Soil Moisture Instruments

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) or 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.

2941174 Electrical Sensing Unit for Measuring Water in Porous Media, L.A. Richards, Jun 14 1960, 338/35; 73/73; 324/689; 338/34 - AC capacitance

Calls:
2729099 Aparatus for Measuring Moisture of Soil, R.Rosenthal (Ind Instruments Inc), Jan 3 1956, 73/73; 236/44.00E; 338/34 - resistive bridge
2740032 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.

Calls:
2636962 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 polarization.

2793527 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.

2834201 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

2859318 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 -

Calls:

US3553481		1971		AUTOMATIC SOIL WATERING SYSTEM
US3882383	Jun 7, 1973	1975		SOIL MOISTURE SENSING SYSTEM
US4132944	Dec 2, 1977	Jan 2, 1979	Simmonds Precision Products, Inc.	Apparatus for monitoring electrical properties of a liquid
US4137937	Mar 10, 1977	Feb 6, 1979	ACF Industries, Incorporated	Adapter for bottom operable tank car valve
US4197866	Sep 19, 1977	Apr 15, 1980		Soil moisture sampler and controller
US4227151	Aug 28, 1978	Oct 7, 1980	Malcom-Ellis (Liverpool) Limited	Measurement and monitoring of the electrical conductivity of liquor samples
US4823087	Jan 16, 1987	Apr 18, 1989	Hideo Sugimori Merbabu Corporation	Salimeter
US4993640	Jun 12, 1989	Feb 19, 1991		Fluid control system
US5040417	Nov 13, 1989	Aug 20, 1991		Electronic relative humidity/temperature measuring system
US5103179	Mar 5, 1990	Apr 7, 1992	Industrial Chemical Measurement, Inc.	Water analyzer with multiple electrodes
US5148826	Sep 3, 1991	Sep 22, 1992		Moisture monitoring and control system
US5179347	Apr 10, 1992	Jan 12, 1993	Irrometer Company, Inc.	Electrical sensor for sensing moisture in soils
US5207380	Feb 26, 1992	May 4, 1993		Irrigation control system

Soil Moisture Instruments

Background

Patents

Related

Links