M C dropped by 2% in 24 hours --too fast ?

[n0legs]

Hi all,
I bought an oak board yesterday, 30mm thick 110mm wide 2.3m long, and brought it into the house to acclimatise.
It's initially air dried down to approx 20% then taken indoors at the timber yard. When I got it home I checked the MC with my moisture meter, in several positions, and got a reading of 17%.
Noted and dated this on the board. Had another look at it today, just a bit of plotting and planning, and I checked the MC again today and found it to be at 15%.
It's not in a particularly warm area, but it is quite a "dry" spot of my home.
Should I be concerned, any comments ?
Thanks in advance.

 

[Random Orbital Bob]

If there's any spare it might be worth cutting an inch off the end and testing the freshly cut centre of the timber just to rule out surface only mis-readings

 

[Jacob]

Surface readings - go up/down fast. Try dunking it in the bath - you'd be surprised! :shock:
The only way to be sure is to weigh samples before and after, to be sure to be sure as they say in the emerald isles.

 

[Sgian Dubh]

As Jacob says, readings near the surface can vary considerably quite quickly. It takes significantly longer for MC changes to work their way through to the centre of a board. Similarly, if you receive a delivery of kiln dried wood, for example, that's been caught in rain, even heavy rain, for half an hour or an hour between timber yard and your workshop the surface might be sopping, but the core will not even have registered a change in MC. Get the stuff into your dry workshop and perhaps rear it up against a wall for about half a day and the surface moisture will have disappeared and the boards will probably be pretty much where they were MC wise when they left the timber yard. Slainte.

 

[n0legs]

If there's any spare it might be worth cutting an inch off the end and testing the freshly cut centre of the timber just to rule out surface only mis-readings

Cheers Bob, I didn't think of that #-o

 

[n0legs]

Surface readings - go up/down fast. Try dunking it in the bath - you'd be surprised! :shock:
The only way to be sure is to weigh samples before and after, to be sure to be sure as they say in the emerald isles.

Thanks Jacob, along with Bobs idea I'll get the scales out

 

[n0legs]

As Jacob says, readings near the surface can vary considerably quite quickly. It takes significantly longer for MC changes to work their way through to the centre of a board. Similarly, if you receive a delivery of kiln dried wood, for example, that's been caught in rain, even heavy rain, for half an hour or an hour between timber yard and your workshop the surface might be sopping, but the core will not even have registered a change in MC. Get the stuff into your dry workshop and perhaps rear it up against a wall for about half a day and the surface moisture will have disappeared and the boards will probably be pretty much where they were MC wise when they left the timber yard. Slainte.

Thanks Richard

 

[ayuce]

The wikipedia page below has a graph showing wood m.c. vs humidity. 0C and 30C curves so close, we can use these lines as reference. The boards moisture level will move to equilibrium point given on the graph. Beside moisture of board, you'd better measure the humidity in the room also. I guess, if delta between current MC and EMC too high some cracks could be.

A graph showing max room temp vs delta M.C. ( current board M.C. - equilibrium M.C.) to avoid crack would be direct answer to your question, but this is a point i also wonder.

http://en.wikipedia.org/wiki/Equilibriu ... re_content

 

[lurker]

As someone who has designed and built commercial relative humidity metering, I can't help but wonder if you all put too much trust in these simple meters with two prongs basically reading localised conductivity.

 

[Sgian Dubh]

As someone who has designed and built commercial relative humidity metering, I can't help but wonder if you all put too much trust in these simple meters with two prongs basically reading localised conductivity.

I treat the readings of typical wood moisture meters as a guide, not as infallible edicts from God. They're handy, and usually their readings aren't too far away from the actual MC of the wood under test, something I've been able to confirm from time to time through the definitive oven drying test.

I agree that you can sometimes be misled by these meters, either the prong type or the pinless variety. Interestingly, it's quite often the case that I can get a fair idea about the MC of the wood by the way it reacts to working with either machines or hand tools - not dead nuts accurate of course, but an indication anyway. Slainte.

 

[memzey]

Is there such a thing as a reasonably priced and accurate moisture meter? I'd like to know because all the sub £100 jobbies I've seen have very poor ratings.

 

[lurker]

I assume these things measure conductivity?

