Moisture Meters

[Vinny]

OK, so my first trip to a timber merchants has now been made and documented on here.

Waterhead posted a link by way of a reply to my OP from a while ago wrt timber merchants and some of the ins and outs (in his experience) - thanks for that, very helpful. Part of his post made mention of moisture meters, which leads me on to my question:

I assume that the moisture meter works out water content (saturation ?) by measuring the resistivity between 2 probes when in contact with the wood - lower resistivity - more moisture and vice versa. However...... measuring at one end of a 6 foot piece of timber could throw the same results back as measuring at one end of a 2 ft piece of timber. Do you put the dims into the meter before measuring so that it can work out the moisture content relative to the size of the piece?

And finally....... is there one meter that is regarded as "the one to have" ? I've had a scoot round the net and theres a massive variation in price, £14 to a hell of a lot more, my preference is for the lower end.

Cheers, hope the above makes sense, it does to me but then I've just returned from 4 pints of Gem up the local :wink:

Vinny

 

[jimi43]

I was lucky enough to pick up one of these cheap at a bootfair last year....

It has needle probes which are used along the grain.

I think Protimeter still make meters although they are now digital and I have recently seen some on fleaBay...quite cheap too.

I have been watching some air dried cherry recently and this is the first time I have used it in anger and we are talking max 3x2" cross section. Once the moisture was down to where I was happy the wood appeared to be about the same all the way through. Remember most of the moisture will come out of the end grain and so the whole piece should dry evenly.

This has been my experience anyway.

Jim

 

[Richard Findley]

Hi Vinny

I've got this one from Aminster: http://www.axminster.co.uk/product....moisture+merter&user_search=1&sfile=1&jump=44

Being digital it is nice and accurate and gives a good clear read out. As you say, you just push the points into the wood and see what it tells you.

HTH

Richard

 

[chipchaser]

Hi Vinny,

There are 2 types of moisture meter commonly available. The most common are the resistive with 2 or 4 pins which as you say simply measure the resistance between the pins and indicate the moisture content.

The best known in UK and very popular with surveyors and architects for many years are the Protimeter range. The timbermaster models are aimed at measuring wood and the surveymaster are general purpose. Protimeter are as good as any but expensive. There are still quite a lot of the old ones with an analogue meter about and working ok. They have plug in probes on a lead which can give problems due to intermittent or poor connections, I know I have one! Still worth buying if you get a good one and treat it well. The new digital ones should be more resistant to rough handling.

The meters without pins are held against the wood and measure the moisture content by the capacitive field between the pad on the meter and the wood itself. Not so common and generally expensive.

Any meter measures only what is immediately adjacent, in the case of capacitive meters, or between the pins, for resistive meters. There is no way to accurately measure the MC of the whole piece of wood other than by taking several readings at appropriate points and estimating the whole by interpolation. None of them will tell you what the MC is deep inside the timber. You could weigh and dry a piece of course.

The better meters come with correction tables for timber species and temperature. If none are supplied the meter is obviously not expected by the manufacturer to be as accurate as those which do have them. Some meters can have the species “dialled in” by group so you don’t have to make the corrections manually. Some suppliers of cheap meters cannot tell you what species group the meter is set for!

A cheap meter can be checked against known resistances which will give an idea of accuracy across the range.

I have seen good reviews of the Comprotec Timber Check Moisture Meter, which Axminster sell, and comes with correction charts. There is information on this meter, the correction charts and meters in general at:

http://www.comproteccanada.com/meter.htm

I have no connection with Comprotec but, maybe naively, infer that as timber is such an important business in Canada they are more likely to “get it right”!

Graham,

 

[maltrout512]

I bought the same one as Richard about two years ago. Nice and straight forward to use.

 

[OPJ]

I have been watching some air dried cherry recently and this is the first time I have used it in anger and we are talking max 3x2" cross section. Once the moisture was down to where I was happy the wood appeared to be about the same all the way through. Remember most of the moisture will come out of the end grain and so the whole piece should dry evenly.

That, for me, is one of the most important points. While the boards may have been stored under cover (protected from the sun and rain), they were still 'outdoors' and will have picked up some moisture from the air. When you first buy the boards and get them home, the average reading is likely to be all over the place - give it a couple of weeks with the boards in-stick and, as this wood was kiln-dried, you'll probably find more regularity in the readings across all boards.

Whether this is at the 10-12% M/C desirable for most interior furniture or not depends on several things... As this wood has been kilned though, it shouldn't be too far off when you buy it.

 

[RogerS]

.....
The meters without pins are held against the wood and measure the moisture content by the capacitive field between the pad on the meter and the wood itself. Not so common and generally expensive.

