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
RH is a topic that we have done a lot of research on and testing as well.
First, as Jeff rightly said, your digital hygrometer likely sucks. No offense intended, in our experience every digital hygrometer except the multi-hundred dollar ones that we tested completely and utterly sucked. Even the very few that did come in close had the issue of range. What I mean by range is that they may be close to accurate in say the 40 - 50% range but above and below this they can be 20% off....
There is no substitute for a wet-blub test and all you need to do this is two lab thermometers, some gauze, something to move air, a few drops of water, and Internet access for the conversion charts. There are also variations on the wet bulb such as sling psychrometers and psycholdynes (spelling error likely...). In both cases it's simply a wet bulb test that is either employing humans to move the thing or electricity to turn a fan.
It's important too to understand that RH is constantly variable. Move across the room, breath in the right direction, and it can skew your readings. This is why when one purpose-builds a Lutherie shop specifically for instrument building doing all that we can to contain the space, insulate it, vapor barrier it, etc. is a good idea.
Below are some pics of some of the hygrometers that we have tested:
What I do is do the wet-bulb test either with a psychro-dyne (got it out to see how to spell the thing...) or use my two lab thermometers and do a wet bulb test. I compare the results approximately twice a year to my Abbeon and then calibrate, if need be, the Abbeon to match the results of the wet-bulb tests. Even though I do this around every 6 months the analog hygrometer does drift a bit over time. My experience has been very little drift, perhaps no more than 2% but it's enough for me to want to recalibrate the thing. You can also send Abbeons to California to the company for calibration including two levels of calibration of which one level is more expensive and to a NIST standard.
Here is my Abbeon and it's what I look at daily when building. Since it's calibrated against an actual wet-bulb I am sure that my readings are pretty close and this gives me great piece of mind. Again, your shop space and doing all that you can to stabilize RH is hugely important too. My shop was purpose built, insulation, vapor barriers, and built on sand, a giant kitty litter box so to speak...
Something else that is worth a good conversation is what RH is right for you, Peter, where you are. I shoot for 42 - 48% here in the midwestern US but in other climates this range may not be correct especially if the resulting instruments are to live in other climates. More specifically what I am saying is that for example instruments built in Arizona where it is very dry and if they are staying in this dry climate a much lower RH range is probably a decent idea. Conversely instruments intended for wet climates and built in same likely would do fine built in a higher RH range. Where we get in trouble is an instrument built at say above 55% RH that moves to a desert climate - I can hear the cracking now....
Anyway RH is important, it's not an easy subject either in so much as it's constantly variable and yes temp does impact it too. RH stability in a desired RH range is the goal and in my experience the only way to get there is a purpose-built area/shop or the perfect climate...
And before anyone suggests that the great violin makers or classical makers of Europe didn't have Abbeons what they likely did have was experience with RH from the school of hard knocks. This experience told them that certain times of year were better for certain things such as during the wet season make your apprentice cut lots of wood and during the "correct" season time to glue-up braces. It was less precise of course but how many of these instruments are still singing?
And lastly a very related topic is seasoning. Seasoned wood is more forgiving with RH swings where unseasoned wood isn't.... As such I won't build a thing unless the wood has been in my stash for at least five years.
My deepest thanks for taking the time for such a detailed response.
I find there are many subjects that I feel I understand, but upon closer examination I realize my understanding is less that I had imagined. So it is with RH, I have been a boat builder my whole life. I own and refer to many reference books some out of print and obtained with difficulty. But as it is with tools, building guitars requires one to step up his knowledge game.
Everyone speaks of building around, and managing RH, but the actual tool is never mentioned. In the last few months I have been attempting to arrive at a reliable method just to measure it and establish an ambient profile as it were. My initial attempt with an inexpensive electronic meter obviously failed. The reference is obviously the sling psychrometer, (as the triple beam is my reference for weight) but is not convenient on a daily basis, an automatic psychrometer ( psychro-dyne?) in my shop is going to need regular cleaning. I determined that the Abbeon was the tool I needed and I bought one. I'm relieved at your confirmation. Did you publish your tests on hygrometers? it would be very informative.
Thank you again
Another method to find accurate RH is by knowing the room temp, dew point, and doing some calculation. What you need are a glass of water, some ice, and a good thermometer. Firstly, read and record the room air temp, then gradually lower the temp of glass water with ice stirred in it until the dew appear and that temp is your dew point. With both temps, you can calculate for RH. There is detailed explanation on this by Mario Proulx in another forum here; http://www.mimf.com/phpbb/viewtopic.php?f=12&t=1701&p=15583....
You can use this method to check the accuracy of your hygrometer from time to time.
Absolutely brilliant, and extremely helpfull. In search for accurate Rh measurementI bought a used abbeon analog hygrometer and a psychcro-dyne to calibrate it, now I can calibrate the reference ( there lies madness)
Here's an ad for a used Hair and an Abbeon together for $160.00.
Not mine, yada yada.
160 is what a new abbeon sells for. I bought a used on on ebay for 40, hope it's not beyond repair