> Peter, I notice the empahasis on the 'insulate' part; should they be or
> should they not?

I'm not Peter, but there are several reasons why insulated sensors might
be better in a particular application.

First, the dielectric value of water is much more stable with respect to
small changes in water chemistry.  A little bit of salt, for example, can
effect the conductivity by orders of magnitude.  Second, corrosion can be
a problem with metral touching water, especially if a current is flowing
thru it.  Third, if the electrodes differ in chemical makeup, there can be
galvanic effects.  This can happen over time if a DC current has flowed
thru the electrodes, even if they started out the same.  Different plating
and corrosion effects happen at the + and - electrodes, thereby changing
their surface composition.

> I had fabricated a sensor using a pair of aluminium strips the height of
the
> tank, separated by plastic spacers ever few inches. The strips
themselves
> were NOT insulated from water. Worked as expected, but the problem came
from
> elsewhere; the sensor was too sensitive to noise, even when connected
using
> a shielded, or co-ax. cable; the readings would go haywire if one
touched
> even the plastic insulation on the connecting wires.
>
> Despite the issues involved, I eventually settled for a conducting type
of a
> sensor; a ladder of 10 100K resistors in series; as the water level
rises,
> it progressively 'shorts' out the resistors underneath. This sensor was
then
> used as the lower half of a potential divider and fed to the ADC on the
PIC.
> Cant complain with the results, its been working for over a year now!

That should be reasonably accurate as long as the effective resistance of
the water is much less than 100Kohms.  That can be true for "dirty" water,
but wouldn't work well with deionized water.


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