Electronics Forum / Design / August 2008

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
1 inch. (3" accuracy is the requirement).  But I can imagine
crud and corrosion will ruin the thing, and it is mechanically
unwieldy.

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

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.