SM0486
air may occur.
(c) A very simple dew point indicator is illustrated in Figure 20. Air
from the squeeze bulb is forced into the tank.
This causes rapid evaporation of
the ether contained in the tank, which in turn cools the ether and the glass tube.
When the air in the tank is cooled to the dew point, dew forms on the glass tube.
When dew formation starts, the temperature of the ether may be read on the
thermometer. This is the dew point temperature.
Figure 20.
Dew Point Hygrometer
(4) Other types of hygrometers.
The gravimetric method of determining
humidity is employed only when calibrating instruments or in determining the exact
water vapor content of air. In the gravimetric method, a measured volume of air at
a known pressure and temperature is passed over a moisture absorbing chemical, such
as phosphorus pentoxide, and its increase in weight is measured.
(a) Other methods involving a color change of cobaltous salts have been
used. A very simple indicator may be made based on the change in color, from blue
to pink, of a cloth of paper impregnated with cobaltous
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