My 45 year old school boy plant physiology knowledge coupled with more recent knowledge of water chemistry and physics suggests a totally different reading if you measure with the grain and across the grain

 

[Sgian Dubh]

Is there such a thing as a reasonably priced and accurate moisture meter? I'd like to know because all the sub £100 jobbies I've seen have very poor ratings.

I can't help you with that request because I haven't owned a working moisture for about six or eight years. Wagner and Lignomat are two brand names that are reputed to be pretty good. My last one was a Lignomat, and it worked well enough for about US$25 when I bought it nearly twenty years ago - until I broke it. Slainte.

 

[Sgian Dubh]

I assume these things measure conductivity?

I believe so. As I understand it pin meters are basically ohmeters that measure electrical resistance. Pinless meters measure dielectric properties to provide readings. The meter, placed against the wood, generates a radio frequency field extending a specific distance. In one version of this type of meter the moisture content of the wood registers via the power loss of the signal. In the other type the change in electrical capacitance according to the wood’s moisture content is measured.

In both forms of meter that I've used, pin type and pinless, the measurement is taken with the sensors set parallel with the grain. I've never come across one where the sensors are set perpendicular to the long grain direction, but it may be that some meters are intended for use like this, although I'm not aware of any. Slainte.

 

[n0legs]

As someone who has designed and built commercial relative humidity metering, I can't help but wonder if you all put too much trust in these simple meters with two prongs basically reading localised conductivity.

I totally agree with you on this.
I have the Protimeter Mini and a Stanley, and it does make me wonder if I should put so much faith in them. The Stanley all ways seems to read 2% more than the Protimeter (or the Protimeter reads 2% less than the Stanley), but they are consistent when the MC starts to alter.
I guess the hyper professional ones are big bucks but are a lot more accurate.
I think the best bet is as Richard says to use them as a guide and then just be satisfied the MC is dropping.
I will definitely weigh this piece and monitor it that way as well. I have an accurate scales and there should be no doubt when the figure becomes less.
Thanks all.

 

[Eric The Viking]

I watched that Rob Cosman video about converting a Cedar. They used a rather odd looking damp meter (jamjar-shaped thing), but couldn't get the same reading twice in the same part of the plank.

I remember years ago someone damp testing our flat with prods and suggesting we had a 'serious' problem. He'd been poking the device through wallpaper, but also through the aluminium foil behind it (wot I'd papered on first).

Jacob's right: the electrical things are a waste of effort.

Besides which, there's a lot of casual conversation suggesting moisture content is linear. Cosman's video had people muttering about 40%, etc. I know trees shift a lot of moisture, but even so.

Take our family-sized teapot: empty, 1lb, 6.5 oz; full-ish 2lb 13oz.

Oddly symmetrical though those numbers are (and those really are the measurements from our kitchen scales), that's a moisture content of only approx. 50% when the teapot is full.

So, how dense is a board with 0% moisture content (by species), and what is the real maximum trees/logs reach?

It would be very interesting to get data from those firms salvaging mahogany from South/Central American rivers and harbours. Whatever moisture was lost between the tree being felled andthe log falling into the water, you'd have to assume that, after decades underwater, they'd have reached saturation. So that must reasonably be a maximum.

Obviously wood needs to dry out to become stable, but it's not that simple.

My dad has a rather nice rosewood and mahogany drop front writing desk that belonged to my grandfather. The drop front is breadboarded and inlaid, and the whole thing was simple but beautiful. I've admired that desk for all my life, and Dad sitting at it is one of my earliest memories. It's was probably made around 1920, and has survived four house moves whilst it's been in our part of the family, but recently my parents moved into a bungalow with underfloor heating.

I was horrified to see that the drop front has now warped horribly, probably beyond any hope of being saved. The change in moisture content after all that time cannot have been very great, but it did huge damage.

 

[Sgian Dubh]

Take our family-sized teapot: empty, 1lb, 6.5 oz; full-ish 2lb 13oz.
Oddly symmetrical though those numbers are (and those really are the measurements from our kitchen scales), that's a moisture content of only approx. 50% when the teapot is full.

You're not comparing like with like, although I suspect you might have been making a joke. Simply emptying a teapot of water is akin to removing the free water from a piece of wood. It's what happens after the free water has been removed and you're removing bound water from the cell walls that causes warping, splitting etc, i.e., shrinkage and distortion only occurs once the wood starts drying below its fibre saturation point (FSP). I think I can reasonably safely assume that you can't really remove a significant quantity of water that might be bound to the chemical structure of a stainless steel or ceramic teapot.

