The psychrometer

SM0486

(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

when the ambient temperature is high and the relative humidity is low.

If the

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

wet-bulb temperature.

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

psychrometers are unreliable and hence

rarely used.

(e) The dry-and wet-bulbs of the psychrometer must be separated.

This

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

temperatures.

(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

difference, the percentage of relative humidity depends upon

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