Re: Soil moisture sensors

["Keith Rathband" <notreal@xxxxxxxxxxx>]

Thanks Marc and Dave,

The links you provided seem to show everything i need to know.

I'm going to be using the sensor in a lawn irrigation system - do you happen
to know if these sensors can be left in the soil all year round ?

Marc - I did see the post you mentioned whilst searching through google
groups - i can't beleive that geezer, obviously got out of the wrong side of
the bed that day.

Thanks again,

Keith.



<MFHult@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:migra199lj4qopou95pqgopu37366k6qv0@xxxxxxxxxx
> On Mon, 13 Jun 2005 16:55:54 +0100, "Keith Rathband" <notreal@xxxxxxxxxxx>
> wrote in message  <d8kacc$oo0$1@xxxxxxxxxxxxxxxxxx>:
>
> >I'm trying to use a Watermark soil moisture sensor in the my home
> >automation project. I believe this sensor cannot be read as a standard
> >resistive sensor using a DC voltage across the sensor, the voltage across
> >the sensor must be A/C.
> >
> >Also, Irrometer who make the sensor don't seem to want to giveout details
> >of the sensor's response curve, and temperature correction details.
> >
> >So,
> >
> >Does anyone have, or know where i can get an interface circuit schematic
> >diagram for the Watermark sensor.
> >
> >Does anyone have, or know where i can get details of the response curve,
> >and temperature compensation details.
>
> >Thanks in advance
>
> >Keith.
>
>
> Irrometer doesn't want to be responsible for calibration of your
instrument
> or the likelihood that the data in the table they provide is not close
> enough to the performance of your soil-sensor system for your purposes.
>
> Irrometer also doesn't conform to a retail market expectations because
they
> don't cater to the retail market directly.. In a previous thread in this
> newsgroup another poster got all bent out of shape because Irrometer
> supplied some of their data in AutoCAD format instead of whateveritwas
that
> the poster wanted.  But they've always been helpful to me and other
> researchers. I've used their watermarkers off and on for ~20 years.
>
> Third-party interface modules for waterwmarks are available, but
> traditionally watermarkers have been hooked up directly to data loggers
> with the data loggers providing the needed pulse excitation.
> I have some tables that it may be OK for me to pass on to you. Also see
the
> linear temperature correction suggested below.
>
> However it is best to calibrate your sensors yourself in _your_ porous
> medium (typically soil). The sensors are semi-quantitative at best but are
> well suited for binary decsion-making in home automation setups
> (water/don't water).
>
> And here are some general instructions quoting from a previous post of
mine
> (Marc Hult) in this newsgroup (comp.home.automation)  4-5 years ago:
>
> Watermark Model 200 sensors can be treated as a resistance that varies
> non-linearly between approximately 550 ohms ("wet" = 0 centibars) to
> 28,000 ohms (200 centibars = very dry). However, the length of time that
> a DC excitation voltage is applied to the sensor must be short enough to
> prevent significant hydrolysis of water and consequent formation of gas
> bubbles on the sensor's internal electrodes.
>
> The manufacturer,  www.irrometer.com , recommends 5 VAC or a pulse
> between 5 and 20 volts of 40 millisecond duration at a current of 200uA
> or less. If repeated measurements are needed, the leads should be
> shorted together or the powered side shorted to ground for 30 seconds to
> remove any accumulated DC potential in the sensor. The sensor will
> self-discharge without shorting in about 10 minutes so several
> measurements per hour are practical without shorting. The sensor
> resistance is inversely proportional to temperature (correction of 1
> percent / degreeF from 75 degreeF)
>
>  Also, in order to avoid ground currents that can permanently damage the
> sensor, neither lead should be connected to earth ground for extended
> periods.
>
> These requirements can be met without adding series capacitors if the
> sensor is connected through a CMOS analog IC switch.
>
> HTH ... Marc
> Marc_F_Hult
> MFHult@xxxxxxxxxxxxxxxxxxxxx