TM 10-6640-239-13 2-11 f. Air System Controls and Indicators.  Figure 2-7 and Table 2-7 list the controls and indicators for the air system. Table 2-7.  Air System Controls and Indicators Index Number Control or Indicator Function 1 Air Compressor ON/OFF Controller Energizes motor start/stop circuit at pressure switch. 2 Pressure Switch Starts and stops compressor automatically at required pressures. Compressor starts at 60 psi, and stops at 120 psi. 3 Safety Valve Provides safety relief if air pressure in tank goes above 150 psi. 4 Tank Drain Valve Provides tank moisture drain. 5 Air System Isolation Valve Provides isolation of air system. 6 Air Pressure Gauge Indicates air pressure in tank. 7 Air Service Valves Provides service access to air system.  Two stopcock valves are located on roadside wall, one on curbside wall, and one in each fume hood.