The ultra sensitive leak detection systems with detection limit as low as 10-15 to 10-16 Pa•m3/s have been developed during the past decade. One permeation type reference leak made by quartz glass with a minimum leak rate in the range of 10-8 to 10-9 Pa•m3/s is commonly used to calibrate the sensitivity of the leak detector. The minimum leak rate of the permeation reference leak element commercially available now is in the range of 10-10 to 10-11 Pa•m3/s, which is used constrainedly for the calibration of ultra sensitive leak detection system through linear extrapolation. The reference leak with lower leak rate is necessary to be developed for the calibration of ultra sensitive leak detection system.
The major influence factors of the reference leak rate for the permeation type leaks are discussed in this paper. The reference leak rate can be decreased by one order of magnitude, which is realized experimentally by selecting hard borosilicate glass as leak element instead of quartz glass, because the helium permeation rate of hard borosilicate glass is about one tenth as low as that of quartz glass under the same temperature.
Furthermore, the linear relationship between the He leak rate and inlet He concentration for permeation leak is proved by experiments and the reference leak rate of the permeation leak can be further decreased by two or three orders of magnitude by using the He-N2 mixed gas and decreasing the He concentration in the inlet gas reservoir.
In conclusion the reference leak rate of the permeation leak can be decreased 3 or more orders of magnitude, which can be used in the real time calibration of the ultra sensitive leak detection technique as well. |