So, how dense is a board with 0% moisture content (by species), and what is the real maximum trees/logs reach?

The density of dry wood varies from species to species, and denser woods generally have a lower FSP than less dense species, some with an FSP of 23% or 24% and some species (softer, spongier ones) have an FSP above 30%. However the average FSP of all species is an accepted 30%, and this is the starting point used for such things as calculating likely shrinkage and expansion as wood takes on and loses moisture.

Obviously wood needs to dry out to become stable, but it's not that simple.

Not at all. Wood is only stable when it's at FSP or above. Below FSP shrinkage and distortion occur.

I was horrified to see that the drop front has now warped horribly, probably beyond any hope of being saved. The change in moisture content after all that time cannot have been very great, but it did huge damage.

But evidently enough to cause the damage you've observed. It may be the case that your parents' new home, in addition to underfloor heating, may have significantly better insulation, fewer draughts, double glazing, etc than their previous homes - that's speculation on my part, but for such damage to suddenly occur after decades suggests something is significantly different in the conditions the bureau experiences in service. Slainte.

 

[Eric The Viking]

Of course you're right.

I wasn't making a joke, as much as considering how much 50% (in that case) moisture content might actually mean. A tree with 40% moisture content will be physically dripping, surely, even though the wood fibres are a lot less dense than our teapot!

I take on-board everything you say, too. Reality seems to be that there is some optimum moisture content at which point unstressed wood (and not stressed is important - not crotch wood, for example) doesn't move. Either side of that, it does. It seems for some species the propensity to move versus moisture follows a rounded "U" curve, but for others, for example the mahogany of my dad's bureau, where it's more like a "V" shape. And obviously it doesn't need to be symmetrical - again the bureau evidently isn't.

The extreme example of this has to be those cellulose sponges one can buy - I assume they're a wood by-product. I have one for cleaning my soldering iron tip. I use it sopping wet, at which point it neatly fits the tray on the soldering iron stand. At this time of year it dries out very fast in the house. Within days it's shrunk in size by around 1/3. I know it's not natural, but the response to liquid water must have some link to what wood does.

Then there's Balsa at the other extreme. Obviously it's not exactly popular for cabinetry (but handy for rafts). The stuff for aircraft modelling seems to have an extremely low moisture content, but oddly, also to be quite stable. It doesn't seem to want to split based on changes in humidity alone ('interesting' landings are another matter).

 

[Sgian Dubh]

Reality seems to be that there is some optimum moisture content at which point unstressed wood (and not stressed is important - not crotch wood, for example) doesn't move.

Below FSP the one condition in which wood remains dimensionally stable, i.e., it doesn't expand and contract, is when it has reached equilibrium moisture content (EMC). This only occurs in conditions of unvarying relative humidity (RH). In this situation wood eventually reaches EMC meaning the moisture content is the same throughout the sample. I can only think of a couple of situations where this might happen: laboratory conditions set up by scientists for some form of study of wood properties, and perhaps storage facilities for valuable artefacts, e.g., museums. Elsewhere atmospheric RH varies all the time, even in climate controlled houses so wood is always in a state of adjusting its MC (and therefore its dimensions) to adapt to the changing conditions and always has some sort of moisture gradient, i.e., drier at the core than at the shell, or vice-versa.

It seems for some species the propensity to move versus moisture follows a rounded "U" curve, but for others, for example the mahogany of my dad's bureau, where it's more like a "V" shape. And obviously it doesn't need to be symmetrical - again the bureau evidently isn't.

I'm not sure I understand the V and U analogies, but shrinkage from FSP to 0%MC is not linear, although for the purposes of estimating shrinkage and expansion in response to changes in MC, the formulae used generally treat it as if it is linear. In reality the greatest proportion of dimensional change occurs between about 20% MC to about 4% MC. Shrinkage is not so marked as wood dries from FSP to 20%, and below about 5% or 4% MC there's not much cell wall distortion left to occur.

I can't help much with the cellulose sponge point you made, and I suspect that you're being somewhat misled about balsa wood simply because it's so light and spongy giving the impression of dryness rather than being any drier than any other species in the same circumstances. The reason it's not prone to splitting is because it's a stable species, i.e., the tangential to radial shrinkage factors are reasonably close to equal. Slainte.