Thread: Determining timber dryness - the kitchen oven experiment

Tramex - Non-Invasive Moisture Meters: Fast, Efficient and Hassle-Free - Moisture Meters, Moisture Detectors, Concrete Moisture Meters, Humidity Meters



Oven drying to determine moisture content by weight?

Study the terms "inter cellular moisture" and "intra cellular moisture" in relation to drying timber...

Yes kiln drying is a form of oven drying - except it takes 3 months (in hardwoods) if you want to prevent structural collapse and limits max temp to 50C!

Are you going to run this oven experiment of yours for 3 months? Why not just nuke it in a microwave and boil the moisture out from the inside?.

My advice (having operated a kiln and drying scheds for years) is get a non invasive moisture meter from Tramex, as linked above and you'll always know your moisture levels before you start in a matter of a minute or so.

The rest (oven experiment) is just dancing around the issue and wasting time & $ and not really achieving much.

My 11 pence halfpenny coz many will tell you I am not the full shilling at times!

Good idea (its actually one of the most accurate methods and often used to resolve disputes when the results of other methods, such as electric moisture meters, are challenged. ) but 150ºC is way too high a drying temperature as it will evaporate and possibly decompose some of the woods natural volatile substances which do not normally evaporate during even kiln drying.
Too high a temperature may even break down some of the wood.
A Microwave is no good either as it does the same thing.

Have a read of these and maybe start again.

http://www.dpi.nsw.gov.au/__data/ass...nt-of-Wood.pdf
At the temps suggested, ~105ºC, it takes around 24 hours to dry the test pieces suggested.

Oven-Drying Wood for Moisture Content Testing

One other thing to watch out for is that most kitchen oven temperatures are not accurate, most read are a bit high so set the temp too low.
If you can find something like a calibrated TC to check the temperature that would be better.

Good Morning Fencefurniture

This one is really testing my memory.

When I worked in the timber industry 30+ years ago, our timber grader used to recallibrate all the moisture meters regularly (monthly, ????) and he meticulously logged the process. Moisture meters were very expensive toys, then.

His process was to take a green offcut, thickness it, then cut it into small blocks - perhaps 150x75x12 mm. He then accurately measured its dimensions, and also measured the moisture content with each MM he was callibrating, stuck the blocks in a laboratory oven - heavy steel thing - Cannot remember the temperature setting, possibly 80<!--[if gte mso 9]><xml> <o:OfficeDocumentSettings> <o:AllowPNG/> </o:OfficeDocumentSettings> </xml><![endif]-->ºC - and then dried it for an hour - checked the moisture content and weight - dried it for another hour - rechecked the moisture content and weight - dried it for another hour - and so on. He graphed the drying curves for each meter and each piece of wood. When the curve was horizontal, and stayed horizontal, he knew the wood was bone dry. Process took two days.

He then examined the shape of each curve and discarded any that were not real smooth:

• human or measurement error, or
• irregularity in the test block (eg hidden hollow or sap pocket).

From these curves, he then did a page of mathematics, which gave him a measure of the accuracy of the meter. He then opened the meter, adjusted it, resealed it with a lead seal, and signed a certificate certifying that meter.

Sorry I cannot be more precise; I was not required to know how to do it - just that all meters had to be "in certificate".

If you google "calibrating moisture meters" then you might find something useful.



Fair Winds


Graeme

Originally Posted by BobL

..... but 150ºC is way too high

Rightio Bob, just turned it down 100°

Originally Posted by Timless Timber

Why not just nuke it in a microwave and boil the moisture out from the inside?.

I've read that it renders the microwave oven somewhat stinky and unusable for a considerable period of time (weeks).

Hey man, it's just a bit of fun (and not to be relied upon - just a guide).

I would like a non-invasive meter, but the last time I looked they were about $500 or something - and that's out of my range atm, especially for the small amount of green timber I have (although perhaps that may change).

Originally Posted by GraemeCook

This one is really testing my memory.

Cheers Graeme, thanks for your thoughts. Not a need for me to be so accurate (and not possible with the equipment I have, but interesting to know the process.

OK, out of the oven now, and just did a quick weight check, and mental arithmetic. Looks like about 10-12% weight loss, but I did weigh them hot - not sure how much the heat weighs.

Later on I'll measure them all and work out the original densities etc using Excel, and post.

I forget exactly now - +1.5% for every 10C over 40C or something.

RESULTS:




OBSERVATIONS:

1. The first three samples showed a fairly similar volume loss, but #4, which was from the absolute core of the billet, had a significantly higher volume loss
2. #4 also had the highest weight loss, indicating that there was more moisture in the core than the other pieces
3. The dimensional reductions in all samples were not uniform - the ends shrank more than the middle (despite drilling the 3 holes in each piece). There was about 0.5mm more reduction in both dimensions of the ends as opposed to the middle of each block. An average of the measurements was used for the calcs. This indicates that there is still more moisture in the middle of each block, and that therefore the volume and weight losses would be higher in each case with longer cooking.
4. What were once nice squared off, butter coloured blocks are now twisted and brown!
5. The kitchen doesn't smell and nor does the oven.

