Not-too-conductive material

There are resistive inks, some of which can be cured at relatively low temperatures, but you'd have to evaluate them for your application.

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Why not just put a fractional cent resistor in series? It will help protect the CMOS MCU as well. Best regards, Spehro Pefhany

If you can find an old-fashioned graphics supply house, the pencil leads (graphite) for the old 8" mechanical pencil might just work. You certainly have enough "mixes" of graphite in pencil lead to try a few and see if it will work for you. You *will* have trouble connecting wires to the graphite.

I'm also not sure if you are measuring the "resistance" of the water between the probes. If so, you'd best be absolutely sure that the water is going to have precisely the same impurities in it every time or your readings will be useless. And, if it is distilled enough to be "pure" water the readings will be infinite at any water level.

Jim

-- "It is the mark of an educated mind to be able to entertain a thought without accepting it." --Aristotle

I'm sure a similar question came up recently...

Anyway, you could consider dipping a pair of insulated wires (with thin insulation) into the water, each of them looped so that both ends are above the surface, and fixed a constant distance apart. The depth of the water will cause the capacitance between the wires to vary. In conjunction with a high value resistor you can make an oscillator and measure the frequency with the PIC.

This method avoids corrosion and will be much less affected by impurities in the water than trying to measure the resistance between two bare metal electrodes. Consistent resistance measurements with dc are almost impossible because the electrodes become polarised by an unpredictable amount depending on many variables. AC resistance measurements are better, but still hard to do accurately. Capacitance measurements have a good chance of being repeatable.

John

I measured a 0.5mm Pentel mechanical pencil lead and it was only a few ohms for a couple of inches. No way that's getting up to 100K.

Best regards, Spehro Pefhany

Regarding the capacitance method: The normal insulation on regular hookup wire is too thick to generate an easily-measured capacitance. I would need to get into the RF spectrum to do it (I've tried), and this device is too cost-sensitive to allow it.

As for calibrating for differences in the water, I have that taken care of. Periodically, the reservior is filled to the top. A third go/no-go probe tells me that. At that time I take a time constant measurement and associate it with the full state. The water I will have to measure will always contain some ionic impurities, so I don't have to worry about non-conductive water.

From the experiments I have done so far, the effective resistance between the probes depends very little on how far apart the probes are, and much more on the total conductive surface area in contact with the water.

And Spehro, how did you measure the .5mm pencil leads? I also measured the same size leads and got more like 100 Ohms for a few inches. I found that DC measurements are invalid because of the DC offsets that appear from ionic activity. But if you use true AC measurements, that should get rid of those offsets. At least it did for me.

Robert Scott Ypsilanti, Michigan

Yes! I did an experiment 2 weeks ago with a piece of Romex. I hot glued one end of a piece 6" long to insulate it, and hot glued it to the inside of a plastic container 5" tall. With the cap meter connected I added water - and got 1 pF per inch change in the reading. I did that several times with the same result.

I still need to experiment with changing the temperature of the water to see whether it changes the capacitance, but the method you suggest shows promise, from both a logical and an experimental perspective.

Ed

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How about several 10K resistors in series? As the water level rise, it will short out the lower Rs but keeping the upper Rs.

For what hardness?

Jim

-- "It is the mark of an educated mind to be able to entertain a thought without accepting it." --Aristotle

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Have you tried it with enamelled wire (transformer wire)? The enamel is only a few microns thick (4 to 10 micron).

but you will want the spacing between the wires to be comparable with their diameter.

-- Bill Sloman, Nijmegen

How about enamel (Formvar, Polyvar, whatever...) insulated magnet wire?

How much spatial (vertical) resolution do you need?

If the answer is "not very much" you could fabricate the probes out of a thin rectangle of PCB material (something water-resistant) with a series of pads etched into it, and solder down to the pads a whole row of SMT resistors (in effect, connected head-to-tail). You'd get a discrete step in resistance each time the water rose high enough to bridge a resistor. Choose the resistance per step to suit your needs.

Some sort of silk-screened conductive ink might also be a possibility?

