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Moisture Meters

I SAID in my April article in CFJ that I would talk about moisture meters in May: but in fact, I ended up telling you about the perils of the CE-Mark – which I thought was a more pressing topic, since there has been an awful lot of that ‘quality’ plywood about (or maybe that should read: ‘a lot of awful quality plywood!’).

But now I return to the subject of moisture in timber, and the way in which it can be identified – sometimes helpfully – with a meter. And the first thing to know about moisture meters is that they do not actually measure moisture.

I know it sounds crackers – but all meters work by measuring electricity: or to be more exact, they measure the electrical properties of timber. The most usual sort of meter that surveyors use, measures resistance; whilst some other types measure capacitance or some other property in wood.

The two main types of meter that I mention above, work in very different ways: although each of them can give an indication of the actual moisture within the timber itself.

Taking the latter type first (since it is less commonly seen in site surveys): it works by emitting an electrical ‘field’ in a zone around the reading head of the meter; it does not in any way penetrate physically into the timber itself.
This ‘field effect’ then interacts with moisture in the wood to create variations in that field, which the meter then detects – a bit like the way a metal detector works, when it finds some hidden metal which disrupts the ‘normal’ pattern of the electrical field put out out by the device.

Now – ignoring the finer points of exactly how it works, I need you to be aware of what it tells you…and also what it does not tell you; which is any difference between a wet surface and a dry core, or vice versa: because it simply ‘averages out’ the moisture and then gives overall reading.

That, to my mind, is not very helpful in the context of site surveys, where detecting differences in the moisture content in the different elements of a component or structure can be crucial to finding out what is really going on. For that sort of information, you need a resistance-type meter, with penetrating probes that can reach into the depot of the wood.

But beware of those meters too! If the probes you stick into the timber’s surface are fairly short and uninsulated (that is, the sort of ‘pins’ which typically come supplied with the meter, or are fixed to its body); then they are more likely to fool you, than they are to help you.

Because if the timber (or wood product) has a wet surface and a dry core, then those short pins cannot differentiate between those two conditions: and so your meter will only tell you it is ‘wet’ somewhere; but not exactly where that wetness is!

Conversely, if it has a very wet core, but it is quite thick material (say 25mm floorboards), then the – typically – 6mm long pins cannot reach to the core and tell you just how wet it is. So you need to use a set of ‘deep probes’ that have insulated shanks.

As a last point, beware of resistance-type meters used on plywood or chipboard. Those sorts of ‘composite’ materials contain resin glues (and other additives) which have different electrical properties to wood – and which can thus alter the readings that the meter gives you: so it may say that the material is ‘wetter’ than it actually is.

A moisture meter is a very useful tool: but it is only one of the tools that a surveyor should have at his disposal. It must always be used with great care and with lots of experience, and it should not necessarily always be believed as being completely infallible.

Jim Coulson is the director of TFT Woodexperts in North Yorkshire

T: 01765 601010
E: tft@woodexperts.com
www.woodexperts.com

This article has been reproduced from the Contract Flooring Journal website. You can find them at www.contractflooringjournal.co.uk.