KILN DRYING HANDBOOK 21 
Humidity may be measured in a number of different ways, but the 
wet and dry bulb thermometer is almost universally used for such 
measurement in dry kilns. (PI.9, A.) This instrument is also known 
as a hygrometer and as a psychrometer. ‘The silk or muslin wick for 
one of the two thermometers of the instrument, kept moist by the 
reservoir of water into which it dips, is cooled a certain amount by 
the evaporation of water from its surface when it is exposed to a 
breeze of nonsaturated air, and in turn it cools the wet bulb it incloses, 
thus causing the wet-bulb temperature indication to drop. The 
amount of cooling is constant for any given temperature and humidity, 
provided that the reservoir contains water enough to keep the wick 
moist and that the velocity of the cooling air is sufficient. If the 
amount of the cooling, called wet-bulb depression, and the temperature 
of the air are known, the humidity can be determined by formula or 
by reference to the chart or the table accompanying the instrument. 
In practice the reading of the dry-bulb thermometer gives the air 
temperature, and the difference between that reading and the reading 
of the separate wet-bulb thermometer gives the wet-bulb depression; 
both thermometers are mounted on one panel. 
To obtain accuracy it is essential that the wick be clean and that 
there be a brisk circulation of air over the wet bulb. A velocity of 
at least 15 feet per second is recommended by various authorities, and 
this velocity is desirable for accurate readings at atmospheric tempera- 
tures. At ordinary kiln temperatures, however, sufficient accuracy 
can be secured with very much lower air velocities. 
With certain types of wet and dry bulb thermometers circulation 
past the wet bulb is produced by whirling the entire instrument. Such 
instruments are known as sling psychrometers. (Pl. 9, B.) Other 
instruments are provided with maximum-reading thermometers, so 
that they can be removed from the kiln and read outside. The mer- 
eury or other fluid column in these thermometers must be shaken 
down before they are used again. They indicate only the maximum 
wet and dry bulb temperatures since they were last shaken down. If 
the temperature and humidity variations have been reasonably great. 
during this time the readings will be misleading. 
Table 1 is a humidity chart for use with wet and dry bulb ther- 
mometers. It is based on the difference between the wet-bulb and 
dry-bulb temperature. The dry-bulb temperatures are in the left- 
hand column and the differences between wet and dry bulb tempera- 
tures are in the top row. The relative humidity is given at the inter- 
section of the row and the column. Suppose the dry bulb reads 140° 
F’., and the wet bulb 130° F.; the difference between them is 10°. By 
reading across the 140 row to column 10 the relative humidity will be 
found to be 75 per cent. 
