The hygrometer is the instrument that measures the amount of water in the atmosphere of your humidor. Because of the delicate nature of the device it is possible for it to become inaccurate over time. Ensuring that your hygrometer is accurate is essential since this is the instrument used to make any necessary adjustments in humidity. If your hygrometer is not accurately measuring the RH, then you may be making adjustments that are unnecessary. This is especially important for new hygrometers as they can become inaccurate just from shipping. If you find that you are having trouble maintaining your ideal humidity, then prior to making any changes you should first perform some of the following steps to test your hygrometer.
There are several ways to test the hygrometer. Following are the more common ways:
The Salt Calibration Test
The Salt Calibration test is one of the more popular techniques and requires common household items. One of the unique characteristics of salt is that when mixed with water and placed in an air tight container, it will maintain a RH of 75%. By conducting this test you can then adjust your hygrometers reading to account for the difference between 75 and what your hygrometer reads.
Ordinary table salt
A bottle cap or cap from a milk jug
A zip lock bag or small Tupperware container
Fill the cap about 3/4 of the way with your table salt.
Add just enough distilled water to make the salt moist. Mix with something like a toothpick. Do not put too much water. It should be moist like wet sand, but not runny. If it is too runny then use a paper towel to dab some water off or add more salt.
Place the cap with the salt and water mixture along with the hygrometer in the zip lock bag or small Tupperware container. Be careful not to let any of the mixture touch the hygrometer as it could damage it. Be sure to seal the container tightly so no air can transfer in or out. If you are using a zip lock bag be sure to leave just enough air in the bag so that the bag is not tight against the hygrometer.
Allow it to sit out of direct sunlight or changing temperatures (away from HVAC vents) for at least 8 hours.
Check the reading without opening the container, if possible. As soon as you open the container, the RH will drop.
The hygrometer should read 75% RH while sealed in the container. Calculate the difference between what the hygrometer's reading is and 75. If your hygrometer has an adjusting screw, then use it to calibrate it to 75. You will need to do this quickly since the RH will drop rapidly once removed from the container. If not, then just make a note of the difference and make the adjustment mentally whenever you read your hygrometer. As an example, if after the calibration test your hygrometer shows a reading of 67%, then you have a difference of 8% (75% - 67% = 8%.) When you read your hygrometer in the future just remember to add 8% to whatever reading it shows in order to have an accurate measurement of the RH in your humidor.
The Wet Rag Test
The wet rag test is less accurate in my opinion, but if nothing else, it can give you a way to confirm your adjustment from the salt test. This test basically involves taking a rag and wetting it with distilled water to the point that it is thoroughly wet but not dripping. Wrap the hygrometer in the wet rag and leave for an hour. The reading after the wet rag test should be 95% RH. Adjust the reading for any differences.
---------------------- Additional Notes from member Baboon:
This article contains a basic knowledge which may become more useful with some experimental practice. Try your hygrometers on both tests (1. wet salt 75% and 2. wet rag 95%). In most cases you will see a different deviation. For example - if you see 70% on a "salt test" on three devices - you may suppose that your devices have "-5% deviation" and must show 91 on "wet rag" test. But wet rag test shows you only 83% on the same device (99% on another, and even 105% on third one). What does it mean? A one simple thing: most of cheap mechanical devices with arrow-pointer based on bimorph spiral spring have a nonlinear dial-scale in reality. But most of manufacturers use a linear (proportional) scale in humidor dashboard because it looks better. So we can trust only in one point of measurements on a gauge. This can be 65%, 70% or 75% - any level you like. To build the 75% etalon is relatively easy task. Use a wet salt as described above. To have 65% or 70% calibration-etalon - use a humidifier together with crystals in one small hermetic glass box or neutral plastic container (or simply use an empty humidor with charged humidifier). If you have exact and lab-checked electronic device - use it for more exact measurement of humidity in your "testing camera". Anyway you must choose what target level you need, achieve this level in calibration camera, and then put your mechanical devices into camera (together with electrical device) for 3-5 hours or more (Some spiral devices require even 24 hours - to finish an exact positioning of arrow completely!). After that you must check current deviations on each of mechanical devices and then correct it by screwdriver. Put the devices to camera again to check that an indicated RH-value is now equal to real RH-level shown by digital device. That's all. You may trust to mechanical devices only at this base point (and in area -5% +5% around base point). It is enough for practical goals.
Also note - a RH-value shown by mechanical devices depends off the initial RH-level which device had before testing. Device placed to salty camera after wet rag test shows 76%, but the same device shows 72% in the same salty camera after a dry room conditions. 4-8% of lapse is normal for cheap devices. Temperature changes provoke similar things with spiral devices too. Test and calibrate your devices at a usual room temperature.
Repeat re-calibration or testing every 3-4 month at first year (and every time after heavy mechanical shocks of device (bump, dirt, etc..)). Cheap devices are not stable at first year after producing. If you get closer measurement values two times successively during a half of year - then you may trust in device more and test it less often.
Conclusion: Simple mechanical hygrometers are not measurement devices. These are only indicators of whether the current internal humidity is at an optimum level or not.
PS: Do not forget to renew batteries in electronic hygrometers.
PPS: Also do not forget to close a hole on humidor while inlay-type hygrometer is out. Use two slices of wide scotch tape.