How do you use hygrometers?
Fill the water bottle and screw it on to the instrument.
Then hang the hygrometer (by the loop at the top) on a
wall or a tree, for instance, for at least five to 10
minutes
before taking the measurements.
Ideally the instrument should be in a well-shaded spot
or in a Stevenson screen (refer to "Making your own
weather station" in Section 4: Contacts and further
information).
The instrument contains two alcohol-in-glass thermometers with a
scale of minus 10 to plus 110 degrees
Celsius, in 1 degree increments. The
end of the 'wet-bulb' thermometer is
covered with a damp cloth dipped in
water. The other thermometer is the
'dry-bulb'.
6 Record the wet-bulb and dry-bulb
temperatures.
6 Calculate the wet-bulb depression
- the dry-bulb temperature minus
the wet-bulb temperature.
6 Determine the relative humidity
(%) using the table (reverse
page). Read down from the wetbulb depression and across from
the dry-bulb temperature.
do's and don'ts
Be careful with the instrument. The thermometers are
made of glass and can easily break.
Empty the water bottle before returning the
hygrometer to its box.
how it works
The hygrometer works on a similar principle to the whirling
psychrometer. The main difference is that the hygrometer
is not rotated, so evaporation from the damp cloth around
the wet-bulb thermometer occurs more slowly.