Here you go Bro....
6.1 Checking your Hygrometer Calibration
Why should I check my hygrometer's calibration?
Even if you don't want to open its case and adjust your hygrometer, you would be well advised to check its calibration. These are not the precision instruments that some people think... Analog (dial-type) hygrometers are often shipped as much as 20% out of spec. Even the often-recommended Radio shack hygrometer is shipped with a factory tolerance of plus or minus 5 points of humidity through the range 40-80%. That means your unit could read anywhere from 65 to 75% while in a 70% environment and still be considered in spec! I've personally seen up to an 8 percentage point difference between units before calibration.
For a calibration source, we're going to use salt - NaCl - plain table salt... Why salt? The following is a brief explanation of the chemistry, originally posted to asc by Peter Shenkin of Columbia University. Don't worry too much about the technical details - the point is that plain salt will, when used properly will maintain an exact 75% humidity in a sealed environment.
A saturated solution at constant temperature & pressure has a fixed composition. [[ Explanation in terms of the Gibbs Phase Rule omitted. ]] Therefore, such a solution has a fixed vapor pressure. Thus, at constant temperature, no matter how much solid NaCl and how much water are present, the concentration of the NaCl in the water is fixed, just as long as both the solution and the solid phase are present. Therefore such a solution has a fixed vapor pressure.
Now, it just happens to turn out that the vapor pressure of a saturated solution of NaCl in H20 is about 75% of the vapor pressure of pure water at any temperature close to room temperature. This means that at equilibrium, if there were nothing present except this solution and a vapor phase in contact with it (no air), the pressure above the solution would be about 15 mm_Hg, in round numbers, using 20 mm_Hg as the vapor pressure of water near room temperature, again in round numbers. As mentioned earlier, the only gas providing the pressure above the solution would be water vapor. We'll see in a few minutes that the same thing happens even when air is introduced.
OK, class dismissed - you've got it now, right?
For more details about the science behind this calibration procedure, check out the excelent report available at:
http://www.pip.dknet.dk/~pip1985/gemarkus.htm
Now to procedures. You'll need a zip-lock baggie or other see-through container, about a teaspoon of salt, and a small, shallow open container for that salt (I use the screw cap from a bottle of milk) Gee, I love this high-tech lab gear.... ;-)
Place a teaspoon or so of salt in its container and add a few drops of water to get it wet. You don't want to dissolve it, just get a good damp pile of salt in your bottle cap. This is important! Just a few drops! you want damp salt, NOT a solution. As the technical explanation above says, you want the salt present in a wet solid phase. Place it in the baggie, along with your hygrometer. CAUTION - DON'T SPILL ANY SALT ON YOUR INSTRUMENT! Be sure the Hygrometer isn't in its "min" or "max" mode, BTW.
Seal the baggie with some air trapped inside (so it's not tight against the hygrometer) and let it sit. Allow this to stabilize for at least 6 hours. (don't rush it!). After the internal "system" has stabilized, check your reading WITHOUT OPENING THE BAGGIE. It should be exactly 75%. If not, note the deviation - this is how much your hygrometer is out. Don't be surprised if your reading up to 5 points out - unfortunately, that's the factory tolerance of this instrument.
Note that several readers have had occasional problems with poorly sealing baggies. Use a small tupperware container, or double-bag your experimental setup will help avoid inaccuracies.
