Re: Soil moisture sensors

[MFHult@xxxxxxxxxxxxxxxxxxxxx]

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