Relative humidity at different temperatures

[Peter Laine]

 I live in Trinidad West Indies, it's hot, very hot, and it rains a lot. I  repair guitars and am getting in to building. I have worried for some time that the humidity here might just place Trinidad geographically out of the prudent guitar building zone.

    However, I have been monitoring the  R/H for some time with an electronic sensor and the range seems to be o.k. today for example it's 92° R/H 32%.   During weeks of heavy rain I have seen as high as 60% but this is not common.    40% would be closer to an average

 Air dried wood seldom goes below 15%-16%  measured with a field type( not prong) moisture meter.

 it got me to thinking.  Because the measurement concept is "Relative" Humidity, there is more actual water in solution at a higher temp ,given the same R/H reading. When an authority gives a range of acceptable R/H    in say New Hampshire, because his ambient temp is lower,   the total gram weight of water in solution is less.    say 75° R/H 33%   against  92° R/H 33%

Which gets me finally to my question. does wood take up water more at higher temperatures ( given equal r/h). or has it been worked out that the  r/h% number is the guide and the wood shrinking and swelling is equal given equal R/h

[harrison phipps]

Saturated warm air holds more water vapor than satutared cooler air, a lot more. Water vapor pressure at saturation is an exponential function of temperature. Building an instrument at 40% RH at 90 degrees is fine unless it gets taken somewhere cooler or dryer. Here in California central velley, I rarely need it, but on occasion I rely on an electric dehumidifyer to get water vapor out of the air in my shop.

[Jeff Highland]

I would suspect that your RH meter is not reading correctly

REALLY

60% in a rainy week is not realistic.

RH is the principal governing factor in what equilibrium moisture content the wood in that environment will attain regardless of temperature, Typically you are looking for about 8% water content in the wood.

[Andrew]

Yeah i would doubt thats accurate too. Here in Ontario if its 90 degrees and raining for a week you can bet on 80 to 100 percent RH.

[Peter Laine]

 which instrument are you using to measure, I doubt this electronic thing is accurate, do I need a sling psycrometer, or a " hair" type

[Peter Laine]

 I could kiln dry wood to 8% but withing a couple of days it would reach equibrium at 15% to 16%

[Kerry Krishna]

If it has been raining in my town, for more than two days, and if my outside door is open a lot, the reading can go up to 80%.

I think your humidity reader is wayyyyy out... 

[Peter Laine]

I believe that is possible, and one question I have is which instrument would be considered adequate to measure this accurately.

however when it rains here, it's usually  for a couple of hours then the sun comes out again quite fierce. so the RH on average might be lower than expected

[Ned Knepp]

Hi Peter, 

 The rain then sunshine is practically guaranteed to boost your RH to high numbers. As Harrison pointed out, the warmer the air, the more moisture it can hold. That "fierce" sun will start evaporating water very quickly and the humidity shoots up. Your RH will probably be less stable because of it but if this is the way it usually is, it will almost always be high. 

[Peter Laine]

Thanks Ned;

    any advice on an instrument to measure it? been looking for a while and it seems to be a very difficult problem sling psychrometer doesn't seem practical, don't know if I can trust any electronic units, a good Hair type is 150 bucks plus, actually a very sticky situation.  I'm concerned because, as I mentioned the humidity might just preclude building guitars here in Trinidad.

[Greg Mirken]

There are reasonably accurate electronic hygrometers. Look at laboratory suppliers or hatchery suppliers. For $100 or so you should be able to get reliable +/- 5% accuracy, which is enough. $20 Radio Shack units are just about worthless.

[Peter Laine]

thanks, I'm looking into an analog abbeon

[Peter Laine]

I guess I should have done a bit more looking here is one answer

http://www.violinist.com/discussion/response.cfm?ID=17369