piezometer (26) by a line that runs from the reference tap connection (21) to the piezometer (26). The pressure/suction manometer (4) registers that pressure or suction. The pressure alS  O registers on the HG manometer (5). NOTE: The HG manometer (5) receives pressure or suction from the piezometer regardless of the position of the pressure selector valve (D). From the pressure/suction manometer (4), the pressure flows through another line trap to a connection at the low-range altimeter (13). It then flows to the reference pressure selector valve (O). W i t h  t h e  v a l v e  ( O )  i n  t h e  A L T I T U D E CHAMBER position, differential pressure is transmitted to the chamber reference port (N/N). Other valves and connections that affect readings on the pressure/suction manometer (4) are the helmet reference tap (24), the suit simulator reference shutoff valve (R), and the pressure equalizer valve (Z). The helmet reference tap (24) is used when you test the full pressure suit helmet. It gives the differential pressure between the respiratory section and the suit section. When you test the full pressure suit controllers, the suit simulator shutoff valve (R) is used in conjunction with the reference pressure selector valve (O) to give the actual altitudes within the suit. The pressure equalizer valve (Z) equalizes the pressure in the pressure/suction manometer (4) as does the pressure selector valve (D) when the pressure selector valve is placed in the HG position. PRESSURIZING THE DIFFERENTIAL PRESSURE INDICATING SYSTEM NOTE: At this point, knowing that 1.0 psig = 27.7 in. H₂O or 2.0 in. HG will be helpful to you. To use the differential pressure indicating system, open the leakage control valve (E) until the pressure/suction manometer (4) indicates 16.0 in. H₂O. Place the leakage selector valve(D) in the HG position. Now close the leakage control valve (E). The system should now be maintaining 16.0 in. H₂O. If the system has a leak, the high-range flowmeter (8) or the low-range flowmeter (7) will indicate it. If the system has no leakage, discon- nect the line between the low-pressure connection (19) and the reference tap connection (21); then remove the plug from the piezometer. T o  b l e e d  t h e  s y s t e m ,  b a c k  o u t  o n  t he low-pressure regulator (N) and open the bleed valve (S). After you bleed the system, close the system bleed valve (S). Leave all valves and connections in their present position. Now you are ready to check the vacuum system. 2-9 VACUUM SYSTEM # This system (figure 2-8) is used to evacuate the chamber to simulated altitudes. It also allows you to draw flows from any item that you have under test. The vacuum pump is considered the heart of the test stand. It is equipped with a vent that is provided to remove any corrosive vapors from the oil used in the pump. Use MIL-L-83767, Type I. Two valves work in direct conjunction with the pump: the vacuum control valve (B_l) and the out- put valve (C). The vacuum control valve (B_l) will directly evacuate the chamber to any simulated altitude necessary to test oxygen components. The output valve (C) draws a flow through the item under test. By using the output valve (C), you can draw a flow through the item under test, through the pie- zometer (26), the flow selector valve (M), the out- put vol-o-flo element, and into the vacuum pump. When you place the selector valve (M) in the SUIT SIMULATOR position and open the flutter dampener valve (J), the flow is identical to the flow that occurs when you use the output valve (C), except the flow is drawn through the suit simulator tank. USING THE VACUUM SYSTEM Prior to using the vacuum system, ensure the vacuum pump vent is open one to two full turns. Now turn the vacuum pump to the ON position and close the altitude chamber door. Be sure the reference pressure selector valve (0) is in the chamber position and open the vacuum control valve (B₁). Ascend to 10,000 feet. Watch the altitude indication on the low-range altimeter (13). When you reach the 10,000-foot altitude, stabilize the altitude by closing the vacuum control valve ( B_l). You should not see any drop in your altitude. Open the chamber bleed valve (K) and descend to sea level. When you reach sea level, close the bleed valve (K) and turn the flow selector valve (M) to the REGULATOR position. Use the output valve (C) to draw a flow of 6.0 in. H₂O. This flow is drawn from the chamber through the output port (23) and is indicated on the output manometer (l). Ascend to 10,000 feet. Close the output valve (C) and watch the low- range altimeter (13). No drop in altitude should be indicated. Open the chamber bleed valve (K) and descend to sea level. Close valve (K) when you reach sea level. By having all valves and connections in their present position, you are set up for the altitude sensing system test.