Also, sensors make poor contact with compost media due to the course texture of compost and composting feedstocks. For all of these reasons, soil moisture sensors cannot readily be used in composting situations. A few commercial moisture sensing instruments have been developed or adapted for composting applications. They tend to be portable, battery-powered instruments with a long stem for inserting into composting materials. 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.