(c) The temperature of the water used to moisten the wick should be at,
or slightly higher than, the wet-bulb temperature.
This is especially important
temperature of the water used to wet the bulb is too high, it may take a long time
for the bulb to cool to wet-bulb temperature.
Before this point is reached, the
water may have evaporated sufficiently so that the thermometer never reaches the
If the moistening water temperature is appreciable lower
than the wet-bulb temperature, the thermometer temperature will climb throughout
the period of ventilation, remaining constant at the wet-bulb temperature only as
long as there is sufficient water to keep the bulb surrounded with a film of water.
If the temperature of the water used for moistening is at, or slightly above, the
wet-bulb temperature, the wet bulb will quickly attain the wet-bulb temperature and
remain at this value long enough to be easily and accurately read.
(d) Ventilation is obtained by
swinging, slinging, or whirling the
thermometers at such rates as to produce
the minimum velocity of 900 ft/min.
Stationary thermometers may be ventilated
with a motor driven fan, so long as
minimum velocity is attained. Unventilated
(e) The dry-and wet-bulbs of the psychrometer must be separated.
is to prevent the air that passes the wet-bulb (and is thereby cooled by
evaporation) from contacting the dry-bulb and causing an erroneous dry-bulb
reading. In the case of a sling, or whirled, psychrometer this may be avoided by
placing the thermometers so that the air will flow across the dry bulb before
reaching the wet bulb. Therefore, a sling psychrometer should be swung in only one
direction, depending upon its construction and the placement of the thermometers.
(f) The heat absorbed by the wet bulb, due to radiation, tends to raise
the wet bulb temperature so that a true depression is not attained.
This can be
minimized by radiation shielding.
One method, as shown in Figure 18, is to
surround the wet bulb with an external primary metal shield and insert an auxiliary
shield with a moist wick. When the thermometer is ventilated, the auxiliary shield
attains a temperature close to that of the wet bulb. This practically eliminates
the source of radiation and conduction due to the difference in dry- and wet-bulb
(g) After the psychrometer is ventilated and the difference between the
dry-bulb and wet-bulb thermometers is determined, a chart similar to Table 9 is
used to compute relative humidity.
Notice that with a given temperature