Official Luthiers Forum!

You would have to explain why my wooden Hygrometers track my digital and my sling psychrometer - at least they do within the confined range of 35% RH - 60% RH. Above or below those figures, I'm not really concerned.

Michael, yours works because you keep it to a relatively narrow RH range. If it begins to be exposed to very high or very low RH environments for extended periods, all bets are off. But the beauty of these is that if/when that happens, you just make a new one, and that's the reason I'm commenting on this thread; these things are fine, and cute and every visitor to your shop will enjoy seeing and learning about it, but don't rely on it as your fail-safe, month to month, year to year hygrometer, because they do have a limited shelf life, and they do need to be checked against a known RH from time to time. So, think of these as an addition to the little "family" of hygrometers I would hope you all have in your shops.

And please, please, PLEASE, find a more accurate "known" RH than the weatherman on TV or the radio! Unless you live next door from the weather station they are getting their figures from, the information they give regarding RH is utterly useless. RH can and will change dramatically within short distances. Ever been driving down the road on a cool pre-dawn morning, and went-through a slight dip in the road, and suddenly had to hit the wipers because your windshield suddenly is covered in moisture? That's dew, and that happened because the air in the 4 foot deep "dip" in the road was a wee tad cooler and had a higher RH than the air in the higher part of the road. See what I mean? If you live even a few feet lower in elevation than the weather station, your RH will be very different. Farmers who live in rolling-hill country know this all too well, too; in droughts, the crops in the lower lands will fair better at first because they get more, and longer, dew in the mornings. The flip side is that the same crops are the first ones to see a frost.... And if you happen to live down the road from the airport where the weather station is, and the land is flat, don't count your blessing just yet; if a neighbor or two watered their lawns overnight, they just raised you RH... See my point? Use a better method of calibration than the radio or TV, fercryingoutloud.

Another thing....

Just because a well-known builder or two appear to have had nice long careers with little care or knowledge, on their part, of RH, doesn't mean that we can all be. Some just happen to live in areas with ideal or near-ideal RH, and very small day to day, season to season RH changes. Good for them!

The rest of us need to monitor our shops' RH and take corrective action when necessary....

I made mine 4 years ago - just after the thread linked in the very first post. Yesterday my digital was reading 48% RH, today the weather has become distinctly colder and the digital is down to 40% RH. My wooden Hygrometer has tracked it and in a manner that it always has i.e. almost the distance between one of my markers. I'm certain that it's still displaying accurately. Of course the Potassium Carbonate may give me further insurance - I'm still waiting for it to arrive. Between the digital, sling Psychrometer and the salt test I guess that they will give me a reasonable indication of RH in that narrow range. - that narrow range is all we need. If the salt test says different. . . . back to square one

Echoing what Mario said about the 'good old days'.

The luthier I knew in Majorca in the '70s didn't have a hygrometer. He did, however, know when the dry season was, and did all of his assembly then. Off season he did binding and finish, and made parts. I'd also be willing to bet that most luthiers as recently as a hundred years ago didn't ship things very far, so the climate conditions were not as much of a variable. Finally, keep in mind that central heat has played hob with instruments since it was introduced. We're a lot harder on them now than the old boys were.

One reason for the hysteresis effect may be that humidity cycling helps to break down hemicellulose, which is the part of the wood that actually absorbs moisture. Over time it gets turned into H2O and CO2, and goes away, but it takes a while. I should try some of the old spruce I got from the covered bridge, that was cut in 1869, for one of these. It might take quite a wile to find out if it's actually more stable, though. Anybody got some ancient Kauri?

I'm in the camp that no matter what kind or type of hygrometer you use, make sure that you have a dependable way of accurately calibrating it at least twice a year. I have always used a psychrometer for calibration, and recently tried Mario's method for checking dewpoint as explained on MIMF. I am now going to try the potassium carbonate... can't ever be too sure.

Chuck

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Well I've tried the Potassium carbonate. After 9 hours the reading seems to be stable - at 45.4% RH. Within 2.5% of the stated 43.16% for that salt. Not bad for a digital hygrometer that is over 4 years old. That supports the readings I was getting from the sling psychrometer and also verifies the accuracy of the wooden hygrometers.

Michael, I just started the potassium carbonate test, amazing how much heat is generated when combined with water. I'd be happy if my digital hygrometers read as close to 43% as yours.

Chuck

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Tonight I tried the method Mario suggested for measuring RH and it worked great.

My thermometer read an ambient temp of 71.6F in the kitchen. Pit water in a thin glass with 5 ice cubes and slowly stirred with the thermometer. Took a couple of minutes but when moisture started forming on the outside of the glass the thermometer read 33.8F.

I plugged these 2 numbers into the spreadsheet I made and it calculated 25%RH.

As a sanity check I looked at the Aprilaire digital humidity gauge I have in the adjacent room and it read 71.8F and exactly 25%RH. Pretty close!

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Formerly known as Adaboy.......

After the Potassium test I tried it with common table salt. The Potassium is result is very close to my default shop humidity and I was concerned that the container wasn't sealed correctly. After 12 hours it read 70% RH - 5% out, a little worse than at the 45% level. At least i know that the container is sealed from the shop environment.