The ASTM E498/E498M-11 Standard Practice for Leaks Using Mass Spectrometer Leak Detector or Residual Gas Analyzer in Tracer Probe Mode, developed by the American Society for Testing and Materials (ASTM), describes procedures for testing and locating sources of gas leaks of 1 × 10−8 Pa m³ / s or greater. These tests can be performed on any object to be tested that can be evacuated and to which helium or other tracer gas can be applied.
Three test methods are described in this standard:
For the purposes of this standard, the terms and definitions in the standard “ASTM E1316-16 Standard terminology for non-destructive testing” apply.
The test methods described in this standard require a helium leak detector capable of detecting a leak of 1 × 10−9 Pa m³ / s.
Test method A is used to helium leak test objects that can be evacuated to a reasonable test pressure by the leak detector pumps in an acceptable time. The object must be clean and dry. Additionally, to handle larger volumes or relatively dirty instruments, auxiliary vacuum pumps of greater capacity than those in the mass spectrometer leak detector can be used with the mass spectrometer leak detector. Under these conditions, leak test sensitivity is reduced.
Test method B is used on leak testing equipment that can provide its own vacuum (i.e., equipment with a built-in pumping system) at least to a level of several hundred pascals or less.
If test method C can produce internal pressures lower than 2 × 10−2 Pa in the presence of leaks , these leaks can be detected and evaluated using a residual gas analyzer or a spectrometer tube and controls from a conventional mass spectrometer leak detector. In this case, it is essential that the leak is within the sensitivity range of the spectrometer from the residual gas analyzer or mass spectrometer leak detector under the existing conditions in the vacuum system.
Test method A is the most commonly used method for leak testing components that can be structurally discharged to pressures of 0,1 Pa. Testing of small components can be correlated with calibrated leaks and the actual leakage rate can be measured, or acceptance can be based on the maximum allowable leakage. For most production needs, acceptance is based on acceptance of components that leak less than an established standard that will ensure safe performance over the expected life of the component. Care should be taken to ensure that large systems are calibrated with a reference leak at a representative location in the test volume. Leakage rates are determined by calculating the net gain or loss from a leak in the test part that would cause failure over the expected life of the device.
Test method B is used to test vacuum systems as a final test step for a new system or as a maintenance application on equipment used for production, environmental testing, or conditioning of parts. Since volumes tend to be large, system sensitivity must be checked as well as response time. Dividing the system volume in liters by the speed of the vacuum pump in L/s gives the response time to reach 63 percent of the total signal. Response times exceeding a few seconds make leak detection difficult.
Test method C should be used only when there is no suitable method of connecting the leak detector to the output of the high vacuum pump. If a helium leak detector is being used and the high vacuum pump is an ion pump or cryo pump, leak testing is best performed during the roughing cycle as these pumps leave a relatively high percentage of helium in the high vacuum chamber. This will hide all but the largest leaks and the trace gas will rapidly saturate the pumps.
Serial leaks with an unpumped volume between them present a difficult, even impossible, problem in helium leak testing. Although trace gas allows helium to enter the first leak quite easily, because the pressure difference of helium across the first leak is approximately one atmosphere, it can take hours for partial pressure of helium to build up in the volume between the two leaks, and thus for enough helium to enter the vacuum system to be detectable by a mass spectrometer leak detector. Such leaks frequently occur under the following conditions:
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