Moisture in solids can be sensed in terms either of the conductivity of the materials, changes in permittivity, or of the absorption of microwaves. For sensing the presence of moisture in materials of comparatively fixed composition (even where some variation occurs, as in masonry), a simple resistance reading between connectors set at a fixed distance is often all that is required. For measurement purposes, readings have to be calibrated, and calibration is a long and tedious procedure that requires samples with various moisture levels to be checked for resistance, then weighed, baked to remove moisture and weighed again. The difference in weights shows the moisture content which can be expressed as a percentage of the original weight of material.
A very common requirement is for moisture content of soil. This is very seldom required to be precise, which is just as well, because the relationship between the resistance of the soil and its moisture content is not a simple one. A simple resistance indication, however, is enough to tell whether a plant needs watering or not, which is the main reason for using moisture indicators. For civil engineering purposes, the use of a soil-resistance moisture meter is only a preliminary indicator, and a complete soil sample analysis would be needed before any decisions on the suitability of soil for foundations were made.
Sensors and Transducers 