Pin or pinless, and what each actually measures
A pin meter drives two sharp electrodes into the timber and measures the electrical resistance between them. Water conducts and dry wood does not, so resistance maps to moisture content. It reads at the depth the pins reach, which makes it good for finding whether a board is wet through or only damp at the surface, and it leaves two small holes.
A pinless meter uses a pad that senses the material below it, typically to a couple of centimetres. It leaves no marks, which matters on finished joinery and flooring, and it scans an area quickly. The cost is that it reads an average through that depth and it is thrown off by anything conductive behind the surface, including foil backing, pipework and fixings.
The two suit different jobs rather than one being better. Survey a floor with a pinless meter to find the wet area, then use pins to find out how deep it goes.
| Pin meter | Pinless meter | |
|---|---|---|
| Marks the surface | Yes, two small holes | No |
| Reads at depth | At the pin tips, selectable with longer pins | Averaged over a set depth |
| Good for | Confirming depth of damp, checking a board through | Scanning an area, finished surfaces |
| Fooled by | Surface wetness on the pins, salts | Metal, foil, pipes behind the surface |
Why the reading needs a species correction
The resistance of wood at a given moisture content varies with the species, because density and chemistry differ. A meter calibrated on one timber and used on another reads high or low, and the error is large enough to matter when you are deciding whether stock is fit to work.
Better meters carry a species setting or a correction table, and using it is not optional if the number is going to inform a decision. Temperature affects the reading too, which is why meters intended for site use offer a temperature correction as well; a cold board reads lower than the same board warm.
Where a meter offers no correction at all, treat its readings as comparative rather than absolute. Comparing one part of a board with another, or today with next week, is still useful even when the absolute figure carries an unknown offset.
The numbers that actually mean something
Wood is hygroscopic: it takes up and gives off water until it reaches equilibrium with the air around it. That is why timber for interior joinery is dried to a much lower moisture content than timber for an outdoor structure, and why bringing wet stock into a heated room causes it to shrink, cup and open joints as it equilibrates.
The threshold most often quoted for decay risk is around twenty per cent moisture content, because that is roughly where wood-rotting fungi become able to establish. Below it, timber is generally safe from rot; sustained above it, particularly with poor ventilation, it is at risk. Mould can appear on a damp surface at lower levels than that.
Acclimatise rather than trusting the delivery. Stack timber in the space it will live in, with air able to circulate between boards, and take readings over several days until the figure stops moving. A stable reading matters more than any single number.
Concrete, screed and grain are different measurements
Meters that offer a concrete or building material mode are giving a relative indication rather than the true moisture content of the slab. That is a genuine limitation rather than a fault: an electrical surface reading cannot tell you the moisture that matters for laying a floor covering, which is the humidity in equilibrium deep within the slab.
For that decision the recognised methods measure relative humidity inside the slab, using an in-situ probe in a drilled hole or a sealed hygrometer box on the surface, left for the time the method specifies. A handheld meter is the right tool for finding a damp patch and the wrong tool for signing off a floor.
Grain and hay meters are a third case again, calibrated per crop because different grains hold water differently. A meter with a crop list is doing the same job as a species correction on a timber meter, and selecting the wrong crop gives a confidently wrong number.
