SM0486
(2) Mechanical Hygrometer.
A mechanical hygrometer indicates relative
humidity directly; it is about 3 percent accurate if both temperature and humidity
are held constant at room temperature. The sensing element is usually a bundle of
human hair that has been put through some process to remove the oil from the hair.
The midpoint of several strands of hair is connected, through a lever arrangement,
to a pointer.
As the humidity increases, the length of the hairs increases and
causes the pointer to move across the dial.
Figure 19 shows this type of
hygrometer.
Figure 19.
Hair type hygrometer
(a) The hair hygrometer indicates relative humidity over a wide range of
temperature, but its reliability decreases rapidly as the ambient temperature
decreases below freezing.
Under changing humidity conditions there is a
considerable lag between the dial reading and the actual humidity.
With
temperature and humidity stable at 77F., a change in relative humidity will require
approximately 5 minutes for the hair hygrometer to indicate 90 percent of the
change. This time lag greatly affects accurate measurement under changing humidity
conditions.
(b) Many organic materials are hygroscopic.
Such materials as wood
fibers and cotton string have been used. None of these seems to be any better than
human hair.
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