Permeation Type Helium Detecting Probe

摘要

Adsorption, absorption, diffusion, solubility, re-emission or desorption are the well-known physical phenomena occurring with gas - metal interacting systems. Permeation incorporates all these processes. Selective permeation of hydrogen through thin palladium or nickel-palladium alloy barriers is well known and exploited in the hydrogen detection and separating systems. In a similar way, the permeation of helium through silica glass membranes is used in 'calibrated helium leaks' for day-to-day calibration of mass spectrometer type helium leak detectors. Commercially viable helium separating systems making use of the passage of the gases through thin polymer membranes (like cellulose acetate & poly-tetra fluoro-ethylene membranes) have also been made. M.Audi of M/S. Varian, Vacuum Products Division, has reported a sniffing type helium detector based on the permeation and described its theoretical basis. He also gave few experimental tips. This device makes use of the increased permeation rate of helium through thin quartz window, maintained at elevated temperatures up to 800℃. The permeated helium increases the pressure in a small volume which is pumped by an ion pump. A similar leak detector is now being marketed (Helitest) commercially by M/s Varian. However, the design details of the above are not available in the literature. In this article, we describe a similar device designed by us based on the above principle and this uses a thin quartz glass membrane as an integral part of the enclosure for a single cell ion pump. This device has been tested for its ability to detect and estimate small changes of the helium concentration in atmospheric air. We also describe these tests and the performance obtained. Since the quartz membrane has to be operated at elevated temperatures - this makes a portable detector more cumbersome- we decided to experiment with polymer membranes that can operate at lower temperatures. Brief design details of the second device - which is still to be evaluated - are also presented here.