T.O. 33B-1-1
5-86
5.6.3.1
TESTING FOR EXTERNAL GAS LEAKAGE WITH THE STANDARD PROBE.
NOTE
Use of the earphones is mandatory in areas where the background noise might
interfere with hearing noise from the speaker. Use of the earphones is
recommended for all tests.
a. Connect the probe to the instrument.
b. If in a noisy area, wear the earphones and attach the earphone connector to the instrument.
c. Set the volume control to its minimum position.
NOTE
During this step, the probe should be pointed away from the assembly to be tested
and away from any other possible leak sources. If no background noise is audible or
the meter indicator does not move past zero, replace the batteries and repeat the
check for background noise and meter indication movement. If noise and meter
indication movement is still not noted, the instrument is not functional and must be
repaired or replaced. Repair should be performed by the PMEL.
d. Turn the instrument on and increase the volume control until the meter indication moves slightly ahead
from zero and a slight background noise is heard from the speaker or the earphones.
e. Use a 500-milliliter polyethylene bottle (NSN 6640-314-2097). Hold it about arm's length away from
the probe with the bottle nozzle facing the probe opening. Rapidly squeeze the bottle and note the
meter reading. The meter reading should be at least 70. If it does not, the instrument is not functioning
properly and must be turned into the PMEL for repair and calibration.
f. Search for leaks by pointing the probe at the test assembly and moving the probe around suspected leak
areas. Note fluctuations in meter readings and volume from the speaker or earphones. When
fluctuations are noted, maximize the meter reading by moving the probe and pointing it straight at the
suspected leak area. The leak will generally be located in front of the probe when the meter reading and
volume is at a maximum.
g. To locate the leak, attach the rubber extension to the probe. This increases the directional response
characteristics of the probe. Keep the probe pointed in the direction where the meter indication and
noise is maximized and move the probe toward the test assembly. Where the extension tip meets the
test assembly with maximum meter reading and noise is the leak location. It may be necessary to
lower the volume control to keep the meter reading on scale as the probe is moved closer to the test
assembly.
NOTE
Under certain conditions background noise detected by the instrument can prevent
the detection of relevant leakage. This background noise can result from many
types of air movement (wind, running of air motors or air cooled motors, running
aircraft engines, operating pneumatic systems, etc.).
h. The leakage rate may be estimated by moving the probe away from the leak to the maximum detection
distance (leakage just indicated, slight increase in meter indication and noise level from normal
