Moisture meter
| Moisture meter | |
| Type | Instrument |
|---|---|
| Measures | Moisture content or a related electrical property |
| Principal types | Resistance (pin) and capacitance (pinless) |
| Calibration | Wood-moisture equivalent unless material-specific |
| Nature of result | Comparative in most building materials |
A moisture meter is a hand-held instrument used to assess how wet a material is. Two types are in common use: resistance meters, which measure electrical resistance between two pins pushed into the material, and capacitance meters, which sense the dielectric properties of the material beneath a flat pad without penetrating it. In leak investigation the meter is used to map the extent and gradient of dampness across a surface, which points back towards a source, rather than to produce an absolute figure for moisture content. No statute or published standard defines the technique, so the description below is of established industry practice and carries no citation.
The two types[edit]
A resistance meter measures the current passed between two pins. Water conducts far better than dry building material, so resistance falls sharply as moisture rises. The reading is local to the pins and to their depth, which makes it precise in position but shallow.
A capacitance meter emits a field into the material and responds to changes in its dielectric properties. It reads to a greater depth and leaves no marks, which suits finished surfaces, but it responds to whatever is within that field: foil-backed plasterboard, embedded metal, pipes and fixings all produce readings that are not moisture.
Interpreting readings[edit]
Most meters are calibrated for wood. Applied to plaster, screed, brick or tile they give a wood-moisture-equivalent value, which is not the moisture content of the material being tested. Salts left by previous damp events conduct and inflate resistance readings long after the material has dried.
The consequence is that a single reading means very little. The information is in the comparison: a grid of readings across a floor or wall shows a gradient, and the gradient points towards the wettest area. That is what makes the instrument useful for narrowing a search even though the numbers are not absolute.
Limits in leak work[edit]
Moisture mapping shows where water has arrived. Water runs along joists, under screed and across membranes, so the wettest point can be some distance from the escape and can be on a different floor. A meter cannot distinguish a plumbing leak from rain penetration, condensation or a historic event, all of which produce elevated readings.
It is therefore used to bound an area and to monitor drying, with acoustic, tracer gas or borescope work used to identify the source within that area.