CONCLUSION:
The timber ain't ready yet, and probably won't be for a good 6-12 months (as I expected).

It should be noted that the original 4x2 billet, which was 400mm long, was from the end of one of the sticks. Therefore the core of the sticks will be even greener (as evidenced by the differences between #3 and #4 which were the core end and the core centre).

ESTIMATION:
I'd be surprised if the core of the sticks was any less than 20-25% moisture content - way to green for a bench.

Interesting thread. Well done fellas!!

Glad I only have to dry the odd pen blank. Easily done in the microwave.

Perhaps I can qualify a few points in relation to Brett's situation. As he was anxious to build a new workbench, I suggested oven drying a test piece as a throw away comment off the top of my head. I was then called away to tea by SWMBO so was unable to make give precise information.

Since then I have referred to my handbook and I would have given this information too. In particular when we discussed temp I said as low as the oven would go but we did discuss around 150 C.

Oven drying in times gone by was the absolute definitive reference for determining Moisture Content (MC) in serious timber seasoning operations. There are in principle three methods available. Oven drying, Microwave and Infra Red. We will deal here with only Oven Drying.

The temperature is critical at 103 C +/- 2 C. The problem Brett has is controlling at these temperatures in his domestic oven. Too low and the moisture is not driven off and too high (>106 C) and the material may begin to char.

The sample should be a small strip weighing about 50g when dry so perhaps 60g green in the case of Tallowwood.

The drying process will take about 24hrs in a coventional oven or 12 hrs in a fan forced oven.

The timber is then weighed and returned to the oven for 4 hrs (2 hrs in a fan forced). This process is repeated until there are two weighings giving the same reading. The timber sample is then regarded as dry.

Moisture content itself can be determined a number of ways but the easiest is to divide the green weight by the dry weight, subtract 1 and multiply by 100.

Having said all that, the object was to see if he could get a rough handle on wether the timber was just too wet to consider using.

I hope that clarifies things a little.

Regards
Paul

Thanks Paul. Good info which I will use in about six months for the next test (unless I buy a meter). Although, running the oven for that long might break the bank. . Wouldn't want to waste too much time and money - one thing I can't stand is standing around watching the oven heat up.

The weight losses you calculate (9-11%) are equal to the MCs - if these are real, that is about what I would expect for air dried wood in Katoomba and it would be very difficult to get it below 8% without kiln drying and then as soon as the wood was removed from the kiln is would reabsorb water and rebound to about 10% anyway.
This means it would appear the wood is dry enough for use right now so I don't know why you think it needs to be further dried.

However, I doubt your MCs are correct because from what I can see it looks like you only have dried the samples for 6-7 hours
The samples must be heated /dried at 105-110ºC until they reach at constant weight (dry - weight - dry - weigh etc and I would shoot for more than just two consecutive readings) and this needs to be demonstrated using a graph otherwise the samples could be anywhere on the drying curve.
This typically takes 18 to 24 hours.

The way I see it the jury is still out on what the MC is for that wood.

Originally Posted by FenceFurniture

Thanks Paul. Good info which I will use in about six months for the next test (unless I buy a meter).

Yeah a meter is a hell of a lot easier.

Although, running the oven for that long might break the bank. . Wouldn't want to waste too much time and money - one thing I can't stand is standing around watching the oven heat up.

I agree with Paul that a domestic oven will have a poor heat control cycle (its called hysteresis) and is likely to cycle by much more than 2º. The last time I tested our Chef oven it was something like 15º.
An empty oven with a couple of small strips of wood in it is likely to be seesawing up and down like a yoyo which is not good for the drying process either. This could be mitigated ( and power costs reduced) by filling the rest of the oven up with something like bricks which will store the heat and buffer the temperature changes, but this may not be sanctioned by other oven users.

Yeah, it's all pretty rough and ready Bob. The background info is that when I restacked about 3 or so months ago there were sticks there that were obviously still quite moist to the touch and eye (because I'd been slack and hadn't stacked it properly before). No idea which stick the sample came from, and it could easily have been one of the drier ones (and bearing in mind that the sample was from the end of a stick).

The object was to find out not so much if it's dry enough to use, but if it's definitely too green to use. There's also a bunch of 8x2 and 4x4 in there......

I'll repeat this again maybe in April or so, and follow the advised steps closely (oven drying it until there's no difference).

Obviously drying will slow down in the cooler months (and we have 6-8 of them per year), but it's also a very low humidity climate, so I wonder how much drying will go on between April and September?

Just on the subject of drying, the duration varies considerably with species. As Timless Timber stated it can take a long time with the more difficult timbers. Some of the species that comprise Tasmanian Oak are amongst the worst. Up to seventy days for Alpine Ash, for example. The board thickness also has a large bearing on length of time required. general degrade and collapse with subsequent reconditioning are all part od commercial drying. Any timber can be dried, but the trick is to dry it so it is in a useable state for it's intended purpose.

Unfortunately I don't have any schedules for Tallowwood.

Out of interest we are talking about hardwoods here, but if we were talking radiata pine, in a high temperature kiln we would be looking at half a day for a 25mm boards increasing to one whole day for a 50mm board!

Regards
Paul