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I have a dehumidifier and a humidifier in my basement shop. The RH gauge always read between 5-7 percentages variance from each other, skewed in the direction of the purpose of each. For example, if I am trying to achieve 40% RH, the humidifier will read 43% while the dehumidifier will read 36%. I think the manufacturers calibrate the machines to read slightly in their favor. I guess I could take the difference between the two as the accurate measurement, but I would like a "third party" involved to get the true RH reading. So, can anyone give me some advice on a good RH gauge? Will a simple Dollar store temp/humidity gauge suffice? Thanks.

I've had good luck with those:
http://www.amazon.com/Western-Humidor-CALIBER3-Thermometer-Hygrometer/dp/B0007W1EA6/ref=sr_1_1?ie=UTF8&qid=1297779412&sr=8-1
And I calibrate those on the Caliber III's:
http://www.amazon.com/gp/product/B0007VWEQA/ref=oss_product
Hygrometers can be all over the place and give conflicting readings depending on the temp. My temp is pretty constant, and all the Caliber III I bought are within 1% of each other at 70ºF, which is pretty good if you ask me. Some will tell you you need to go the sling psychrometer route, which is fine. Personally I do not think a couple of % points are critical for building, as long as you maintain a constant RH in the 40-50% range.
Avoid cheap hygrometers at the $ store, WallMart or homebuilding/hardware stores, most of them are grossly inaccurate.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Search for psychro-dyne on e-bay and you can find lab grade psychrometers for half price. Right now there don't seem to be any super cheapies on there but I bought mine for around $30.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

I just believe whichever one is telling me what I want to hear.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

I have two thermometers, and one has a piece of cloth wrapped around the bulb. to get a reading, I wet the cloth covered bulb and blow air across it with a small fan. I look up the difference in temperature on a chart that tells me the RH. I also have a little wall hanging hygrometer thing that looks like a clock, and has 0-100% on it. I calibrate it with my readings, but it tends to stay somewhat accurate for weeks at a time, before it starts to vary, ie., if I assume the wet-bulb-dry-bulb method is always accurate.





I have a basement shop also. It's in a 12' x 12' room down there. One wall, the one that I built, has a vapor barrier on it. But humidity can waver greatly, here in Michigan. I'm constantly vigilant.

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"...you have to get over your strict adherence to your largely imagined notion of absolute perfection..."

I apply this principal to all areas of my life. I am a happy, care free guy as a result.

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http://www.CostaGuitars.com
PMoMC

That's true, but perhaps it's worth mentioning that the absolute accuracy of the thermometers is quite important?
I seem to remember that a small change in the observed temperature (dry bulb) changes the reading from the tables quite a lot.
So while the temperature difference is based on the "match" between the two of them i.e. they both read the same temperature in the same environment, that even less than 1 degree out of calibration can make a significant difference to the apparent RH.

I have been using the wet/dry bulb method like yourself to check my mechanical and digital hydrometers, but will soon be going the Potassium Carbonate saturated solution route as I found out that it was much less sensitive to temperature reading errors http://www.natmus.dk/cons/tp/satslt/satsalt.htm
I managed to source it recently, so I just got to find a big enough transparent ziplock bag now!

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I have 4 thermo/hygro's in my shop--collected from various projects (I build wine cellars). I can tell you that the digital one's you buy from Cigar stores are all over the map. +/- 5% within a few feet of each other.

I bought a digital one that allows itself to be calibrated and has an accuracy of +/- 1%. It was marketed by a cigar retailer. I then also bought what is called a Humidipak one step calibration kit. It's a bag with a packet in it that will provide 75.5% constant humidity inside when sealed. Place the instrument in and wait 24 hrs, whatever it reads will now show the variance from 75%. In my case I adjusted the instrument, in yours note the discrepancy and mark how much to add or subtract for a true reading. You are now at the mercy of the accuracy of the instrument. The gauge on the dehumidifier seems to be about +/- 3%for accuracy and about 5%low compared to my calibrated one. I also found by taking careful measurements around my assembly room that RH varies quite a bit by location and some areas are far more stable than others when the humidifier or dehumidifier are running. I chose the most stable location ( under a workbench believe it or not) to store my wood. Hope this helps.
Brian

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

There's always more than one way to achieve good results, but my wet-bulb/dry-bulb psychrometer has never let me down in thirty some odd years. I'm lucky that I get to compare its readings with a friends sling psychrometer every year, just to make sure they agree. Any scientific store should carry these. I use the wet-dry bulbs to calibrate my German made analogue hygrometers every month or so, as the dust makes them read falsely high. I agree with Laurent though ... a couple of percentages off doesn't make a ton of difference.