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CHAPTER 2
THEORY
The PS-425 test box consists of one manual vacuum pump, one manual pressure pump,
reservoirs for the pressure and vacuum, and a digital display for indicating altitude and
airspeed.
• Pneumatic Theory
Metering valves allow the operator to control the rate of flow of the air from the pressure
or vacuum sources, exiting out through the vacuum and pressure ports. The vent valves
allow for metering the produced vacuum or pressure back into the atmosphere. One
crossbleed valve permits the operator to isolate the pressure and vacuum sides, or to
allow the balancing of pressures, or even to reverse the normal “flow” of pressure.
The two round gauges allow the operator to monitor the level of pressure or vacuum that
is stored in the tester’s reservoirs.
When the operator wishes to simulate an altitude, the Crossbleed, Vacuum, and Vacuum
Vent valves are gently closed. The vacuum cylinder is pumped until sufficient vacuum is
in the reservoir, as indicated on the Vacuum Reservoir Gauge. How much vacuum will be
needed depends on the desired altitude, and is also affected by how long 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 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.
Similarly, 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 pumping the pressure cylinder. 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.
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