26 BULLETIN 1186, U. S. DEPARTMENT OF AGRICULTURE 
obvious that if either instrument fails to function, humidity control 
will also fail. Several instruments have been developed to over- 
come this inherent defect. In one type the operation of the con- 
troller depends upon the difference in pressure between a dry-bulb 
system and a wet-bulb system, both incorporated in the same instru- 
ment, and in another type changes in the weight of a hygroscopic 
material, such as wood shavings, operate the controller. 
STEAM JETS 
Humidity controllers almost without exception operate valves 
controlling steam jets, just as temperature controllers operate valves 
in the heating system, and the same kind of valves are ordinarily 
used, each one being adapted to the needs of the particular service 
it is to render. Since the use of humidity controllers on steam-jet 
lines presupposes the necessity of always increasing the humidity, 
means must be provided to insure this need. Ordinarily in natural- 
circulation kilns the fresh-air inlets and the moist-air vents are 
open sufliciently to require humidification of the kiln atmosphere. 
If necessary in special cases, the controllers can be made to operate 
dampers of various sorts and also to regulate the flow of water in 
condenser pipes. Humidity control in the various kiln types will 
be considered more in detail later. 
SPECIAL CONTROLLERS 
Several special types of temperature and humidity control instru- 
ments have been either designed or adapted for dry-kiln use. Prin- 
cipal among them are the humidity recorder-controllers (pls. 10, 11, 
and 12), which are generally of the air-operated direct-set type 
and combine, in a single case, two recording thermometers:and two 
temperature controllers. Only two tube systems are required, a wet- 
bulb system and a dry-bulb system. Hach tube system operates 
its individual recorder pen and its own air valve. The instrument 
serves to control temperature and humidity and to record both the 
wet-bulb and the dry-bulb temperature, and obviates the need for 
other recorders or controllers on the kiln. Air-operated humidity 
controllers, without recorders, which combine two single controllers 
in one case, are suitable for use where temperature records are not 
desired or where they are secured by recording thermometers. 
Several special forms of humidity controllers have been developed 
for special kinds of drying schedules. One of these controllers, 
operated by the rise and fall of the humidity in the kiln, produces 
periodic oscillations of humidity between predetermined limits. 
Another, operated by clockwork, serves to steam the kiln charge at 
set intervals; the time between steamings and the length of the 
steaming period are adjustable. 
AIR CIRCULATION IN THE KILN 
Air circulation performs several important functions in kiln 
drying. It serves to bring the heat to the lumber and to carry 
away the evaporated moisture. Upon its briskness and uniformity 
depends, to large degree, the uniformity of temperature and of 
