MP306 Moisture Probe Operation Manual
2.2.3 Cable Length
The standard cable length is 4.5 m. This may be extended by using suitable cable.
Multi Core Polypropylene Insulated Irrigation Control Cable, specifications 9 core (9 x 7/0.30) has been
tested successfully over 500 m. Two wires were connected for power supply positive from the data logger to
the MP306.
2.2.4 Power
The MP306 is normally powered by a voltage in the range 7.0 to 18.0 volts using 18 mA of power. It can
function satisfactorily within the voltage limits of 7.0 to 18.0 volts provided 18 mA is maintained.
1 x MP306 = 32 mA (maximum power use)
5 seconds of warmup/hour = 5/3600 x 32 = 0.04 mAh
Total of 16 x MP306 = 16 x 0.04 mAh = 0.64 mAh
Logging for 24 hours on an hourly frequency = 0.64 x 24 = 15.36 mAh/day
Gel Cell Battery 7 Ah capacity. Then = 7/0.01536 = 455 days
In other words a Gel Cell battery is capable of supplying enough power for about 455 days. The battery
would need to be recharged within about 30 days as it will self discharge in this time.
2.3 Theory of Operation
2.3.1 Theory
The MP306 has a high frequency moisture detector which uses the standing wave principle to indicate
the ratio of two or more substances forming a body of material, each substance having a different electric
constant (Ka).
The moisture measurement of the material is based upon the fact that in a water: soil: air matrix, the
dielectric constant is dominated by the amount of water present. The dielectric constant of water is
approximately equal to 80 whereas the dielectric constant of soil is approximately equal to 3 or 4 and air is
equal to 1. Therefore any changes in the volume matrix ratio of water will result in a substantial change in
the dielectric constant of the matrix. Then the soil water content can be measured exactly because changes in
water content of the soil result in changes in the dielectric constant of the soil.
The material that can be measured by the MP306 is often soil but can be any composition of non-metallic
powdered, liquid or solid phase substance into which the needles are inserted.