Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!ncar!unmvax!sandia!wllarso
From: wllarso@sandia.UUCP (Bill Larson )
Newsgroups: sci.electronics
Subject: Re: moisture sensors
Message-ID: <459@sandia.UUCP>
Date: 14 May 91 21:26:13 GMT
References: <1991May10.173139.1371@solbourne.com>
Distribution: usa
Organization: Sandia Natl Labs, Div. 9224
Lines: 31

In article <1991May10.173139.1371@solbourne.com>, daveh@solbourne.com (Dave 
Helms) writes:
|> does anybody know how I can buid some sort of moisture
|> sensors to insert at different locations in my yard ?

If you were to look back in Sunset Magazine three or four months, there was
an article about soil moisture sensors and controller systems.  The sensors
they referred to were all commercial items, but from their description they
are VERY simple.

Using the resistance between two conductors with a water (solution) dielectric
is quite common in chemistry.  The problem is that this technique can be very
difficult due to the possibility of various contaminants in the solution.
If you were to bury wires directly in the soil and measure the resistance
between them, the conductance would be provided by ions from salt, fertilizer,
various organic acids, etc.  I would suspect that this would make any attempt
at a calibrated reading almost impossible.

What the commercial sensors do is to in-case the conductors in gypsum, better
known as Plaster of Paris.  This way the sensors will absorb water until they
are in equilibrium with the moisture in the soil and the conductors will have
a much more controlled environment, just calcium and sulfate ions to provide
the conductance.

The normal technique in chemistry to measure resistance/conductance is by using
an AC Wheatstone bridge to avoid polarization effects in the solution.  With 
all of you brilliant EEs out there, do you have any cheap solutions to make
a simple AC resistance/conductance measuring technique?  Something that would
use a six volt transformer to supply the current and/or voltage, and one or
two op-amps to convert the conductance/resistance to a proportional output
voltage.