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Accurate RH gauge?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=31120		 Page 1 of 1
Author:  Goodin [ Tue Feb 15, 2011 8:30 am ]
Post subject:  Accurate RH gauge?
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.
Author:  Laurent Brondel [ Tue Feb 15, 2011 9:26 am ]
Post subject:  Re: Accurate RH gauge?
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.
Author:  Andy Birko [ Tue Feb 15, 2011 9:40 am ]
Post subject:  Re: Accurate RH gauge?
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.
Author:  Howard Klepper [ Tue Feb 15, 2011 12:49 pm ]
Post subject:  Re: Accurate RH gauge?
I just believe whichever one is telling me what I want to hear.
Author:  enalnitram [ Tue Feb 15, 2011 1:24 pm ]
Post subject:  Re: Accurate RH gauge?
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.

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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.
Author:  Tony_in_NYC [ Tue Feb 15, 2011 2:19 pm ]
Post subject:  Re: Accurate RH gauge?
Howard Klepper wrote:
I just believe whichever one is telling me what I want to hear.



I apply this principal to all areas of my life. I am a happy, care free guy as a result.
Author:  Colin North [ Tue Feb 15, 2011 2:53 pm ]
Post subject:  Re: Accurate RH gauge?
enalnitram wrote:
ie., if I assume the wet-bulb-dry-bulb method is always accurate.


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!
Author:  rfm [ Thu Feb 17, 2011 7:54 pm ]
Post subject:  Re: Accurate RH gauge?
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.
Author:  B. Howard [ Thu Feb 17, 2011 8:14 pm ]
Post subject:  Re: Accurate RH gauge?
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
Author:  David Wren [ Thu Feb 17, 2011 8:14 pm ]
Post subject:  Re: Accurate RH gauge?
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.
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