'POT' issues with long cable

[ardhuru]

I have designed a circuit around the 16F887, with a resistor/capacitor combination on PortA.6 being read with the 'pot' command.

Earlier, I had these connected to PortA.5, had wierd results, and realised that that pin was a Schmitt input.

Now, things are fine on the workbench. However, the capacitor is to be located remotely as a sensor (about 50 meters), and when I did this using a standard non-shielded 2 core wire, the readings are no more consistent.

My question, before I buy the cable, is would shielded cable perhaps help?

Or would co-ax cable, the kind used for cable TV be even better?

Or neither, I need to look at another solution altogether?

Also, if I use either of the above, can this cable be run in close proximity of the mains without the readings going haywire again?

Any advice would be highly appreciated, folks!

Regards,

Anand Dhuru

[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.