Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 7 
AN AIR-OPERATED THERMOSTAT 
Pressure changes in the bulb a, which is placed in the dry kiln, are transmitted through the exten- 
sion tube b to the pressure-sensitive element in the case c of the instrument. ‘The pointer d serves 
to set the instrument for the desired temperature. Air enters from the right, passing first through 
a reducing valve h and a drip chamber 7, which is provided with a drain cock k. <A safety valve i is 
supplied to prevent excessive pressure within the system, and a filter e keeps dirt from entering and 
clogging the fine air passages. The pressure gauges f and g show, respectively, the air pressures in 
the supply line and in the motor head of the valve. In the instrument illustrated, the valve n is 
of the reverse-acting type; that is, the valve in its normal position is closed by the action of the 
spring m, and is opened by air pressure in the motor head /, which is made in two sections. The 
purpose of such reverse action is to obtain automatic shutting off of the steam supply in case the 
air pressure fails. 
