electrodes used in water

Platinum is the preferred material for conductivity electrodes. Graphite is good - as has already been mentioned in this thread - but you can't solder to it.

Stainless steel often works reasonably well, but it is "stainless" because it forms a thin, coherent and modertely conductive layer of chromium oxide on its surface when exposed to oxygen - under the wrong conditions this can get thick enough to have appreciable resistance, and you can set up conditions where one "stainless" steel electrode wil corrode Iif it is made from the wrong sort of stainless steel).

As has also been mentioned in this thread, use AC excitation to measure the conductivity - with DC excitation you can get polarisation, electrolysis and (with bad luck) electrophoretic deposition of an insulating layer of crud on one or other electrode (sometimes both).

-------- Bill Sloman, Nijmegen

if it is possible to place a metal strip or wire on the outside of the caustic materials container then perhaps you might consider using a capacitive sensor for the fluid level.

It's easy to plate copper onto graphite with copper sulfate solution.

One can also strip most of the copper off of copper-clad welding carbons with ferric chloride (or a grinder), leaving a band at one end for a solder connection.

As others have said, a capacitive sensor with insulated electrodes is better. The insulation can be chemically inert, and insulation that doesn't "wet", like teflon or silicone, won't collect crud nearly as quickly. Further, water can vary enormously in conductivity but doesn't vary much in dielectric constant with ionic impurities present.

Analog devices offers chips that directly convert capacitance to digital output:

Better yet, use an ultra low duty cycle pulse (AC). If you have a microcontroller in charge, use a timer to poll the condition of the water as infrequently as your application can tolerate. Then the effect of running current through the probe will be cut to a minimum because most of the time the probe will be idle.

-Robert Scott Ypsilanti, Michigan

stainless steel how ever, water can have deposits that will collect on the rods and thus effect it, ect.. what you should try is is a capacitive method using 2 rods insulated from the water. or , use a piece of PVC pipe that has a pressure diaphragm in it at the top. mount the pipe vertically on the open end into the water.

Also - use an AC test signal, not DC.

.......... Phil

If you want to cheat, battery carbons have crimped caps which you can solder to - or you can electroplate the end of the carbon with copper and solder to that.

Try graphite, but whatever conductive material used, do it with AC measurements.

Try carbon from a dead drycell (Leclanche-type (zinc, carbon, salamoniac), NOT an alkaline cell. Cheers, Roger

Very expensive platinum.

Or else use a float mechanisim inside of a glass tube. Detect the position of the float in the glass tube. Since you didn't supply details, I'm assuming stationary, standard room temp./pressure operation.

Al

platinum.

And even better: remove the cause of this electrolysis. He is obviously measuring with a DC voltage, which causes electrolysis. Measuring with an AC voltage prevents this. Also, carbon electrodes (pencil fillings) work well.

Meindert

Stainless Steel lasts fairly well and is much cheaper than gold or platinum. that's what they use in swimming pool chlorinators.

on the other hand if you encapsulate the electrodes in some insulating membrane and measure the capacitance between them you'll detect a signifcant increace in capacitance when there's water between them.

or you could attach a magnet to a float and use a reed switch etc to detect the level.

Bye. Jasen