diaphragm.  The  third  port  is  located  on  the opening  side  of  the  diaphragm.  A  sensing  line  is attached to the third port to connect the cabin pressure  regulator  control  and  the  cabin  pressure exhaust   duct.   The   pressure   admitted   to   the diaphragm  through  the  third  port  is  equivalent  to cabin air pressure. The difference between the pressure on either side of the diaphragm causes the pressure regulator valve to modulate between the open and closed positions. CABIN   PRESSURE   REGULATOR SAFETY VALVE.— The pressure regulator safety valve  is  an  independent,  pneumatically  operated, balanced type of poppet valve that limits cabin- to-ambient  pressure  differentials  to  7.07  (+0.2 and – 0.0) psi. If the difference between cabin pressure   and   ambient   pressure   reaches   the calibrated  limit,  the  change  in  pressure  acting  on the  limit  control  diaphragm  overcomes  the metering  valve  spring-load  and  allows  the metering valve to open. This also opens a passage in the cabin pressure safety valve head, which causes the head pressure to be slightly lowered. Since  the  cabin  pressure  is  greater  than  head pressure,  it  opens  the  pressure-balanced  main poppet to allow cabin air to be vented overboard. When the cabin pressure differential is restored to normal, the limit control metering valve closes, and  the  pressure  safety  valve  returns  to  its normally closed position. CABIN  PRESSURE  REGULATOR  CON- TROL.—  The  pressure  regulator  control  is  a pneumatic control that provides four modes of cabin pressure operation. In addition to the modes of operation, a test valve is included with three manually set positions: FLIGHT, DIFF ON, and ALL OFF. The test valve is normally lockwired in   the   FLIGHT   position   for   all   cabin   pres- surization modes. The DIFF ON position permits aground test of the normal delta-P setpoint. The ALL OFF position permits a ground test of the setpoint  of  the  pressure  safety  valve.  These  tests are   accomplished   with   pressure   supplied   by support  equipment. Four  pneumatic  ports  are  provided  on  the pressure regulator control for use with various sensed pressures and the pressure regulator valve. These ports are different sizes to prevent improper plumbing    connections. The  pressure  regulator  control  contains  an isobaric  bellows,  which  is  calibrated  to  maintain an aircraft cabin pressure of 5,000 feet while the aircraft is flying at altitudes between 5,000 and 24,000  feet.  The  isobaric  bellows,  which modulates   a   control   pressure,   use   cabin   air as  a  pressure  source  and  low  pressure  in  the environmental control system compartment as a negative pressure. Control pressure is delivered to   one   side   of   the   pressure   regulator   valve diaphragm, and cabin pressure is connected to the opposite   side.   Because   control   pressure   is normally less than cabin pressure and will decrease relative to cabin pressure, the pressure regulator valve  becomes  more  open  to  decrease  cabin pressure. The   pressure   regulator   control   contains provisions   for   controlling   the   rate   of   cabin repressurization  when  recovering  cabin  pressure after  using  the  cabin  dump  mode,  or  during  a rapid  descent  in  altitude.  The  control  pressure modulated  by  the  isobaric  bellows  is  further modulated by the repressurization diaphragm to limit  cabin  repressurization  to  an  equivalent  4,000 feet per minute change. The pressure regulator valve  is  held  open  until  normal  pressure characteristics are sensed. CABIN  LOW-PRESSURE  SWITCH.—  The low-pressure  switch  is  installed  below  the  center console to sense the cabin absolute pressure. The normally  open  low-pressure  switch  closes  at 13,000 ± 500 feet and reopens at 11,000±500 feet. The cabin pressurization indicator light on the annunciator  panel  illuminates  when  the  low- pressure switch closes. The indicator light goes off when the low-pressure switch reopens. CABIN AIR PRESSURE SENSING FIL- TER.— The air pressure sensing filter is located in the line that connects the cabin exhaust air duct, the cabin pressure regulator control, and the cabin pressure regulator valve. The replaceable filter element, which is connected to the air sensing tube,  is  mounted  with  clamping  rings  on  the fuselage frame. The filter element is a cylindrical plug  of  treated  paper  and  fabric  in  a  metal housing.  The  clamping  rings  confine  the  air entry to the dome-shaped end to trap the entry of tobacco tar and dust particles greater than 10 microns in diameter. MAINTENANCE  AND  INSPECTION Very little maintenance is required on most pressurization and ACSs other than making the required  periodic  inspections  and  operational checks. Many of the components are repairable at the depot level of maintenance rather than at 3-34