I build sensors to measure conductivity of seawater, and we have similar
problems with electrolysis etc.  The ultimate way to do it is to
eliminate electrodes by putting a coil in the soil and measuring the
impedance as it is affected by the soil making a shorted loop.  This is
not easy but it gives the best results.

Sherpa Doug

> -----Original Message-----
> From: Lawrence Lile [mailto:llile@toastmaster.com]
> Sent: Monday, January 28, 2002 3:47 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]:Wetness sensor
>
>
> I have messed around with them:
>
> 1.  Soil is wierd stuff.  A DC signal in soil will work like
> a pair of leads
> in an electrolyte.  Straight DC soil resistance will change
> constantly over
> time regardless of dryness.  Try sticking your ohmemeter in a
> bucket of
> dirt, and checking it every 15 minutes for a day, it will
> vary all over the
> place.
>
> 3. AC signals work better for measuring soil resistance.
>
> 4. Some retail soil meters use conductance, rather than
> resistance.  This
> will be a galvanometer across two dissimilar metals.
>
> 5. Soil resistance is affected by fertilizer levels (which
> are ions) as well
> as water.
>
> 6. Jameco sells a cheap kit that get's you over a lot of
> these problems
> painlessly.
>
> Good luck!
>
> --Lawrence
>
>
> ----- Original Message -----
> From: "Shirollah Ghadirian" <Shirollah.Ghadirian@ENERGY.INVENSYS.COM>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Monday, January 28, 2002 2:36 PM
> Subject: Re: [PIC]:Wetness sensor
>
>
> > Hi all,
> > Has anyone had experience with a cheap soil wetness
> (moisture) sensor
> > please.
> > Something that could give me some rough idea of soil
> wetness condition.
> > This is for my home water irrigation project.
> >
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