Any meter measures only what is immediately adjacent, in the case of capacitive meters, or between the pins, for resistive meters. T.....,

Is that strictly true? My understanding was that the resistive pin meters were a tad limited in that they only measure between the pins...as you say. But the contactless ones actually sensed the moisture level up to a depth of about an inch or so and therefore gave you a much better indication.

 

[Mike.C]

I have the one Chipchaser mentions from Axminster http://www.axminster.co.uk/product.asp? ... e=1&jump=0
and on testing it up against a few other brands (both pin and pinless) that friends have, it has always gave roughly the same readings. So I am very happy with mine.

Cheers

Mike

 

[head clansman]

hi vinny

The one's you see being advertised at axminster.
http://www.axminster.co.uk/product.asp? ... =1&jump=44


I bought one identical from e bay had to come from china at less than half that price took about 15 days to arrive but works fine . hc

 

[Chris Knight]

I use the Wagner capacitative (inductive?) type. I like it because I can sweep along a board quickly and it's surprising how much a board's moisture content can vary across its width/along its length.

I normally take to the yard with me and then once the wood is in my shop, use it periodically to check how the wood is drying. I now have a fair collection of different wood species that have been in my shop long enough to act as handy references for newly bought wood.

 

[chipchaser]

Hi Roger,

I believe your understanding is correct and I failed to specify what I meant by adjacent

However, I understand that layers of timber closer to the surface touching the pad have a greater effect on the capacitance meter reading than the deeper layers. The surface of wood to be tested might be wetter than the centre, from rain or dew/condensation or perhaps the surface layers of kiln dried wood have started to take up water from a damp environment. In this case the reading will be higher than it should be if it were a true average reading of the full 1” depth and no better than a reading from a resistance meter. That may be the reading you need to judge whether a particular piece is ready to use for a bit of cabinet making but maybe not if you are checking progress of DIY timber drying.

If you want a deeper reading from a pin meter you can drive nails or panel pins to whatever depth you wish. And touch the pins to the nail heads. The reading will be indicative of the wettest piece of timber between the nails but practically that is often what you need to know. If you need a reading at a given depth, perhaps to avoid a wet surface layer, you can drill clearance holes to that depth and drive nails with a bit of insulation slipped over them into the holes. Not convenient if there is nowhere that you can afford to make nail holes but is this often the case?

It comes down to personal choice based on intended use, budget etc. I have nothing against the capacitance meter and can see it could be better or more convenient in some circumstances but could not justify spending that much myself.

Graham

 

[jimi43]

As I said earlier...the majority of the moisture exits from the end grain. It is like many straws packed together and full of water....

This is why end grain is waxed on larger pieces to slow down the moisture release as the stock dries to minimise rapid shrinkage which can crack the stock in an adverse way.

I think top layer readings can give you a relative indication of the drying out of the wood not the absolute overall moisture content. Average readings and reference materials of known performance assist this as Graham rightly says.

At the end of the day...any capacitive or resistive meter will work but it is the calibration of these instruments against known samples and internationally produced tables that gives close to absolute values. The tables in my Protimeter have been updated since its manufacture and an online table gives a greater selection of species and reading correlation.

Jim

 

[OldWood]

And what is wrong with the cheap ones ? All the resistive ones do is to measure the resistance between two terminals and as all electrical / electronic engineers will tell you, that is a very, very basic function. I could do it with my general purpose digital meter and some probes but it is preferable to have something dedicated to show a figure in %. So why not just spend £10 on Ebay ?

For calibration if considered necessary all you need to do is to find something around the house that is a similar type of wood and use that for a comparison.

Yes I know that is not quite suitable if you are checking wood in a timber yard, but the accuracy of that measurement has already been queried so one does then have to rely on the integrity of the seller that the timber has been dried properly as a whole.

Somebody has to do a bit of iconoclasm !

Rob

 

[OPJ]

Rob, I bought a cheap pocket-sized meter out of curiosity last summer (Kamasa, about £20 inc. p&p on eBay). I compared it with the Protimeter we have at college and it was reading 2-3% higher, on average (not a huge problem, I admit). After a couple of months of occasional use, it died on me... Before that, I managed to break the tip off one of the pins (I doubt you can get replacements) - this was when the readings started going through the roof (30% and climbing! :?). So, I've had to bin and said I will save up for a much better model, next time.

No doubt, if you pay a bit more, you might not have this problem and you may well be able to buy spare pins.

 

[OldWood]

Well like all tools it is horses for courses.

