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
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
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
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
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
The relative humidity in the sealed bag with the salt/water
mix should be 75 percent.
How to Calibrate a Hygrometer - Adjusting After
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