Vacuum envelope tests are integral leak tests using helium as the test gas, in which the test specimen is enclosed either in a rigid (usually metal) enclosure or in a light plastic envelope. The helium which enters or leaves (depending on the nature of the test) the test specimen is passed to a helium leak detector, where it is measured. Envelope tests are made either with the test specimen pressurized with helium (Fig. 5.4c) or with the test specimen evacuated (Fig. 5.4a). In both cases it may be necessary to convert the helium enrichment figure (accumulation) to the helium standard leak rate.
a: Integral leak detection; vacuum inside specimen
b: Local leak detection; vacuum inside specimen
c: Integral leak detection (test gas enrichment inside the enclosure); pressurized test gas inside specimen
d: Local leak detection; pressurized test gas inside the specimen
To determine overall leakiness of a test object pressurized with helium, the object shall be enclosed in an envelope which is either rigid or deformable (plastic). The test gas leaving the leaks accumulates so that the helium concentration in the envelope rises. Following an enrichment period to be determined (operating period) the change in concentration inside the envelope will be measured with a sniffer connected to the helium detection unit. The overall leak rate (integral leak rate) can be calculated following calibration of the test configuration with a reference concentration, e.g. atmospheric air. This method makes it possible to detect even the smallest overall leakiness and is suitable in particular for automated industrial leak testing. Due to gas accumulation, the limits for normal sniffer techniques are shifted toward lower leak rates and the ambient conditions such as temperature, air flow and sniffer tracing speed lose influence. When using plastic envelopes it is necessary to take into account helium permeation through the plastic envelope during long enrichment periods.
When the test specimen, pressurized with helium, is placed in a rigid vacuum chamber, connected to a helium leak detector, the integral leak rate can be read directly at the leak detector.
The evacuated test specimen is surrounded by a lightweight (plastic) enclosure and this is then filled with helium once the atmospheric air has been removed. When using a plastic bag as the envelope, the bag should be pressed against the test specimen before filling it with helium in order to expel as much air as possible and to make the measurement with the purest helium charge possible. The entire outside surface of the test object is in contact with the test gas. If test gas passes through leaks and into the test specimen, then the integral leak rate will be indicated, regardless of the number of leaks. In addition, it is necessary to observe when repeating testing in enclosed areas that the helium content of the room will rise quite rapidly when the envelope is removed. Using plastic bags is thus more advisable for “one-off” testing of large plants. The plastic envelope used here is often referred to as a “tent”.
The use of a solid vacuum vessel as the rigid envelope, on the other hand, is better for repetitive testing where an integral test is to be made. When solid envelopes are used it is also possible to recover the helium once the test has been completed.
The “bombing” test is used to check the tightness of components which are already hermetically sealed and which exhibit a gas-filled, internal cavity. The components to be examined (e.g. transistors, IC housings, dry-reed relays, reed contact switches, quartz oscillators, laser diodes and the like) are placed in a pressure vessel which is filled with helium. Operating with the test gas at relatively high pressure (5 to 10 bar) and leaving the system standing over several hours the test gas (helium) will collect inside the leaking specimens. This procedure is the actual “bombing”. To make the leak test, then, the specimens are placed in a vacuum chamber following “bombing”, in the same way as described for the vacuum envelope test. The overall leak rate is then determined. Specimens with large leaks will, however, lose their test gas concentration even as the vacuum chamber is being evacuated, so that they will not be recognized as leaky during the actual leak test using the detector. It is for this reason that another test to register very large leaks will have to be made prior to the leak test in the vacuum chamber.
Industrial leak testing using helium as the test gas is characterized above all by the fact that the leak detection equipment is fully integrated into the manufacturing line. The design and construction of such test units will naturally take into account the task to be carried out in each case (e.g. leak testing vehicle rims made of aluminum or leak testing for metal drums). Mass-produced, standardized component modules will be used wherever possible. The parts to be examined are fed to the leak testing system (envelope test with rigid envelope and positive pressure or vacuum inside the specimen; see respective sections above) by way of a conveyor system. There they will be examined individually using the integral methods and automatically moved on. Specimens found to be leaking will be shunted to the side.
The advantages of the helium test method, seen from the industrial point of view, may be summarized as follows:
Use of the helium test method will result in considerable increases in efficiency (cycling times being only a matter of seconds in length) and lead to a considerable increase in testing reliability. As a result of this and due to the EN/ISO 9000 requirements, traditional industrial test methods (water bath, soap bubble test, etc.) have now largely been abandoned.
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References, sources and further reading related to the fundamental knowledge of vacuum technology