ASTM E498-11, titled “Standard Practice for Leaks Using the Mass Spectrometer Leak Detector or Residual Gas Analyzer in the Tracer Probe Mode,” was released this month. It is one of Document Center’s most widely used standards for the non-destructive testing of components.
This practice covers procedures for testing and locating the sources of gas leaking at the rate of 1 × 10−8 Pa m3/s (1 × 10−9 Std cm 3/s) or greater. The test may be conducted on any object to be tested that can be evacuated and to the other side of which helium or other tracer gas may be applied.
Three test methods are described:
1. Test Method A—For the object under test capable of being evacuated, but having no inherent pumping capability.
Test Method A is most frequently used in leak testing components which are structurally capable of being evacuated to pressures of 0.1 Pa (approximately 10−3 torr). Testing of small components can be correlated to calibrated leaks, and the actual leak rate can be measured or acceptance can be based on a maximum allowable leak. For most production needs acceptance is based on acceptance of parts leaking less than an established standard which will ensure safe performance over the projected life of the component. Care must be exercised to ensure that large systems are calibrated with reference leak at a representative place on the test volume. Leak rates are determined by calculating the net gain or loss through a leak in the test part that would cause failure during the expected life of the device.
2. Test Method B—For the object under test with integral pumping capability.
Test Method B is used for testing vacuum systems either as a step in the final test of a new system or as a maintenance practice on equipment used for manufacturing, environmental test or for conditioning parts. As the volume tends to be large, a check of the response time as well as system sensitivity should be made. Volume of the system in liters divided by the speed of the vacuum pump in L/s will give the response time to reach 63 % of the total signal. Response times in excess of a few seconds makes leak detection difficult.
3. Test Method C—For the object under test as in Test Method B, in which the vacuum pumps of the object under test replace those normally used in the leak detector.
Test Method C is to be used only when there is no convenient method of connecting the leak detector to the outlet of the high vacuum pump. If a helium leak detector is used and the high vacuum pump is an ion pump or cryopump, leak testing is best accomplished during the roughing cycle as these pumps leave a relatively high percentage of helium in the high vacuum chamber. This will obscure all but large leaks, and the trace gas will quickly saturate the pumps.
This is the first revision of ASTM E498 since 1995 (that edition was reapproved in 2005). It is available from Document Center Inc., along with all ASTM standards, at our website www.document-center.com. Or call us at 650-591-7600, fax us at 650-591-7617, or send us an email at info@document-center.com. We can provide not only the current editions but many of the obsolete documents and revisions as well.