60
Figure 4-5. Variation of ambient pressure for 13 hours at test condition.
The error correction factor due to the variation of ambient pressure was calculated
by performing the permeability test without applying the upstream pressure. An
aluminum plate is used for the test to ensure that there is no gas permeation to the
downstream chamber. Since there was not much variation in the ambient temperature,
only ambient pressure causes changes in the position of the capillary indicator. After the
outlet valve of the downstream chamber was closed, the displaced position of the
capillary indicator and ambient pressure were measured simultaneously as shown in
Figure 4-6.
Barometric Pressure
10293 Average Barometric Pressure 185
S -o- Slua Position
', 180
I I I IIl- 1754
2 Z5
M2 ,, Ii' I
-170
10286
0 500 1000 1500
Time (Sec)
Figure 4-6. Variation of barometric pressure and indicator position as a function of time.
The correction factor k is calculated by k = Ah /Ap where h is the moving distance
of the capillary indicator andp is the ambient pressure. The average correction factor is
found to be 0.21 mm/Pa. Therefore, the corrected moving distance of the capillary
indicator is calculated as follows:
corrected = actual + k Ap