Yes, that would give me enough resolution. But this probe may need to be cleaned occasionally with a damp rag. Chip resistors would catch on the rag.

--unless I put the resistors on the back side of the PCB and put vias to the pads that touch the water on the other side. But a PCB is still not the ideal form factor. Something shaped like a dowel would be easiest to clean.

As for the range, it is 6 inches from full to empty. The resolution only needs to be . 5 inches.

Robert Scott Ypsilanti, Michigan

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Put the 10K's in series above the max water line, and have vertical tracks going downwards from each junction, make the tracks stop at different heights.( or depths)

martin

Cute. It will drift over time, though, because plastic is permeable to water. Metal and glass are much better, so if you can get some glass tubing and fuse it around a piece of wire, you'll have a much more stable capacitance probe.

Cheers,

Phil Hobbs

Oof. At that resolution I'm not sure I'd bother using a resistance type of sensor at all. I'd probably just run a bunch of individual sense wires down the inside of a rod to little contact on the rod surface, and use CMOS inverters or something like that to detect a discrete open/closed state for each.

Or, if you still want to use resistance sensing methods, here's a possible way to fabricate it which will give you an easily-cleaned, waterproof sensing rod.

Get yourself a block of Plasticene. Reshape it into a rectangle 8" long or so, a couple of inches thick, and a couple of inches wide. Press an acrylic dowel/rod around 3/4" in diameter halfway down into it and then pull out the rod, leaving yourself with a trough as long as you need.

Buy a box of "escutcheon pins" at your local hardware store... they look like short round-headed brads made out of brass. Even better would be round-headed copper pins, if you can find them, or silver- plated PC-board turrets.

Push a series of escutcheon pins, head-down, gently into the bottom of the trough in the plasticene... just deeply enough for the head to stick in the plasticene and not fall over. Place one every half-inch or so (at your desired resolution).

Mix up, and pour into the trough, just enough epoxy (or polyester) resin to cover the heads and first parts of the shafts of the escutcheon pins. Allow to harden... this will hold the pins in place during the next part of the procedure.

Trim and shape the leads of a bunch of 1/8-watt resistors (leaded, not surface-mount), and then go down the length of the rod, placing each resistor between adjacent escutcheon pins and bendings its leads around the pin to hold it. Crimp the leads gently into place with needle-nose pliers.

When all of the resistors are in place, either solder them quickly to the pins (use an active rosin solder, as brass can be hard to solder), or apply a dab of silver-bearing conductive epoxy and allow it to harden. Also, solder a wire lead to the pin at one end of the trough and run it up out of the trough.

When you're done, measure the resistance from the wire lead to each pin in sequence (making sure that the contacts are OK). When you're satisfied, mix more epoxy or polyester resin, and fill the trough up to the top (covering the ends of the pins, and encapsulating the resistors). Allow to harden. Remove from the plasticene. Clean off the slightly-projecting round heads of the escutcheon pins.

Thanks! I'll have to try that. The experimentation is a variation on another experiment where I was using a PTO to measure tide height. A "bit awkward" mechanically - a large coil (42" long) with a threaded rod in the center. The frequency alters as the rod penetrates the coil more or less with tide height change. It measured well for the 36" that was required, accurate to within

But I figured it could be as simple mechanically as affixing a piece of romex (or similar) to an existing pier and measuring the capacitance. That's what prompted the initial "probe" experiment. I didn't know about plastic being permeable to water, so you've saved me some aggravation as the experiments proceed!

Ed

Sounds like this would do the job:

Probably HB. Measuring two other samples (Staedtler 0.5mm HB micro carbon, and Selectum (Taiwan brand, bought there) 0.5mm HB) I get around 5 ohms for a couple of inches.

Best regards, Spehro Pefhany

Thanks - nice link!

Ed

Don't use resistive probes for water level. If you totally insist on doing it electronically (i.e., no sight glass, no floats, no nothing?) then use a capacitive sensor - two concentric pipes with a little gap between them, and measure their capacitance - it should be directly proportional to the water depth; I think water's dielectric constant is a lot more predictable than the water's conductivity would be.

Good Luck! Rich