A voltage is applied to the composting material between the electrodes of the sensor at the tip of the probe. Moisture is related to the electrical conductivity, which increases with increasing moisture levels. However, conductivity also changes with other factors like salt concentration and the nature of the substrate. Therefore, the meter is intended as a relative indicator of moisture level. It is calibrated by inserting the probe into a saturated sample of material and then setting the meter to 10.
Moisture is measured by drawing a sample of gas from pores of the pile or windrow.
Moisture content is correlated with the relative humidity of the gas sample which, in turn, is determined by the sensor's electrical capacitance. After about a minute, the device yields an estimate of moisture content between 20 and 80 percent in increments of five percent. As the five percent increment implies, the device provides an estimate of moisture content rather than a precise reading. Other moisture sensing technologies have the potential to improve the accuracy of electronic sensors for organic materials. For instance, work is being conducted jointly by Pennsylvania State University and the University of Delaware on moisture sensors for mushroom growing beds, which are essentially compost.