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Calibrating Your Hygrometer
Can you trust your Hygrometer?
While one can easily feel if a thermometer is reading too warm, tell if a wind direction indicator is incorrect, or see your barometer does not correspond to a nearby airport, how do you know if your hygrometer is reading correctly? Since air moisture is difficult to "feel" and there is no reference authority (airport, The Weather Channel, for example) to compare to your indoor relative humidity conditions, you should periodically test and adjust your hygrometer in order to trust it's readings. While most hygrometers ship to the consumer calibrated at the factory, it is possible that your hygrometer, due to excessive handling, long shelf life, or regular use may need a simple adjustment. But before any adjustment is made, you need to test the hygrometer for accuracy. Try any of these common tests to check accuracy.
Salt Test
Place a small amount of salt in a small shallow open container, such as the cap to a plastic bottle. Add a few drops of water to the salt only to get it moist. It should damp, not watery. Place the container with the salt solution in an air tight sealed container such as a Tupperware container of Zip-Lock type plastic bag.. Next, place the hygrometer in the container and seal. Careful - do not spill any salt or solution on your hygrometer, it could harm the finish of your instrument. Allow to stabilize for at least 6 hours. Without opening the container, check your reading. It should read exactly 75%.
Damp Rag Test
Wrap the hygrometer in a damp (not dripping) rag and place into a sealed container, After a few hours, the hygrometer should read >95% RH. This test is not as accurate as the salt test, nor is it always advisable as the hygrometer case comes in to direct contact with the moisture which can be tarnished and or damaged, but it is quick and cheap.
Sling Psychrometer Test
Arguably the best and most accurate device for checking the relative humidity is with a sling psychrometer. Operate the psychrometer to measure the humidity near where your hygrometer is placed. Compare the readings. If you don't have one, they can be bought online here.
How to adjust the Hygrometer
If one of the above tests produces significant deviation, you should make an adjustment. Most analog (mechanical dial type) hygrometers have an adjustment screw on the back of the case. If there are multiple screws present, it will be the one different from the rest. These other screws are usually screws holding the case together and should be ignored. Turn your adjustment screw slightly to see the corresponding change in indicating needle until it matches the correct relative humidity.
Most digital hygrometers I have seen cannot be adjusted. If your is able to be calibrated, you are in luck. Otherwise, simply note the deviation when making readings.
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