Electronic Theory; Electrical Theory - PRESTON PRESSURE PS-500 User And Maintenance Manual

the tubing and lines are between the test box and the altimeter. The vacuum and
pressure reservoir gauges serve only as general references of reservoir vacuum and
pressure; they are not used in the actual calibration of any instrumentation, and do
not require calibration. Once sufficient vacuum is obtained in the reservoir, the pump is
turned off, and the Vacuum Valve is gently opened to allow air to be suctioned into the
system through the vacuum port. Once sufficient altitude simulation is obtained, the
Vacuum Valve is gently closed. Reduction in altitude is accomplished by gently opening
the Vacuum Vent Valve and allowing atmospheric air pressure to enter into the lines.
If the operator needs to go to an altitude below atmospheric pressure, pressure must be
introduced into the pressure reservoir. Opening the Crossbleed Valve will enable pressure
to flow from the pressure reservoir to the altimeter. Open the Crossbleed valve, and then
the Pressure valve to pressurize the vacuum side.
When airspeed is to be simulated, the Crossbleed, Pressure and Pressure Vent valves will
first be closed until the operator establishes a pressure build-up in the pressure reservoir
by turning on the pressure pump. Then the Pressure Valve will be gently opened,
allowing air pressure to flow through the pressure port to the airspeed indicator. To
reduce pressure, the Pressure Vent valve is opened to leak the air pressure into the
atmosphere.
 Electronic Theory
The main circuit board of the PS-500 incorporates all the electronics for the altimeter and
airspeed displays. The power for the altimeter and airspeed circuits is regulated from 12
VDC to 5 VDC and 3.3 VDC.
The sensors used in the PS-500 test box are proprietary. The pitot sensor is not a
differential pressure sensor as is most typically used in airspeed measurements. Instead,
the pressure information from the static sensor is sent digitally to the microprocessor
where it is subtracted mathematically from the pitot pressure, thus giving the differential
pressure value. This design eliminates several components with their associated costs and
maintenance. It also allows the Altimeter side to be used at any altitude with the
Crossbleed valve closed, without damaging the airspeed sensor.
Warning: Even though the pressure sensors will not be damaged due to a high
differential pressure between the vacuum and pressure sides, damage can still occur
in external instruments connected to the test box.
 Electrical Theory
The internal gel-cell lead-acid battery provides the power to the PS-500 test box. The
battery is charged via the power connector on the front of the test box. Full charging time
is about 10 hours. The test box can be used while it is being charged.
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