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With Winter here, the cool, crisp conditions have likely altered the environment inside your humidor, so you'll want to keep a close eye on it. But how do you know that your hygrometer is reading the humidity accurately? A simple calibration will do the trick. Here are a couple of ways to calibrate your hygrometer:
Dampen a towel (not dripping wet, but good and damp), then wrap the hygrometer in the towel for 30 to 45 minutes. Then unwrap it and read the humidity (quickly). If your hygrometer is perfectly calibrated (few are) it will be reading exactly 100% humidity. Most likely, it will be reading somewhere between 80 and 90%. At this point, whatever the hygrometer reads, you can either set the needle to exactly 100% immediately after the test, or if you're a little lazier, make a mental note of how far over or under the actual humidity is from the reading from your hygrometer.
Luckily, as nature would have it, when salt and water (NaCl and H2O for you studious types), are in a saturated solution at equilibrium, the resultant humidity is 75%. This gives a fantastic reference point to calibrate our hygrometer. There is an easy way to determine if your hygrometer is accurate. Here's the procedure you should use: you need a Ziploc bag, a screw-on beer bottle cap (or other small container) a small amount of salt (regular 'ole table salt), and water.
After 8 hours in the damp salt environment, the actual humidity inside the bag will be 75%. Compare it to your hygrometer, your hygrometer should also read 75%. If not, you will then know exactly how far off your hygrometer is. If it's off, note the amount and direction that it actually reads and be sure to add or subtract that amount when reading the hygrometer. If the hygrometer has a control to adjust it (either the needle or the display), you can set the hygrometer to 75% immediately after the test.
You should salt test your hygrometer every 6 months or so to be sure of the accuracy.