PG-4 Indicates Low Readings Consistently The 0-15 psig pressure gage PG-4 measures extremely low pressures from the item under test. This gage is protected from high pressures by gage guard GP-1, which is set between 11 and 14 psig. To locate any leaks in this system, you will need to perform the leakage test described earlier for the test stand. You will not be required to perform the leakage test for the accessories section. In most cases by tightening the necessary fittings, you will be able to remedy the low readings on the PG-1 pressure gage. Differential Pressure Gage (DF-1) Indicates Low The 0-100-in. H₂O differential pressure gage is a bellows-operated gage used to indicate differential pressure when the pressure closing and pressure opening valves are tested. The probable cause for low readings on this gage is a leaky shut off differential pressure valve V-8. If you are lucky, you can correct it by tightening the fittings. If this does not solve the problem, you will need to replace the valve. Bell Jar Leakage You may also have problems with the bell jar. The bell jar is used for testing components having more than one possible area of leakage. The bell jar consists of the bell jar itself, a relief valve with a range of 5 to 15 psig, and a bell jar coupling. The relief valve is designed to be leakproof at 5 psi and set to relieve at 10 psig. To perform the leakage test on the bell jar assembly, proceed as follows: 1. Remove hose assembly 3 and nipple assembly 14 (figure 1-5) from the bottom bell jar coupling C-1. Disconnect the opposite end of the hose from differential pressure connection NIP-7. 2. Ensure differential pressure bleed valve V-7, test-pressure-gage-to-bell-jar valve V-2, and system bleed valve V-5 are closed. Open differen- tial pressure shutoff valve V-8. 3. Place the bell jar on the fixture and secure it with a clamp. Plug bell jar top coupling C-2. 4. Slowly open oxygen supply valve V-6 until 100 in. H₂O is indicated on differential pressure gage DF-1. Close valve V-6. Leakage, indicated by a drop in pressure on DF-1, shall not be more than 2 in. H₂O in 10 minutes. 5. Close the oxygen supply cylinder valve and open system bleed valve V-5 to bleed the system. CAUTION When the test stand is secured, all valves with the exception of system bleed valve V-5 will be closed. Valve V-5 is left open to prevent accidental build-up of pressure in the system. 6. Secure all test stand valves. Leave system bleed valve V-5 open. REPAIRING AND REPLACING PARTS Anytime you have a defective or damaged part, it must be repaired or replaced. Informa- tion on part numbers can be found in the NAVAIR 17-15BC-20 manual. Y o u  m a y  o n  o c c a s i o n  f i n d  y o u  h a v e  a defective piece of tubing. To replace any tubing installed on this test stand (59A120), remember that you are dealing with high-pressure oxygen. Therefore, you must use tubing with a minimum wall thickness of 0.049 to replace any defective tubing. This tubing may be cut to length and flared to replace any defective portion of tubing. W A R N I N G When you work with oxygen systems, never use any parts that have been in con- tact with oil, grease, or any other material that is not approved for use in the presence of high-pressure oxygen. Fire or explosion may result when even the slightest trace of combustible material comes in contact with pressurized oxygen. Heat Exchanger Panel If the heat exchanger panel is defective, it may be replaced. You may replace the panel by discon- necting its connections and removing it’s seven retaining screws. If a new heat exchanger is used, you may drill or punch holes not exceeding 11/32 inch in diameter in the perimeter, beyond the outer seam welds, for use in mounting. When the holes are drilled at installation, you should be careful to prevent the drill from puncturing the seam welds. Lubrication The test stand nor its components require lubrication. 1-11