KILN DRYING HANDBOOK 15 
provided by means of an adjustable spring or of sliding weights, and 
the instrument (pl. 6) is set for the desired temperature by changing 
the tension of the spring or the position of the weights; the setting 
is made by the slow method of trial and error. 
The principal advantages of the self-contained thermostat are that 
no auxiliary source of power is required for its operation and that its 
first cost is comparatively small. An important disadvantage hes in 
the fact that instruments of this type fail to respond quickly to 
changes in temperature, with the result that they do not work well 
under conditions requiring wide fluctuations in the amount of steam 
supplied. In addition, changes in the temperature of the motor head 
may cause changes in the setting of the instrument, so that it will 
operate at temperatures higher or lower than the one desired. This 
irregularity occurs when the temperature of the head is as high as 
that of the bulb; the head accordingly should not be placed in an 
operating room the temperature of which is likely to approach that 
in the kiln. Further, the self-contained type is not quite so sensitive 
as the auxiliary-operated; although the manufacturers claim regu- 
lation within 2° F. of the temperature for which the instrument is 
set, this range is sometimes exceeded where the circulation is inade- 
quate. The principal field of usefulness of the self-contained ther- 
mostat 1s a progressive kiln, where the temperature at the control 
bulb is intended to be constant, rather than in a compartment kiln, 
the temperature of which is varied from time to time. The auxiliary- 
operated instruments, on the other hand, are supposed to control with 
a variation of only 1° F. and in kilns having ample circulation 
usually maintain this accuracy. 
Auxiliary-operated thermostats are made in various types. Some 
use electric power, some water or steam pressure, and some com- 
pressed air, and again certain of them work under various com- 
binations of these means. Most of the auxiliary-operated thermostats 
in dry-kiln service, however, are of the air-operated type. The 
temperature-sensitive element may be bimetallic, but in kiln work it 
is usually the extension-tube type, with bulb, capillary tube, and 
pressure-sensitive hollow spring or capsule filled with liquid or with 
vapor. (Pl. 7 and fig. 3.) The movement of the free end of the 
hollow spring or of the capsule top in response to temperature 
changes in the kiln is transmitted to a small valve connected on one 
side with a supply of compressed air at about 15 to 25 pounds pres- 
sure per square inch and on the other side with a diaphragm-motor 
valve on the steam main—the diaphragm is sometimes called a bel- 
lows. The 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 diaphragm-motor valve. 
This forces the diaphragm down, closing the main vaive and shut- 
ting off the steam 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, 
again admitting steam to the kiln. Reverse-acting diaphragm valves 
are so constructed that the air pressure opens them and the springs 
close them. Direct-acting and reverse-acting valves can not be used 
interchangeably with the same thermostat. 
