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 TM 55-1905-220-14-8
4-29. AIR CONDITIONING SYSTEM - MAINTENANCE INSTRUCTIONS (Continued).
(h) Connect vacuum indicator to charging valve. Hold vacuum for about five hours and observe
vacuum indicator reading hourly (by opening charging valve) to see if system is maintaining
vacuum.
(i) If system holds vacuum, disconnect indicator and connect a refrigerant-12 drum to charging
connection. Introduce enough refrigerant to break vacuum (about 5 psig (34.5 kPa)). The
system is now ready for charging and operation.
q. Opening System.
(1) When a refrigeration system is to be opened for service or repairs, avoid the admission of air and
moisture. Before opening a charged or functioning system or part of a system, pump down or
evacuate the part to a pressure slightly above atmospheric (1 to 2 psig (6.9 to 13.8 kPa)).
(2) If the final evacuation reaches a pressure lower than zero psig, bleed enough refrigerant into the
evacuated part to raise the pressure to about 2 psig (13.8 kPa). Connections may then be broken
and the replacement part installed. First make one connection on the part. Second, sweep out any
air or other foreign gas through the free end by purging with refrigerant gas bled from the charge in
the system. Then quickly make the other connection or connections. If more than a few minutes
must elapse after breaking connections, plug the free ends of the system.
(3) Purge refrigerant or oil charging lines (though small and short) with refrigerant gas immediately
before charging.
r. Refrigerant Charge and Leaks.
(1) Refrigerant Charge.
Refer to paragraph f for refrigerant charging and removal procedures, discussion of refrigerant
overcharge and undercharge, procedure for transfer of refrigerant from one receiver to another in
same system, and discussion of refrigerant flooding.
(2) Testing for Piping Leaks.
(a) The most positive method for finding leaks in a refrigerant-12 system is with an electronic or a
halide leak detector. Testing with oil or soap suds at joints will only detect the larger leaks
and, therefore, is ineffective in determining the tightness of a system. An electronic or a
halide leak detector, on the other hand, will also locate the smaller leaks.
(b) Do not attempt a leak test in a compartment where a leak is suspected until the compartment
has been well ventilated. The sensitivity of the test is lessened if large concentrations of
refrigerant are present in the air.
4-900
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