This thread has been quite enlightening. It got me curious enough to check among my various hygrometers, and predictably, there was a range. Starting from lowest to highest:
The last one is an interesting device, although I've always believed its accuracy to be suspect. I bought it at a yard sale several years ago for probably a buck. It has a small water reservoir in the bottom, which gets wicked up to the wet bulb on the right. There is a series of square boxes on the left side of the roller on top that show the difference in degrees between the two bulbs. You dial in that number and then read off the RH from the scale on the roller beneath the dry bulb temp shown above it. It's reading about 48% in the photo.
I have Hoadley's book,
Understanding Wood. On page 143 it shows a sling psychrometer. Once I saw what it is, I realized the above device could closely approximate it, so I placed a fan in front of the hygrometer and let it blow on the wet bulb, until the temp stabilized. After doing this, it gave a reading of 42%, agreeing exactly with my indoor digital hygrometer. Hmmm . . . maybe it's not all that inaccurate after all.
I believe the differences shown between the two digitals is due more to the fact that the one reading 35% is located outside my shop door, whereas the one reading 42% is located about 15 ft away, inside my shop. Because I'm in my shop about 5 ft away from the second one, and perspiring slightly, I'm raising the RH a bit over ambient. There's also a lot of wood in here, with its bound moisture. However, I'm right next to the analog one and it is now reading 34%. Go figure. But then, I've never particularly trusted that one either, another yard sale special.
At least it has an adjustment screw on the back, though.
Refering to Hoadley's book again, on p.113 is a chart showing equilibrium moisture content vs. RH @ 70F. Correct me if I'm wrong, but it is my understanding that builders like to have the equilibrium moisture content of their wood somewhere between 6% and 10%. On Hoadley's chart, that corresponds to a RH ranging between 35% and about 56%. And that corresponds to the
line his chart shows, and not the acceptable range which extends above and below it. If one uses the acceptable range, the figure expands from 25% to about 64%. So, it would appear that maintaining a precise RH is not all that critical as long as it stays within the moisture content bounds one prefers.
Interestingly enough, the wood Hoadly used to establish this chart was white spruce. The caption of the chart emphasizes its importance to Hoadley. He writes "This may be the most important information in this book. Hang a copy of this graph on you shop wall and look at it every day. . ."
Best,
Michael
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