12 . BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 
of the hand valve makes only a small change in the amount of steam 
supplied. If the kiln is provided with automatic control, the con- 
trol valve will usually be located next to the kiln. The use of auto- 
matic control valves is recommended for practically all kinds of 
kiln drying, because a more even temperature may be maintained, 
injury from excessive temperatures avoided, and loss of time from 
unnecessarily low temperatures prevented. 
AUTOMATIC TEMPERATURE CONTROLS. 
There are two classes of automatic temperature control in common 
use in dry kilns. These are known as self-contained and auxiliary 
operated. The self-contained thermostats are operated by means of 
the direct pressure of vapor or liquid upon the valve stem. The 
action is very similar to that of the recording thermometers already 
described. A large bulb in the kiln is connected by means of a 
capillary tube to a diaphragm or capsule in the head of the valve 
located in the steam line. The temperature variations in the kiln 
change the pressure inside the bulb, which in turn causes correspond- 
ing pressure changes in the capsule. This results in the opening and 
closing of the valve, the stem of which bears upon the capsule. The 
valve itself is usually of balanced type to provide ease of movement. 
A counter force or pressure is provided by means of an adjustable 
spring or sliding weights, and the instrument is set for the desired 
temperature by changing the tension of the spring or the position of 
the weights. (See PL V.) The principal advantages of the self- 
contained thermostat are that no auxiliary source of power is re- 
quired for its operation and that the first cost is comparatively small. 
This type is not so sensitive as the auxiliary operated type. The 
manufacturers claim regulation within 2° of the temperature for 
which the instrument is set, but in kiln operation the variation is 
often much greater than that. The auxiliary operated instruments 
are supposed to control with a variation of only 1° and in kiln 
operation usually maintain this accuracy. 
The auxiliary operated instruments using air as the operating 
medium are usually provided with a small bulb inserted in the kiln 
and connected to a capsule in the instrument by means of a capillary 
tube. (See PL VI.) The movement of the capsule top in response 
to temperature changes in the kiln is transmitted to a small valve 
connected on one side to a supply of air compressed to 15 pounds 
pressure and on the other side to a diaphragm-motor valve on the 
steam main. Sometimes a bimetallic system is used in place of the 
capsule to operate the air valve. This small air valve is so arranged, 
in instruments using direct-acting diaphragm valves, that as the 
temperature rises, air pressure is admitted to the head of the dia- 
phragm-motor valve. This forces the diaphragm down, which closes 
the valve and shuts the steam off from the kiln. As the temperature 
falls, the air pressure is shut off, and a means of escape is provided 
for the air in the valve head. The valve then opens through spring 
action and admits steam to the kiln. Reverse-acting diaphragm 
valves are so constructed that the air pressure opens them and the 
springs close them. The air valve must be modified accordingly. 
The advantage of the reverse-acting type is that a failure of the 
air supply causes the valves to shut, which prevents a dangerous 
rise in temperature. The same effect may be secured in a battery of 
