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A hygrometer is an instrument used to measure
relative humidity.
Relative humidity is a measure of how much
water the air is holding, relative to the maximum amount of water the air
could possibly hold at a given temperature. Because of the delicate design
of hygrometers, it is easy for them to become inaccurate (e.g. with
shipping or just over time). A simple method of calibrating your
hygrometer will help ensure you are measuring the relative humidity as
accurately as possible. Hygrometers should be calibrated yearly.
Hygrometers are available in both digital and dial models, and both should
be checked for calibration.
Some dial types will have a small screw or
adjustment on the back so you can adjust them after calibration.
Tip: give a hygrometer about 2 hours to
stabilize before taking a reading, as changes in the relative humidity may
take a while to register accurately on a hygrometer.
To calibrate a hygrometer you will
need:
1/2 cup table salt
approximately 1/4 cup water
coffee cup
hygrometer
large re-sealable freezer bag
Place 1/2 cup of salt in the coffee cup, and
add the water. Stir for a bit to totally saturate the salt. The salt won't
dissolve in this amount of water; instead, the salt should have the
consistency of wet sand.
Carefully place the cup containing the salt/water mix in a re-sealable
plastic bag. Place the hygrometer in the bag, away from the cup of salt
and water. Note: make sure none of the salt/water mix comes in direct
contact with the hygrometer, or the hygrometer may be damaged. Completely
seal the bag.
Place the sealed bag aside at room temperature for 8-12 hours. Pick a
location free of drafts, out of direct sunlight, and away from heating or
cooling vents. The temperature should be fairly constant.
After being in the sealed bag for 8-12 hours, check the reading of the
hygrometer. It is best to read it while still in the bag, since if your
house air is dry the reading may go down quickly once you take the
hygrometer out of the bag. The relative humidity in the sealed bag with
the salt/water mix should be 75 percent.
How to Calibrate a Hygrometer - Adjusting After Calibration
If yours is the adjustable type, adjust the screw or setting so that it
would have read 75 percent. You will have to do this very quickly, or
remember how much you need to adjust the setting (e.g. if it read 72
percent when it should have been 75 percent, you would need to set it
ahead by 3 percentage points).
You may want to put the hygrometer back in the
bag for another 8 hours to double check your adjustment.
Remember: always give a hygrometer about 2 hours to stabilize before
taking a reading, as changes in the relative humidity may take a while to
register accurately on a hygrometer.
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Hygrometer Setup
