94. BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
No satisfactory instrument for making a direct record of the 
relative humidity in a dry kiln is available, but a record of the wet- 
bulb and dry-bulb temperatures forms a reasonably good substitute, 
and is the expedient usually employed. It is obvious, of course, that 
such a record can be secured by the use of two separate recording 
thermometers, one suitably equipped with a wet wick over the bulb. 
It is just as obvious that a better arrangement would be to have both 
records on the same chart, and this is the way most humidity record- 
ersare made. ‘These instruments are known also as-wet and dry bulb 
recording thermometers, recording psychrometers, and recording 
hygrometers. In principle humidity recorders are exactly like 
recording thermometers, there being two complete though component 
instruments in a single case. (Pls. 3 and 4.) 
Two types of wet bulb are used in dry-kiln work, the well-known 
wick-and-water-trough type and the porous-sleeve type. In the 
latter (pl. 4, B) a porous sleeve of alundum or other suitable material, 
which surrounds the wet bulb, is kept filled with water. The water, 
gradually seeping through the porous walls, is evaporated on the 
sleeve surface, producing the necessary depression of temperature in 
the sleeve and the contained bulb. Both types are thcroughly reli- 
able and are fully satisfactory under proper ‘operating conditions. 
Hard water soon clogs up the porous sleeves, just as it encrusts the 
wicks, but the sleeves can be cleaned very easily by immersing them 
in muriatic (hydrochloric) acid, and the wicks can be changed at 
slight trouble and expense. 
CONTROL OF KILN HUMIDITY 
The humidity within a kiln may be raised or lowered in two ways: 
(1) Water vapor may be directly added to or removed from the kiln ~ 
atmosphere or (2) part of the air may be removed from the kiln 
and may be replaced by wetter or by drier air from the outside. To 
some extent both of these processes are continuously going on in the 
ordinary dry kiln, and it is not possible to exercise full control over 
certain parts of them. Thus, the evaporation of water from the 
wood is continuously adding water vapor to the kiln atmosphere, and 
air leakage into and out of the kiln is continuously tending to lower 
the humidity, since outside air almost invariably has a lower abso- 
lute humidity than kiln air. When air of a given absolute humidity 
is warmed, its relative humidity decreases, since its capacity for 
moisture increases, and the same weight of water vapor then repre- 
sents a smaller percentage of its total capacity for moisture. 
DEW POINT 
A controllable method for removal of moisture from the kiln 
depends upon the dew: point of the kiln atmosphere. The tempera- 
ture at which any atmosphere, upon cooling, becomes saturated is 
known as the dew-point temperature. This temperature is a fixed, 
determinate one for any given set of conditions. Ordinarily it is 
somewhat lower than the wet-bulb temperature, with which it must 
not be confused; at saturation, however, the dew-point temperature, 
the Ber bulb temperature, and the dry-bulb temperature are all 
equal. 
