Soil Moisture Measurement Equipment

November 11, 2010

There are many ways to measure soil moisture.  The most accurate is to take a volume of soil, weigh it, (air) dry it and then weigh it again. The impracticality of this method, particularly in the field has led to the development of other techniques.

The best known was the neutron probe which used a radioactive source lowered into the soil by way of an aluminium access tube. Pulse bounces were counted by the device to record soil moisture very accurately. With its demise, on safety issues and the fact that it was incapable of measuring top layers, TDR (Time Domain Reflectrometry) became a safe replacement and, almost as accurate, alternative.

TDR measures the ‘dielectric constant’ of a material to determine the moisture. The dielectric constant is a complex quantity with a real number that characterises the moisture and with an imaginary component as a measure for energy loss and electrical conductivity. Both parts depend mainly on frequency so that the measuring frequency of an electromagnetic technique is a decisive criterion.

TDR systems are the favoured tool for researchers and very recent technology has allowed TDR moisture sensors to be manufactured which are priced to almost match the less accurate capacitance, frequency domain based alternatives.

After 25 years experience of accurately measuring soil moisture we have chosen to distribute the award winning TRIME TDR range of systems including the new PICO-BT Bluetooth Module. This equipment is small, portable and provides immediate soil moisture readings as a percentage by volume. The system can be networked for automated soil moisture measurements from several sensors across a site with just one readout device and one module to read any quantity of sensors.