First check to see if your hygrometer can be adjusted.
Look at the back to see if the small center post has
a slot for a small screwdriver. If it does you'll
need to have a screwdriver ready to make any
adjustments as soon as you open the jar. If it
doesn't, don't worry, this procedure will still allow
you to use your hygrometer more effectively.
Half-fill a shot-glass or other small dish with table
salt and add enough water to wet the salt completely
but not dissolve it--it should be like wet sand. Put
the shot-glass and hygrometer in a wide-mouth jar, and
seal it airtight. After six hours (or more), the
humidity in the jar is 75%. The hygrometer should read
somewhere around 75%. If it does not, take the
screwdriver and turn the screw on the back of the
hygrometer until the meter reads 75%.
If your hygrometer is non-adjustable and reads other
than 75% in the jar, you must correct all subsequent
readings by this difference. Example: if the
hygrometer reads 67% in the jar, it is reading 8% low.
Thus, it will always read 8% less than the actual RH.
Using this example if you wanted the humidity to be
60-70% then the reading on your hygrometer would have
to be 52-62%.
Dial RH gauges can be off by 20% or more! They are
also usually only accurate to +/- 5% and often take up
to 2 hours to finish stabilizing to a new reading if
the humidity changes. Please remember that it is
simply the nature of the beast and NOT necessarily
poor quality or manufacturing. The spring mechanism
is delicate and goes through a lot (shipping, etc)
before reaching your home.
