Thread: Relative humidity in Australia

16 sounds a tad high.

If you were in Wagga you'd want it a lot lower

And possibly higher on the Goldcoast

Mate it depends on the time of year, and which way the wind is blowing in Sydney. Our Relative Humidity is all over the shop. Usually it's pretty humid, which is why you get powdery mildew on the pumpkin vines in the veggie patch.

BTW, I make acoustic guitars as a hobby, so I have to monitor the RH all the time I'm building and only do certain jobs when the RH is right, 'cause I don't yet have a controlled environment in my workshop.

Anyway, this one morning I go out to the shed and check the hygrometer and it's saying something like 85%RH, and less than an hour later I hear this loud cracking noise coming from the direction of my half finished guitar, I look at it and there's a bloody great crack in the top. I look at the hygrometer and now it's saying 15%RH. The thin guitar top plate just couldn't cope with such a sudden change.

What had happened was that the wind changed, coming off the ocean at first, then swung around bringing one of those hot dry westerlies we sometimes get. Relative humidity plummeted, and bloody fast.

I don't have an answer for you on the moisture content of wood, but don't assume that Sydney has a fairly constant RH. It doesn't by any means, it's just one of those places that don't.

Cheers,

Paul

I suspect that you'd get the same variation in RH through the US too.

Coincidently, I've just been browsing through Donna Menke's Bandsaw Box book. She says she aims to use wood at about 13% - and she is in Texas, I believe - so would be picking up humidity from the Gulf, I'd expect.

(I can't see why bandsawn boxes would be different to other uses, but maybe I'm wrong.)

Actually, in some parts of the US the RH is pretty rock solid, or so I'm told by a bunch of fellow guitar makers over there. You're right tho, in some places in swings around a lot.

G'day. We supply hardwood flooring all over Australia. It is dried to AS2796 standards 9 to 14 MC%. Average is 10.5%. this suits virtually all areas of installation from Melbourne to Noosa. Further north needs higher. West needs lower.

R.H. means nothing. EMC is the one you need to know. It is the one that has the influence on timber stability.

Well, Trevor, in the lutherie world, the common wisdom is that RH means everything. You probably know much more about wood than I do, so I'll defer to you. However, with 16" wide 2mm thick guitar wood they respond quite rapidly to changes in RH even when the wood has been stickered and stabilised for literally years, I guess flooring or thicker pieces don't do that.

Originally Posted by glock40sw

G'day. We supply hardwood flooring all over Australia. It is dried to AS2796 standards 9 to 14 MC%. Average is 10.5%. this suits virtually all areas of installation from Melbourne to Noosa. Further north needs higher. West needs lower.

R.H. means nothing. EMC is the one you need to know. It is the one that has the influence on timber stability.

Trevor could you explain more please?

Shane.

EMC (Equilibrium Moisture Content) Is a measure of the condition where wood and the insitu micro climate will be in Equilibrium.
If the RH is 50% and the Temp is 25Deg C then EMC will be 10.5%. This 10.5% is what the wood moisture content would need to be so that no timber dimensional changes happen. If the wood MC is higher than the EMC it will shrink and dry out until it gets to the EMC (10.5%). If it is lower than EMC, it will expand and pickup moisture until it reaches EMC (10.5%).

However, It the wood is dried to 8%MC and is placed in a 10.5% EMC environment, It will not expand as much as wood that was dried to 9%.
Why? Cause the wood has already been to 9% on its way to 8% and thus will only display minor dimensional change.
We have done exhausting testing on this and it is one of the reasons socalled wood science isn't. Drying timber has a hell of a lot of Black Art Voodoo associated with it. I and my 2 kiln operators have 105 years of hardwood drying experience between us.

We are still learning every day. As the rescource changes, so we have to change the way we dry and the drying schedules have to be tweeked to maintain uniform drying.

Relative humidity on its own cannot be used to indicate drying conditions. It needs Temperature as well. RH is constantly changing from hour to hour where these changes can mean no change to EMC.
EG: 50% RH & 25 Deg C = 10.5 %EMC
65%RH & 19 Deg C =10.5%EMC
See how the RH makes no difference to the EMC.

This is what I am talking about. RH Needs Temp to control EMC which in turn controls the quality of timber stability and moisture content.

BWT. I must need a new job or a long holiday. It is scary that all this stuff just rolls out of my head. Been doing this ( drying hardwoods) every day for 30 years.

I might have a look and see if anyone needs a trainee brothel manager .

Interesting stuff Trevor.

So in that case, how should we prepare timber then? Get it down to below 10% and then seal the timber to stop the temp and humidity from affecting it as much?

With your emc rating above is there a formula that you use to work it out or done by a meter?

I thought about that as a career change once before as well. Unfortunately the boss at the time overheard my joke and as a highly religous type wanted a please explain as to why i still should keep my job.

Shane.

Go to the FWPRDC website and download the Drying best practice manual.
It has everything you need.

Sealing the timber will not stop the moisture induced movement. It might slow it down, but will not stop it.

Originally Posted by BobR

Comments please

With the amount of rain Australia has had the last few years and the levels of water restrictions alot of areas are now experiencing I would have thought our Relative Humidity on average in this grand country of our would be a big "fat" 0.

Trevor, I understand what you have explained. I guess I am looking for some input from forum members who take MC into account when making furniture. As mentioned in my original post, my research in this area (which is mostly US based) indicates a MC of 6-9%, in a workshop that is environmentaly controlled to ensure minimal change during manufacture. It also implies that the "normal" environment for the housing of the finished product is generally within this range (EMC), which made me believe that most US homes/offices are airconditioned/heated. Where a finished piece is to be housed in an environment different to this then the MC should be such that it can live in this location and will therefore require the manufacturing environment to be changed. Sounds like a lot of trouble, but I understand why this may be necessary. I am not aware of any environmentally controlled workshops in Aus (not to say they don't exist), hence my request for local response.

I have friends who purchased good quality wood furniture in Sydney prior to moving to north west NSW. Did not take long for cracks to appear.

BobR,

Quite right we look for 6 to 9 percent MC in wood to be used for furniture manufacture. Whilst almost all production shops here are controlled environment, many hobbiests (like myself) do not have an EC shed. So, the problems you encounter may vary by the part of the country in which you live. Arizona, where I lived previously, had an RH that varied only from 5% to 15 or 20% most of the year. But my current location runs typically 30% in winter to 85% in summer, all over the place. It makes building furniture problematical at times and always looking for joinery that allows for movement.

That being said, it IS true that almost all US homes are environmentally controlled (that is, both heat and AC). So, once you get the piece indoors, it will be in a fairly stable environment. And that environment will have generally 30% RH or less.

Bob, thanks for your input. It confirms my suspicions, but still leaves me with my original delima. The MC of the wood in my shop varies substantially. I can only put this down to the characteristics of each piece in an unstable environment. As mentioned in my initial post, the RH for the east coast varies greatly within a day, and even more over the seasons. I don't think I am going to get the answer I would like to hear, just confirmation of a problem I am going to have to continue to live with.