16 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 
HUMIDITY MEASURING INSTRUMENTS. 
Since the humidity in the air determines the drying characteristics 
at any given temperature, the control of humidity in the kiln is of 
prime importance. It is essential that the moisture be removed 
from the wood surface at the maximum safe drying rate. If the 
humidity is too low the wood will dry too fast and will be injured; 
if the humidity is too high the drying will be slow and expensive. 
Humidity is measured by an instrument variously named a 
hygrometer, a psychrometer, or a wet and dry bulb thermometer. 
Modifications of the instrument adapted to dry-kiln use bear trade 
names, but the principle underlying all modifications is the same. A 
wet surface exposed to a breeze of nonsaturated air will be pooled a 
certain amount by the evaporation of water from the surface. The 
amount is constant for any given temperature and humidity. 
Knowing the amount of cooling, called wet-bulb depression, and the 
temperature of the air, the humidity can be determined by formula 
or by reference to a humidity chart. The wet and dry bulb ther- 
mometer consists of two separate thermometers, mounted on a panel. 
One, the dry bulb, registers the temperature of the air; and the 
other, the wet bulb, registers the air temperature minus the wet-bulb 
depression. The wet bulb is equipped with a silk or muslin wick 
dipped in a water reservoir. The wick surrounds the bulb of the 
thermometer and keeps it wet by drawing water from the reservoir. 
The evaporation of this water from the bulb produces the cooling or 
wet-bulb depression. 
To obtain accuracy it is essential that the wicks 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, 
but this is more than is needed, except for the most accurate work. 
With certain t37pes of wet and dry bulb thermometers circulation is 
produced by whirling the entire instrument. Such instruments are 
known as sling psychrometers. Other instruments are provided with 
maximum-reading thermometers, so that they can be removed from 
the kiln and read outside. The mercury or fluid column in the^e 
thermometers must be shaken down before they are replaced in the 
kiln. They record 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 deceptive. 
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 temperatures. The dry-bulb temperatures are in the left- 
hand column and the difference between wet and dry bulb tem- 
peratures in the top row. The relative humidity is given at the in- 
tersection of the row and 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. 
Most of the kiln humidity recorders are wet-bulb instruments with 
extension tubes. They differ from dry -bulb recorders (recording 
thermometers) in that the sensitve bulb in the kiln is provided with 
a wick and a water reservoir. Any type of recording thermometer 
