Surveying equipment and tests

A capacitance meter can be used to speedily check surfaces, but there is sometimes a tendency not to follow up a trail of evidence because this instrument is commonly used by surveyors undertaking the more cursory inspections.

Some meters offer, in one instrument, the opportunity to use the capacitance function (sometimes referred to as 'search mode') or the resistance function. If a material is checked by both modes, it may be possible to determine whether moisture exists just at the surface or more deeply into it, because in capacitance mode it offers insight into moisture conditions a little deeper into the wall, typically to depths of 10–20mm. Another advantage is that this instrument does not cause puncturing of joinery, masonry or plaster surfaces.

However, hidden metals, such as cables or metal foils influence the instrument and it can also detect surface moisture from bathroom tiles. A flat surface is needed for the instrument to work, and it may scuff wallpapers and linings if not used carefully.

TRADA predicts that

'capacitance-type meters are less accurate [than the electrical resistance type] in general use (probably +/-5%) but are capable of improved accuracy with systematic use involving direct calibration against the oven drying method.'

Using a capacitance meter

A capacitance meter is easy to use. The bulge at the back of the instrument is placed against the material under investigation; but, as with the resistance meter, it is the interpretation of readings that requires the skill.

The device uses radio frequency emissions to measure dielectric properties; it does not measure 'dampness' directly. In some instruments, readings go up to 1,000 and give an indication of the potential moisture beneath a surface at shallow depth. All readings from the capacitance meter are relative readings.

Some manufacturers claim that a reliable judgment as to dampness cause can be made using a resistance meter and capacitance meter in combination. Whichever instrument you use, readings that are higher than expected for a material under investigation should initiate further investigations.

The methods for using the meter on site are the same as for the resistance meter described above and in the Invisible damp case study.