Atmospheric pressure Pressure inlet Microswitch 10-12 Figure 10-20.  A pressure switch can be used in addition to, or  instead of, a pressure gauge.  Figure 10-19.  Vacuum suction gauge.  Figure 10-21.  A normally open pressure switch positioned in an  electrical circuit causes the circuit to be open as well. The switch  closes, allowing electricity to flow when pressure is applied beyond  the switch’s preset activation point. Normally, closed pressure  switches allow electricity to flow through the switch in a circuit but  open when pressure reaches a preset activation point, thus opening  the electrical circuit.  diaphragm. Upon starting the engine, oil pressure increases  and the pressure against the diaphragm is sufficient to hold  the contacts in the switch open. As such, current does not  flow through the circuit and no indication of low oil pressure  is given in the cockpit. Should a loss of oil pressure occur,  the pressure against the diaphragm becomes insufficient to  hold the switched contacts open. When the contacts close,  they close the circuit to the low oil pressure indicator, usually  a light, to warn the pilot of the situation.  Pressure gauges for various components or systems work  similarly to those mentioned above. Some sort of sensing  device, appropriate for the pressure being measured or  monitored, is matched with an indicating display system. If  appropriate, a properly rated pressure switch is installed in the  system and wired into an indicating circuit. Further discussion  of specific instruments occurs throughout this handbook as the  operation of various systems and components are discussed. pressure meets or exceeds a preset level designed into the  structure of the switch does the diaphragm move far enough  for the mechanical device on the opposite side to close the  switch contacts and complete the circuit.  [Figure 10-21] Each switch is rated to close (or open) at a certain pressure  and must only be installed in the proper location.  A low oil pressure indication switch is a common example  of how pressure switches are employed. It is installed in  an engine so pressurized oil can be applied to the switch’s  Some of the most important flight instruments derive their  indications from measuring air pressure. Gathering and  distributing various air pressures for flight instrumentation  is the function of the pitot-static system. Pitot-Static Systems 