Official Luthiers Forum!

Mike-
Minor clarification: the 'relative' in RH refers to the idea that the RH% describes how much water is present in the air, compared to the maximum that the air could 'hold' at that temperature.
The idea of air 'holding' water is not actually correct (water vapor is a common component of air ), but is a convenient simplification.
So, if you have a room at 20 deg C and 50% RH, and seal the room (so the amount of water in the room is fixed), and let the temperature drop, the RH will increase (since cool water forms a vapor less easily). Of course, if you get the room cold enough the water will condense into visible droplets.
So just changing the temperature in the room will affect the RH reading.

One way to reduce the swings in humidity is to make sure that there are a lot of 'sinks' in the room- keep a big stock of wood in the room, for instance. (That's my excuse for the wood collection, anyhow! )

Cheers
John

Relative and Absolute Humidity:Besides the usual information sources like Wikipedia, there are some online RH calculators at:
http://www.natmus.dk/cons/tp/atmcalc/atmocalc.htm
http://www.lenntech.com/calculators/humidity/relative-humidity.htm

The first calculator shows that if you have your shop at 44%RH at 20 deg and let the temp drop to 16 degrees, the RH will be 57% with no water added or taken from the room.

Cheers
John

John,
Excellent suggestion, I'm going to include that into my plan.
I can easily plan out the build so I can brace, the next morning
I can voice / groom and glue the top to the rim that day.

Guys, you've all been great, I've said this before, with out the help
and support of this community I would be in big trouble,
Thanks !!

_________________
Gregg

Thanks for the clarification John. So, let me try again. my chemistry and physics if rather old though.

So, regardless of the cause, RH changes. I believe that an increase in RH slow the rate at which water will evaporate and similarly, a decrease in RH increases the rate things dry out. That's why my towel in Denver is always dry and when I visit NJ, my towel is never dry. What does that curve look like? Is a change of 5%RH points equal across the range, or is a change from 5% RH to 10% RH equivalent to a change from 40%-80%. In other words, does a given change in a low RH environment effect a larger (or faster) movement of wood than it does in a high RH environment?

I'm not sure what to google to find the answer myself.

Thanks for your patience.

Mike

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Mike Lindstrom

Mike-
I didn't know the answer but knew what to look for (I think)....
Equilibrium Moisture Content of wood is the topic to search on...
There's a good explanation and EMC calculator at
http://www.csgnetwork.com/emctablecalc.html

More academic treatment at Wiki http://en.wikipedia.org/wiki/Wood_drying#Wood-water_relationships

Plugging a few numbers into the calculator, shows that there is a bit of difference in the change in EMC depending on whether you're looking at a low or high humidity environment (ie look at 30-40%RH; compare with 80-90%RH), but it's not that significant.

Your basic point was entirely correct- that a change from 30% RH to 60% RH is a doubling (ie 100% change in RH), and shouldn't be thought of as just a 30% change. The EMC calculator shows that at 72F the EMC would go from 6.2% to 10.9%, a 76% increase.

This stuff gets quite complicated, as not all the water in the wood is behaving the same way, green wood vs dry wood, etc etc...

Cheers
John

Thanks John. It's another one of those things that is easy to understand the basic idea, but when you really try to understand, it gets complicated on a hurry.
Mike

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Mike Lindstrom