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Calibrating Your HygrometerCan 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 the 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.
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
One of the fastest and most traditional devices 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. Sling Psychrometer
Humidipak One-Step Calibration Kit
We now sell a great tool for checking the accuracy of your hygrometer. The Humidipak One-Step Calibration kit for hygrometers utilizes the patented Humidipak two-way humidity control packet within a 5" x 5" zip-seal bag. This packet is guaranteed to produce a 75.5% RH within the bag which you can compare your hygrometer to. Humidipak Calibration Kit
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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