calibrating a hygrometer


just got some cheap humidor hygrometers, I wanted to place them around the house in various locations, mostly to ensure that I wasn't using dehumidifiers any more than necessary in the interest of saving energy. DAGS reveals many links to calibrating a hygrometer using a capful of damp salt in a sealed container; supposedly this will cause the relative humidity to stabilize at exactly 75%. Very convenient, but why/how does this work? just curious...
nate
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http://www.veriteq.com/humidity-temperature-data-loggers/dataloggers - faq.htm
tells about using salt solutions;
Is there a quick and easy way to calibrate relative humidity in the field?
Relative humidity is a difficult variable to calibrate, primarily because of its sensitivity to even minor temperature changes. Effective relative humidity calibrations require highly stable humidity and temperature conditions, an accurate reference device, and lots of patience. Unfortunately, this kind of calibration is difficult, if not impossible, to perform properly outside of a carefully controlled lab environment.
Some suppliers offer field humidity calibration kits using saturated salt solutions that create "fixed" relative humidity reference environments. Although convenient to use, the kits are meant for field use and can be significantly affected by ambient temperature conditions, making it very difficult to achieve repeatable or reliable results. For information on performing humidity calibrations using salt solutions, refer to ASTM Standard E104-85 "Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Solutions".
If you have a simple thermometer that measures in the range of room temps,you can use the "wet bulb/dry bulb" method.Record the temp with the thremometer,then put a sleeve of cloth over the thermometer's bulb and saturate it with water. Then you circulate air over the thermometer(small fan) for a couple of minutes and get a 'wet' temp. Then you need a chart to give you the RH.
http://en.wikipedia.org/wiki/Psychrometrics
I had to calibrate RH chart recorders in cal labs.
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Jim Yanik
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We can measure the dewpoint very accurately by stirring a glass with some water and an ice cube with a good thermometer and noting the water temp when condensation starts to appear on the outside of the glass.
Then find RH = 100e^(-9621(Ta-Td)/Ta/Td), where Ta = Troom(F)+460 and Td = Tdp(F)+460. For instance, if Troom = 70 F and Tdp = 50.5, RH = 100e^(-9621(530-510.5)/530/510.5) = 50%. The exponential function e^() is the inverse natural log (shift ln) on an Casio fx-260 calculator ($9 at Walmart.)
Nick
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I read somehting that made sense, a while back. Put the hydr in a box or bag with a bunch of paper towels soaked with water. Wait half hour or so. Figure that's got to be essentially 100% humidity, and set the gage for 100.
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Not easy but I've determined RH using a Psychometric Chart and a couple of fairly good thermometers. Wrapped the bulb of one with some wet (wrung out) tissue and held right it in front of a fan to get a good, consistent reading. Got reasonably good correlation when compared to a hygrometer that I have on the wall. MLD
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That is basically what my Bacharach sling psychronometer does but it has a slide rule style calculator as part of the case to get to RH. http://www.bacharach-inc.com/sling-psychrometer.htm
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Taylor stated to wrap the unit in a wet rag and set to maybe 95% on my unit every 6 months. Calibration is important, at stores you will see easily a 15% difference inbetween units. I find electronic units much more accurate
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I would suggest putting them all on the kitchen table. Let them sit there for a hour or more and see how close they are. Pick on with a reading toward the average and then adjust the others to it. You should be really close. Having the exact humidity is far less important that having what works for you (like above the point of growing mold.
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