>Here's another; use a length of coax cable,
>with the outer jacket removed, so the braid forms the long rod, and the
>inner conductor, exposed only at the bottom, forms the reference electrode.
>Just have to make sure the inner conductor is sealed well where it enters the
>insulation. A weight attached to the end will hold it vertical.

Continuing the refinement of the probe assembly:

Suspend a small diameter tube (to feed the wire for the third sensor) inside
a larger diameter tube. These tubes are kept separate from each other by a
non-conducting mesh similar to the tube mesh used for bundling cable wires.
You should use as open a mesh as possible.  The third sensor is attached at
the bottom of the inner tube but is electrically isolated from either of the
tubular sensor tubes.  The top end of the probe assembly must be vented to
allow the water to rise inside.

Now you have a small diameter compact probe assembly.

The close distance between the sensors afforded in this design lowers the
overall resistivity between the sensors.

I believe you could find such tubes in a hobby shop.  I've seen them in
places that specialize in R/C airplanes.  The length may be a problem though.

Rich

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