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TM 1-4920-458-13&P
Section III. Principles Of Operation
1-10.
PRINCIPLES OF OPERATION
A small high-speed pump serves dual functions by producing system vacuums up to 29 inches of mercury and system
pressures up to 6.5 psi to simulate aircraft atmospheric and airspeed conditions. These vacuums and pressures are
delivered to the respective aircraft system and tester instruments through oil reservoir, oil sumps, and a network of
hoses, rigid pressure lines, fittings, adapters, valves and ports on the tester along with external hoses and adapters
supplied with the tester.
The pump serves the dual function of developing pressure as well as vacuum for operation of the Tester. In the pressure
section, a mixture of air and oil is pumped into the reservoir where the oil and air are separated. The separated oil drops
to the bottom of the reservoir where it is bled to the pump for lubrication purposes, the air being forced out at the top of
the reservoir under pressure. The pressurized air, after going through the check valve whose function is to prevent oil
from entering instrument lines during operation, enters an oil sump and passes through a filter which is an integral part of
the sump. Air from the sump blows through two pressure control valves. The pressure INCREASE valve controls the
amount of air permitted in the system and the pressure DECREASE valve opens the line to the ambient air, allowing
system pressure to bleed off. The available pressure level, as fixed by the control valves is sent to the pressure relief
valve and PRESSURE selector valve. Setting the PRESSURE selector valve at the desired test position completes the
circuit to the instrument under test and the master instrument to one of the pressure relief valves. The pressure relief
valve is in the line at all times and protects the instruments from pressure overload when the pump is producing vacuum
which passes through the check valve, and the sump which prevents oil from entering the instrument lines. The vacuum
INCREASE valve controls the amount of vacuum in the system and the vacuum DECREASE valve opens the line to the
ambient air allowing the system vacuum to bleed off.
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