Cheap hygrometers are notoriously inaccurate, especially those from
[retail stores dealing in 'hobby grade' instruments'].  They might be
okay for watching relative changes but these things never seem to be
calibrated at all.  Buy two, put them side by side and you may be
shocked.

Accurate measurement of humidity requires either a well made (read
'expensive') calibrated instrument or a wet/dry bulb thermometer type
which can be very cheap or even home made and will be very accurate.

The absolute best is a sling psychrometer which you should be able to
find on E-Bay or elsewhere for not a lot of money.  This can be used to
calibrate a cheap one if you want something with digital convenience.
Just make a chart comparing the psychrometer reading to the cheap gauge
at various times and make a calibration chart.

You can also calibrate a hygrometer to 100% humidity, at least, and get
an idea how wrong it might be.  Put the hygrometer in a closed container
with a wet rag and leave it there for a while being careful, of course
to not get the instrument itself wet.  It should read 100% or very close
to it.  If it doesn't I wouldn't be too confident in any other part of
the scale.

Best regards,
Spencer Chase