Re: Re: Moisture sensor

["David Gumbrell" <david.gumbrell@xxxxxxxxx>]

Sounds like this explains why I kept killing pot plants despite using
the
spike :-0

Cheers,

David

On 4/17/07, Karam Karam <kzkaram@xxxxxxx> wrote:
>
>   I should imagine its a similar idea to the gypsum based sensor but
> just using a pair of stainless steel probes. The trouble with
> measuring moisture levels using conductivity is that you don't really
> know what's in the water from one location to another and this can
> effect conductivity tremendously. So the gypsum sensor tries to
> ensure the local ionic conditions are always the same (ie. dominated
> by the level of solution of gypsum ions, so mostly dependent on
> moisture level only). I've no doubt that other ions from the
> surrounding soil must eventually get in with the gypsum ones but the
> effect is probably negligible.
>
> The steel probe approach will no doubt tell you whether the soil is
> moist or not but bound to be a much cruder result. Perhaps with some
> regular method of callibration and for a relatively self contained
> location it might be ok to get something a little better than just a
> logic indication. So this might suit the DIY approach.
>
> In either case an ac signal should be used to avoid polarisation
> (remember school chemistry ;-) ). Though I've seen comments that the
> frequency (and so harmonic content of non sinusoids) affect the
> measurement I'm not too sure about whether this means affecting the
> repeatability or just the range - back to the electrochemistry books
> I guess...
>
> I would guess that for relatively crude measurements a simple
> alternating square wave @ 1KHz should be ok and need only be switched
> on at polled measurement times.
>
> Karam
>
> --- In ukha_d@xxxxxxx <ukha_d%40yahoogroups.com>, "David
Gumbrell=
"
> <david.gumbrell@...>
> wrote:
> >
> > Getting back to moisture sensors - I wonder what is inside those
> cheapo
> > "spike" types with the meter on ? Anyone broken one
open ? How
> accurate
> > does this application need to be (especially as it could rain
> torrentially
> > immediately after automatic watering if met forecasts are not
> integrated) ?
> >
> > Cheers,
> > David
> >
> >
> > On 4/15/07, lexfordparc <paul@...> wrote:
> > >
> > > Hi Karam,
> > >
> > > I have found a possible sensor which has a UK distributor
and
> costs
> > > <=A325.
> > >
> > > http://www.irrometer.com/landcat.htm#Watermark
> > >
> > > I have read through the blurb and it seems to require 5VAC
to run
> and
> > > the resistance will vary according to moisture from 500 to
30,000
> Ohms.
> > > Could you suggest what would be needed to get this working
with my
> > > Idratek system?
> > >
> > > Many thanks,
> > >
> > > Paul
> > >
> > >
> > >
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
>=20=20
>


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