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13 Metrology Technology of Lunar Lander
The difference between single point leak test and system leak test was the volume
and size of the covered space. When the covered space was less than 2 mL, the
helium mass spectrometer sniffer method could implement quantitative test of the
single point leakage rate, and the inlet pressure of the leak detector was irrelevant
to volume of covered space. However, the volume of covered space and the inlet
pressure of the leak detector would affect the reaction time of the leak test system.
When the inlet pressure was higher and the volume of cover space was smaller, the
leak test time by helium mass spectrometry sniffer method will be shortened. The
leak test equipment of helium mass spectrometer sniffer consisted of helium mass
spectrometer leak detector, sniffer, positive pressure standard leak and single point
cover tool. Helium mass spectrometer leak detector was the core of single point leak
test. In order to achieve that the leak test sensitivity of the helium mass spectrometer
sniffer for single point leak test was better than 10
−8 Pa•m3/s, the sensitivity of the
leak detector should be better than 10
−11 Pa•m3/s, and leak detector should be reliable
and stable.
The sensitivity of non-vacuum accumulation leak test technology by helium mass
spectrometer was affected by two factors which were the container volume and
the accumulation time of hardware in stand still [13]. The smaller the volume of
collection container was and the longer the accumulation time was, the higher the
sensitivity of leak test system was. Whereas, the larger the volume of collection
container was and the shorter the accumulation time was, the lower the sensitivity
of leak test system was. At the same time, the volume of collection container was
limited by spacecraft size and the accumulation time was limited by development
cycle, various factors should be considered to establish the leak test system. Up to
date, for 100 m
3 volume of collection container which was sealed and stood still
for 24 h under atmospheric environment, its leak detection sensitivity could reach
10
−5 Pa• m3/s.
The flexible helium hood method for quick leak positioning was applied for
complex system that could not be dissembled and the leak was uncertain. When
the accumulation leak test was conducted, the complex system should be partitioned
and flexible coating was applied to cover and formulate a simple hood. By use of
flexibility, the covered space was reduced as far as possible. The helium mass spectrometer sniffer was applied to test covered space. If there was leak, the concentration
of helium in the measured space would gradually increase to indicate that there was
leak point in the covered space. Otherwise, the leak point was not within the covered
space. Then space with leak was continued to be partitioned until the leak point is
positioned. The reverse idea was applied in such technology to solve the issue of leak
positioning when the total leakage rate didn’t meet the requirement. It was suitable
for search and identification of unpredicted leak when the leakage rate was lower
than 10
−3 Pa•m
3 /s.
3. Analysis of Accuracy
The flexible helium hood method for quick leak positioning was successfully applied
to implement leak positioning in system assembly process of the Chang’E-3 lunar
13 Metrology Technology of Lunar Lander
The difference between single point leak test and system leak test was the volume
and size of the covered space. When the covered space was less than 2 mL, the
helium mass spectrometer sniffer method could implement quantitative test of the
single point leakage rate, and the inlet pressure of the leak detector was irrelevant
to volume of covered space. However, the volume of covered space and the inlet
pressure of the leak detector would affect the reaction time of the leak test system.
When the inlet pressure was higher and the volume of cover space was smaller, the
leak test time by helium mass spectrometry sniffer method will be shortened. The
leak test equipment of helium mass spectrometer sniffer consisted of helium mass
spectrometer leak detector, sniffer, positive pressure standard leak and single point
cover tool. Helium mass spectrometer leak detector was the core of single point leak
test. In order to achieve that the leak test sensitivity of the helium mass spectrometer
sniffer for single point leak test was better than 10
−8 Pa•m3/s, the sensitivity of the
leak detector should be better than 10
−11 Pa•m3/s, and leak detector should be reliable
and stable.
The sensitivity of non-vacuum accumulation leak test technology by helium mass
spectrometer was affected by two factors which were the container volume and
the accumulation time of hardware in stand still [13]. The smaller the volume of
collection container was and the longer the accumulation time was, the higher the
sensitivity of leak test system was. Whereas, the larger the volume of collection
container was and the shorter the accumulation time was, the lower the sensitivity
of leak test system was. At the same time, the volume of collection container was
limited by spacecraft size and the accumulation time was limited by development
cycle, various factors should be considered to establish the leak test system. Up to
date, for 100 m
3 volume of collection container which was sealed and stood still
for 24 h under atmospheric environment, its leak detection sensitivity could reach
10
−5 Pa• m3/s.
The flexible helium hood method for quick leak positioning was applied for
complex system that could not be dissembled and the leak was uncertain. When
the accumulation leak test was conducted, the complex system should be partitioned
and flexible coating was applied to cover and formulate a simple hood. By use of
flexibility, the covered space was reduced as far as possible. The helium mass spectrometer sniffer was applied to test covered space. If there was leak, the concentration
of helium in the measured space would gradually increase to indicate that there was
leak point in the covered space. Otherwise, the leak point was not within the covered
space. Then space with leak was continued to be partitioned until the leak point is
positioned. The reverse idea was applied in such technology to solve the issue of leak
positioning when the total leakage rate didn’t meet the requirement. It was suitable
for search and identification of unpredicted leak when the leakage rate was lower
than 10
−3 Pa•m
3 /s.
3. Analysis of Accuracy
The flexible helium hood method for quick leak positioning was successfully applied
to implement leak positioning in system assembly process of the Chang’E-3 lunar