16 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
The advantage of the reverse-acting type is that a failure of the 
air supply causes the valves to shut, which prevents the possibility of 
a dangerous rise in temperature. The same effect may be secured in 
a battery of direct-acting thermostats by putting a single reverse- 
acting valve in the steam main and connecting it direct to the air 
supply. 
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FicturRe 3.—Cross section of an air-operated thermostat and a direct-acting dia- 
phragm-motor yalvye. This diagram shows the method of operation and illustrates 
the details of a common type of air-operated thermostat, a type in which the flow 
of air to and from the diaphragm-motor valve is controlled directly by an air 
valve, without the use of an intermediate valve and bellows. Opening and closing 
oi the diaphragm-motor vaive is accomplished through the medium of compressed 
air, at a pressure of about 15 pounds to the square inch. When air is admitted to 
the chamber m it forces downward the diaphragm g and with it the valve and 
valve stem w; the valve, seating in the valve body o, shuts cff the flow of steam. 
When the air pressure is released, the spring p, aided by the steam pressure acting 
on the under side of the valve, raises the valve, thus opening the steam passage. 
The supply of air to the diaphragm-motor valve is controlled by a small valve e¢, 
as follows: When the temperature of the bulb 7, which is in the kiln, rises in 
response to increasec kiln temperature, increased pressure is transmitted through 
the capillary extension tube & to the spring capsule 7, which expands, its top ris- 
ing as a result, carrying with it the adjusting screw i and the lever h#. This 
movement allows the air pressure from the supply line a@ to raise the valve ¢ 
and the valve stem d. Air then flows around the valve through the pipe 7, into 
the chamber m. When the temperature of the bulb 7 falls, the reverse action takes 
place, and the valve ¢ is seated, shutting off the air supply to the diaphragm-motor 
valve. The pressure of the air in the chamber m is then relieved through leakage 
of air around the valve stem d, which is made a loose fit for this purpose. As the 
pressure is relieved, the valve and valve stem nm rise. The gauge e indicates the 
pressure of the air supply, and g that acting on the diaphragm gq 
