lead in, sprinkler wire, twisted doorbell wire . . . to another PIC . Compared to shielded wire and fancy Stainless steel rods this should be cheap. You could put the wire array in a piece of plastic pipe and insert it vertically into the Well, sump, or whatever it is you are checking.

[ardhuru]

Hi ardhuru,
All that stuff you mentioned in Post 1 sounds pretty expensive, and Al's SS Rods do too.

Actually, its not. I just use 10 0.1 uf capacitors strung (stringed?) together in series. A bit of labour involved, but the material cost is nothing.

Hi ardhuru,
and install a PIC circuit there, powered by a couple of solar cells easily removed from 2 cheapie Yard Lights, and yes use the nicads that come in those too, then send some form of slow serial data using relatively inexpensive wire, TV lead in, sprinkler wire, twisted doorbell wire . . .

I was also toying with the separate-pic-for-the sensor idea. In fact, I thought I would use Melanie's trick of data-over-power from an earlier thread to be able to do this over just 2 wires. Wasnt sure how reliable the serial comms would be with my already laid twisted, non-shielded cable.

Will give this a try.

[Melanie]

I did have an earlier version with the resistance being a variable, using the ADC, but this tends to get corroded within a few weeks.

The secret is to have AC excitation at the probes... electrolyis is then minimal if not completely eradicated. 1kHz HPWM through say a 100nF Capacitor is just perfect for the source probe.

[mackrackit]

Go to your local welding shop and talk them out of a piece or two of SS tig wire.
And with a little practice and some silver solder you can make the connections that way.

[ardhuru]

The secret is to have AC excitation at the probes... electrolyis is then minimal if not completely eradicated. 1kHz HPWM through say a 100nF Capacitor is just perfect for the source probe.

This would be the best, easiest to adapt/incorporate in my earlier design, with a minimal of hardware/code change if I can get it to work for my app. I didnt realise I could use a PWM'ed signal in conjunction with ADC.

I just hope I am able to resolve the 10 steps I'd like to have in the sensor reliably.

Anand