The Salt Calibration Test procedures are:
Materials required: Tablespoon of Plain Table Salt (NaCl)
Two Ziplock-style baggies
Bottle Cap or other suitable small container
Swizzle Stick or other such item to stir with
Distilled Water
Your Hygrometer
Place the tablespoon of salt on the bottle cap. The wide mouth ones that are used on individual serving juice or tea bottles are ideal.
Slowly add distilled water to the salt while blending with the swizzle stick. You want to add just enough water to moisten the salt so that it begins to meld into a thick paste. Do not add enough water to dissolve the salt!
Place the bottle cap with salt gently into the first ziplock bag and then add you hygrometer. Make certain the sensor is exposed and free of being block by the bag.
Seal the bag while capturing some air inside of it. Then place into the second bag and seal again. The double bags are needed to ensure that you really have a good seal as the test will not work if there are any leaks.
Then place the bag in a place that is free of direct sunlight and is of a stable temperature.
Leave undisturbed for a minimum of 8 hours.
Check the reading on the hygrometer through the transparent baggies and it should read on or near 75% RH.
Remember most small inexpensive hygrometers are only accurate to within 3% so do not be surprised if it reads 72% or 78% RH. It is exactly 75% RH within the confines of the bag due to the salt paste reacting with the air, and what your hygrometer reads differently is the amount of error.
What to do about off-reading depends on the circumstances. If your hygrometer has an adjustment potentiometer then by all means try to tweak it to exactly 75%. You should repeat the Salt Calibration Test again after making any adjustments. If your hygrometer doesn't have the ability to be adjusted and the reading is close then don't worry about it ­ just remember that your hygrometer is X% off either high or low.
If the reading is grossly in error and you are unable to adjust it, then I suggest you replace it.
And finally, let me say that hygrometers are not really necessary. You will find over time that you will gain the experience to be able to judge the relative humidity within your humidor by simply touching and smoking your cigars.
-CigarNexus
