An electrical resistance moisture meter is mostly used for measuring timber moisture content. It has two pins, or 'electrodes', which are hammered or pushed into the timber.

An electric current is passed between the electrodes and the amount of charge that gets through is measured.

Moisture meters work on the principle that water is a good conductor of electricity, but wood is not. So the higher the charge, the more water there must be in the wood.

It's a simple rule that works reasonably well for timber, but you need to remember that the meter is measuring electricity, not moisture.

This means that any other substances that conduct electricity, such as certain chemical additives, will give false readings.

It's also the case that moisture meters are calibrated to measure Douglas fir (also called oregon).

So you have to apply a conversion factor when you measure other species to get an accurate reading.

The result also needs to be adjusted to allow for different temperatures in the wood, because higher temperatures tend to lower electrical resistance.

Another important factor is the depth of the electrodes in the wood.

If the wood is still drying, the moisture content will be highest near the core, so you have to make sure you've pushed the electrodes in far enough to reach this area of the piece.

The moisture content range in timber subfloors should be between 10% and 14% for resilient floor coverings and 9 to 14% for timber floor coverings.

Note that in air conditioned buildings it could be less, particularly if the windows are sealed and there is no direct access to outside air. This is because air conditioners tend to de-humidify the atmosphere.

The levels could also be less in some dry inland areas where the atmospheric RH is typically low. By contrast, in tropical areas where the humidity remains high for long periods of time, the EMC may be as high as 17% during the 'wet season'.

In these cases, you should discuss the local conditions with the builder or local council before proceeding.

Moisture meters and concrete

In the past, electrical resistance moisture meters were sometimes used to test concrete; however, this is no longer permitted under AS 1884-2012.

One of the reasons is that the electrodes won't penetrate far enough into the concrete to provide an accurate RH measurement.

Another problem is that the meter can give distorted readings if there are additives in the concrete.