How are helium leak detectors used for integral leak tests
Envelope test, aka "integral leak test"
An envelope test is an integral leak test using helium as test gas.
In case of this method, the test object is enclosed in a rigid (usually metal) enclosure or in a light plastic envelope. The helium which flows through the leaks of the test object is detected using a helium leak detector.
Envelope tests are carried out according to the vacuum method (test object under vacuum, Fig 4a) as well as the positive pressure method (test object under positive pressure, Fig. 4c). In both cases it might be necessary to carry out the conversion to the helium standard leak rate.
Fig 4 - Usage options for a vacuum leak detector based on the vacuum method (a, b) and based on the positive pressure method (c, d)
- a: Enclosure test (integral leak detection)
- b: Spray technique (local leak detection)
- c: Enclosure test (integral leak detection)
- d: Sniffer technique (local leak detection)
Envelope test according to the positive pressure method
Envelope test with concentration measurement and subsequent leak rate calculation
In order to determine the overall leakage of the test object, it is enclosed with an envelope which may be rigid or flexible. The helium which flows through the leaks of the test object accumulates in the envelope so that the helium concentration in the envelope increases. After an accumulation time (operating period) to be determined, the change in concentration inside the envelope will be measured with a sniffer connected to the helium leak detector (Fig. 4c). The overall leak rate ( = integral leak rate) can be calculated following the calibration of the test configuration with a known concentration, e.g. atmospheric air (5 ppm).
This method makes it possible to detect even the smallest overall leakage 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.
Direct measurement of the leak rate with the leak detector - rigid envelope
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.
Envelope test according to the vacuum method
Envelope = “plastic tent”
The evacuated test object which is connected to the leak detector is enclosed with a flexible, light (plastic) envelope. Before the envelope is filled with helium, one presses it against the test object, in order to remove as much of the existing atmosphere air as possible.
The entire outer surface of the test object has contact with the test gas (helium). If test gas enters the test object through the leaks, the integral leak rate is measured without knowing the exact number of leaks.
In case of repeated tests in closed rooms, it must be noted that the helium concentration in the room will increase rather rapidly after removing the envelope. Using plastic bags is therefore more advisable for „one-time testing“ of large systems.
Rigid envelope
Rigid envelopes should be used in case of test series for determining integral leak rates. Furthermore, this has the advantage that the helium can be recovered after the test has been carried out.
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