Soil Moisture Sensing and their limitations

The WaterGreat SR-1 controller is just perfect for surface soil moisture control.  While we cannot publish our trade secrets, we would like to provide a technical summary of the many other techniques used to measure soil moisture.   The WaterGreat SR series sensor is superior to these approaches!  We apologize up front if this is too technical, but this information is provided for all interested in the technology behind WaterGreat.

An Innovative Approach to Heat Dissipative Moisture Sensors:

After over a decade of research, WaterGreat presents a heat dissipative sensor that utilizes exact heat pulses and digitally filtered results using hundreds of discrete samples to deliver a consistent energy to a thermally conductive medium, namely the small sensor body and the surrounding soil with its water. Using its proprietary methodology, ordinary low precision elements are automatically calibrated to produce an accurate dependable reading for nearly all types of soil and temperature conditions, under a variety of salt or fertilizer concentrations. Also, the issue of dissipative sensors not capturing the changes in thermal conductivity of the soil due to temperature have been eliminated. What you are left with is an extremely durable, consistent sensor that is extremely easy to use.

Prior Art:

There are presently in existence a variety of probes and detectors for use in the detection of moisture present in a medium such as soil.  The prior art probes have been found to be inaccurate due to affectation by a variety of soil parameters such as pH variation, contact and cohesiveness with the soil, soil temperature and soil salts.   More expensive techniques such as time domain reflectometry or neutron probes offer good accuracy, but at an expense an order of magnitude greater than this invention.  Unlike the prior art moisture detection probes, the present invention is unaffected by those parameters in soils which have previously created inaccuracies when soil moisture is measured.

Among the prior art moisture probes which claim reasonable accuracy there are those which utilize electrolysis or conductivity to determine the presence of moisture and those which measure the surface tension of the medium in which the moisture is present.   Those probes which utilize electrolysis or conductivity in order to measure soil moisture content can be seriously affected by change in ionic concentrations and soil salts as electrical conductivity during electrolysis is affected by ionic variation.  The variations in ion concentrations cause pre-measurement calibrations to be inaccurate making it difficult to accurately measure moisture content in a particular soil.   Those probes using conductivity that attempt to mitigate the changes in ionic concentration by providing a conducting buffer such as gypsum suffer from clogging of the soil particulate and have an inherent slowness to respond to rapidly changing soil conditions.

These electrolysis and conductivity probes depend on electrodes which are placed in the soil for obtaining readings.  The electrodes are linked to electrical circuitry.  The circuit is completed by the presence of moisture in the soil enabling an electrolytic ion transfer to take place.  This method for measuring moisture content is inaccurate especially when the moisture content of the soil is low.  In addition, variation in acidity or alkalinity of the soil increases or decreases conductivity depending upon which of these phenomena occur.  As the readings for moisture content are conductivity dependent, variations in conductivity due to these effects introduce significant error into the moisture content readings.  

Moisture probes which rely on measurement of surface tension in soil mediums are generally inaccurate.   Soil tension is not useful around the upper root zone, where most watering decisions are made.   Earlier heat dissipation sensors which operate on the time required to return a sensor to its original temperature after heating the sensor to an elevated temperature consume higher heat, require special shielding and anticorrosive plating and do not account for the changes in soil thermal conductivity at different temperatures – diminishing the moisture sensor accuracy over temperature ranges.   The additional heat requirement of the prior art makes small solar powered control assemblies impossible as well, and the higher heat attracts ants and other ground organisms.  

Capacitive sensors, although providing for indirect contact with the soil, are strongly influenced by moisture and air gaps in closest proximity to the soil, which affects the time domain reflectometry systems as well.   Lower frequency capacitive sensors, under 20 megahertz, are affected by soil salinity.   Most of these previous sensor designs fail when physical corrosive or conductive properties change.   This invention is highly resistant to physical deterioration.  

We believe this product to be a superior choice for your irrigation needs.