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  | ![]() MIL-S-51368B(EA) 
The shelter shall maintain a positive pressure in all modes, i.e. 
recirculation, cooling, and heating at all times.  During an entry or 
exit, the shelter low pressure alarm shall not activate when tested in 
accordance with 4.4.4.1.  During operation in the heating and cooling 
modes (without entry or exit) the shelter and entrance pressures shall 
fall within the limits specified in table I. 
3.3.3 Entry and exits. A minimum of twelve entry and exits shall be 
made during the four hour operating mode.  When tested in accordance with 
4.4.4.1 pressures in the shelter and entrance shall stabilize to the 
values specified in 3.3.2 after an entry or exit.  NOTE:  An entry or 
exit is defined as going from the outside through the entrance into the 
shelter or going from the shelter through the entrance to the outside. 
3.3.4  System leakage.  With 160 cubic feet per minute (cfm) maximum of 
make-up air being introduced into the system and with the shelter and 
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entrance maintaining at least minimum pressures (3.3.2), the purge-air 
effluent, measured at the damper valve in the outside entrance door shall 
be no less than 110 cm when tested in accordance with 4.4.4.2. 
3.3.5 Entrance purge.  The entrance shall be capable of purging 
dioctylphthalate (DOP) to give a 1000 to 1 (3 log) reduction in 5.0 minutes 
maximum when tested in accordance with 4.3.3.1. 
3.3.6 Shelter purge.  The shelter shell be capable of purging DOP 
to give a 1000 to 1 (3 log) reduction in 60 minutes maximum when tested 
in accordance with 4.3,3.1. 
3.3.7 Noise.  Noise inside the shelter shall be determined for vent, 
cool and heat modes using applicable instrumentation and shall not 
exceed the noise criterion curve NCA-60 of the ASHRAE Guide and Data 
Book when tested in accordance with 4.3.3.2. 
3.4 Reliability. 
3.4.1 Preproduction and reliability qualification.  Prior to the start 
of regular production, a preproduction sample of the M51 shelter system 
shall be produced in accordance with this specification for examination 
(4.3.2.1), test (4.3.2.2), and reliability qualification (demonstration) 
phase testing (4.3.2.3). 
3.4.1.1 Reliability qualification (demonstration) phase. The M51 
shelter system shall conform to the following Mean Time Between Failure 
for both the overall system reliability and protection 
reliability when tested as specified in 4.4.4.3 and evaluated for account- 
able failures (3.4.3) in accordance with table I. 
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