KILN DRYING HANDBOOK. 19 
trollers by providing the bulb or sensitive element with a suitable 
wick and water supply. The Forest Products Laboratory has 
used with success air-operated wet-bulb controllers of both the 
extension bulb (vapor rilled) and the bimetallio or differential 
expansion types. Self-contained thermostats can also be used, 
but their sensitiveness is not so great as that of the air-operated 
instruments. Wet-bulb controllers can keep the wet-bulb tem- 
perature constant. If the dry-bulb temperature is also kept con- 
stant, the humidity will remain constant. If it does not, how- 
ever, the humidity will vary, even if the wet-bulb temperature 
is accurately controlled. To overcome this difficulty a differential 
type of self-contained humidity control has been developed. In 
this instrument there is a dry bulb as well as a wet bulb; the two 
bulbs are connected to their respective motor diaphragms on the 
body of a balanced steam valve so that an increase in the wet- 
bulb temperature will close the valve and an increase in the dry- 
bulb temperature will open it. Balance between the two is se- 
cured by a lever and sliding weights. This system provides for 
a constant difference between the vapor pressures in the wet and 
dry bulb motor diaphragms, no matter what the dry-bulb tem- 
perature may be. This results in an approximately constant differ- 
ence between the wet and dry bulb temperatures. 
A glance at the humidity table shows that, with a constant 
difference between wet and dry bulb temperatures, even quite a 
considerable variation in the dry-bulb temperature has but little 
effect upon the relative humidity. 
To secure satisfactory service from wet-bulb thermostats, care 
and attention should be given especially to the wicks, which should 
be changed as often as they become hard and dirty. 
Humidity controllers almost without exception operate valves 
controlling steam jets, just as temperature controllers operate 
valves upon the heating system. The same kind of valves are 
ordinarily used, each valve being adapted to the needs of the partic- 
ular service it is to render. As the use of humidity controllers 
on steam- jet lines presupposes that the humidity will always need 
to be increased, means must be provided to insure this need. Ordi- 
narily in ventilated kilns the fresh-air inlets and the moist-air vents 
are open sufficiently to require continuous humidification. If 
necessary in special cases, the controllers can be made to operate 
dampers of various sorts, and also to control the flow of water in 
condenser pipes. Control in the various kiln types will be con- 
sidered more in detail later. 
Several special types of temperature and humidity-control instru- 
ments have been designed or adapted for dry-kiln use. Among these 
are double-duty air-operated instruments which have two sensitive 
bulbs and extension tubes with but a single case, in which are housed 
the capsules and air valves. These instruments can be used for 
temperature and humidity control, or for temperature control and 
the removal of condensation from the heating coils. This latter 
use is not common in dry kilns. 
The recorder regulator has already been mentioned under temper- 
ature control. This air-operated instrument provides for the con- 
trol of either wet or dry bulb temperature and for a graphic record 
of the controlled temperature. 