I've always taken the policy with the purchase of a new type of tool (there must be exceptions to this - yes battery tools!), that I buy at the bottom end to begin with on the basis that anything more expensive is going to be better when the cheap one dies, and if I haven't needed this particular tool beforehand then how much use am I really going to get from it.

So half a dozen quid from Ebay a couple of years ago for the humidity meter - it gets used for the turning wood and the fire logs, so the use isn't heavy, it tells me what I want to know so is therefore excellent value for money.

I really cannot see what is the point of spending more unless you are a professional or seriously-into-it amateur. Fails - buy another one a fiver; how many fivers go into 20,30,40 or more quid?

Rob

 

[woodbloke]

I have been watching some air dried cherry recently and this is the first time I have used it in anger and we are talking max 3x2" cross section. Once the moisture was down to where I was happy the wood appeared to be about the same all the way through. Remember most of the moisture will come out of the end grain and so the whole piece should dry evenly.

That, for me, is one of the most important points. While the boards may have been stored under cover (protected from the sun and rain), they were still 'outdoors' and will have picked up some moisture from the air. When you first buy the boards and get them home, the average reading is likely to be all over the place - give it a couple of weeks with the boards in-stick and, as this wood was kiln-dried, you'll probably find more regularity in the readings across all boards.

Whether this is at the 10-12% M/C desirable for most interior furniture or not depends on several things... As this wood has been kilned though, it shouldn't be too far off when you buy it.

If the timber has been stored kept outside (regardless of whether it's been kilned or not) after a period of time, depending on thickness it will eventually settle at around 20% MC, which is too high for furniture indoors.

Regardless of the MC (and my timber is kept in the 'shop, but it's pretty dry to start with) once a piece has been designed I rough out the timber and cut oversize, leave it in stick for a month or two, and forget about it. Then it's re-machined again, left in stick and finally brought down to finished sizes after another month.

I use a bit of elm for much of my stuff and it's notorious for warping (at WF factor 9 :wink: ) given half the chance, but to date, I've never had any problems...and I never use a moisture meter.

Treat the timber gently and with consideration and it ought to behave itself, it's when you rush it and conditions aren't right that you'll encounter problems - Rob

 

[jimi43]

Or get one at a bootfair for £12 (I found the price sticker on the back)...or fleaBay where the analogue ones are "not vogue" anymore...

MARCONI ONE ON FLEABAY...GOTTA BE GOOD! - A TENNER!

Like Rob says...it is only measuring resistance between two static points...any test meter will do that. This is great for relative values against a known sample...say some known dry wood of that species...which is what most people use it for.

If you are a wood yard who has to estimate the absolute value of the moisture content for control and sales then you need a reliable source of doing this. This is what Protimeters and their like are designed, calibrated, recalibrated and used for. This, accompanied by up-to-date, internationally recognised, species tables, allows importers and retailers to quote resonably accurate values to their clients.....you!

J

 

[chipchaser]

I am very fond of cheap solutions too but many digital multi meters are not sensitive enough to read the high resistances found in dry wood. I have two, of which the smaller, “large pocket” size model, has a 20M Ohm range while the chunky job, for poacher’s pockets, has a 200M Ohm range. The only table of moisture content and resistance by species I have been able to find on line is in the USDA report published in 1988. This lists only 36 species and is biased towards timber available in USA.

The 200M Ohm meter will measure down to 14% in all the species in that list but only 23 of the species to 12%. The number it can cope with drops off rapidly below 12% and no species in the list can be measured below 10%.

The 20M Ohm meter will only measure down to 19% in all the species in that list, 26 of the species to 16% and only 13 to 15%.. The number it can cope with drops off rapidly below 15% to only 2 species at 12% and nothing below that.

Note that the resistance values given by USDA are averages but no figures for deviation from the norm are supplied.

John Sankey (http://www.johnsankey.ca/wetwood.html) provides a short table of moisture % and electrical resistance for one typical species (unidentified but probably American Red Oak) but recommends: “Don't rely on measurements by electrical methods to better than ±2% absolute moisture content, no matter what a manufacturer claims.” He goes on to suggest measuring thicknes with a caliper as a good means of checking when the wood has reached equilibrium.

Protimeter sell their “Calcheck” device, for about £7 which they say should show a reading of 17-19% on their Mini or 17.8-18.3% on their Timbermaster. So even with a fixed resistance the best they expect is plus or minus 0.25% for the Timbermaster and plus or minus 1% for the Mini. Add in the natural variability of timber within a species and plus or minus 2% sounds reasonable.

So OK for a guide but as Rob (woodbloke) says condition your timber. Wood technology is as much art as science which is probably why we like it.

Graham