You didn't list conductivity type probes. I have done a few of these
and I've posted to the PIClist on the topic before. Your application
could be well suited to this method, especially because you have a
grounded metal tank and non-explosive atmosphere.

You need AC bias on a probe to prevent electrolysis. Basically you
use an oscillator through a resistor then a capacitor to the probe
and then an input from the capacitor to your micro. Frequency can be
just about anything, suggests a few hundred hertz. Conductivity
through the liquid to ground attenuates the signal, your micro
detects the lost signal as liquid contact on that probe. That's it.

I love using CMOS schmitt trigger parts for interfacing these things.
Gives a more defined trip point for each probe. The 40106 is a hex
schmitt inverter which is cool for a 6 level probe or use one
inverter as the oscillator and you have 5 inputs. I can post
schematics if you're interested.

The same scheme can be used to make a capacitive probe. Just increase
the frequency and resistor value and make the probe an insulated
plate.

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Brent Brown, Electronic Design Solutions
16 English Street, Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/txt: 025 334 069
eMail:  brent.brown@clear.net.nz

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