Soil Moisture Instruments
© Brooke Clarke 2012
The factors that effect soil
resistance are: moisture, salinity, density and temperature.
Another factor is that when dry the soil contracts and may move
away from a fixed electrode.
There are a number of aspects to measuring soil
moisture. The probe can be designed to be buried and
stay there or like a spike that can be used in a portable
measurement system. The measurement can be made at a single
depth or at multiple depths. The early probes were a block
of Plaster of Paris with a couple of electrodes imbedded.
The moisture in the Plaster of Paris would be the same as the
surrounding soil and the resistance between the probes was
proportional to the moisture content.
DC systems of measurement suffer from polarization and
electrolysis. AC systems avoid those problems. I would
think that measuring the AC impedance
either at a magic frequency or over a band of frequencies would be
the best method.
Other things that are similar are measuring the pH (Wiki: pH
, pH Meter
Total Dissolved Solids Wiki: TDS
of a liquid. The electrical TDS meters depend on the
dissolved solids forming ions that increase the
conductivity. So a TDS meter is closely related to
soil moisture meters. pH is measured using a specialized
electrode probe that depends on free hydrogen ions.
Electrical Sensing Unit for Measuring Water in Porous Media, L.A.
Richards, Jun 14 1960, 338/35; 73/73; 324/689; 338/34 - AC
Aparatus for Measuring Moisture of Soil, R.Rosenthal (Ind
Instruments Inc), Jan 3 1956, 73/73; 236/44.00E; 338/34 -
Electric Hygrometer, G.J. Bouyoucos, Mar 27 1956, 338/34; 73/73;
324/696; 338/331 - talks about plaster of Paris molded probes
but they have short life spans.
Soil Moisture Meter, G. Bouyoucos (Mich. State Board of
Agriculture), Apr 28 1953, 338/34; 73/73; 73/335.05; 324/443;
324/696; 338/208; 338/328
- uses AC for resistance meas to eliminate electrolysis and
Soil Moisture Testing Apparatus, W.J. Turner Jr. & R.H.
Raymond (Rayturn Machine Corp), May 28 1957,
73/73; 324/448; 324/696; 338/34;
338/35; 338/229; 338/333 - driven probe has 4 sample sections
at different depths.
Apparatus for Measuring Humidity Conditions in Materials, C.E.
Ohlheiser (American Inst Co), May 13 1958,
73/73; 73/29.02; 324/689;
324/696; 338/35 - grain elevators
Electrical Humidity Sensing Unit, C.E. Ohlheiser (American Inst
Co), Nov 4 1958, 338/35; 73/335.05; 324/696; 338/226 -
5430384 Temperature Compensated Soil Moisture Sensor, L. Hocker
(Onset Computer Corp), Jul 4 1995, 324/694; 137/78.3 -
||Jun 7, 1973
||Dec 2, 1977
||Jan 2, 1979
||Simmonds Precision Products, Inc.
monitoring electrical properties of a liquid
||Mar 10, 1977
||Feb 6, 1979
||ACF Industries, Incorporated
bottom operable tank car valve
||Sep 19, 1977
||Apr 15, 1980
sampler and controller
||Aug 28, 1978
||Oct 7, 1980
||Malcom-Ellis (Liverpool) Limited
monitoring of the electrical conductivity of liquor
||Jan 16, 1987
||Apr 18, 1989
||Jun 12, 1989
||Feb 19, 1991
||Nov 13, 1989
||Aug 20, 1991
relative humidity/temperature measuring system
||Mar 5, 1990
||Apr 7, 1992
||Industrial Chemical Measurement, Inc.
with multiple electrodes
||Sep 3, 1991
||Sep 22, 1992
monitoring and control system
||Apr 10, 1992
||Jan 12, 1993
||Irrometer Company, Inc.
sensor for sensing moisture in soils
||Feb 26, 1992
||May 4, 1993
Back to Brooke's Products
for Sale, PRC68, Alphabetical Index of web pages, Home web page
[an error occurred while processing this directive] page created 9 Mar