BS EN ISO 20486:2018,Non-destructive testing – Leak testing – Calibration of reference leaks for gases.BS EN ISO 20486:2018 pdf free download here.
BS EN ISO 20486:2018 specifies the calibration of those leaks that are used for the adjustment of leak detectors for the determination of leakage rate in everyday use. One type of calibration method is a comparison with a reference leak. In this way, the leaks used for routine use become traceable to a primary standard. In other calibration methods, the value of vapour pressure was measured directly or calculated over a known volume.

The comparison procedures are preferably applicable to helium leaks, because this test gas can be selectively measured by a mass spectrometer leak detector (MSLD) (the definition of MSLD is given in ISO 20484).
2 Normative references
ISO 20484, Non-destructive testing – Leak testing -Vocabulary.
Aperture leak (orifice)
Orifices are seldom used as reference leaks in practice, as they are difficult to manufacture and even more prone to blocking than capillaries.
5. 2. 3 Compressed powder leak
This type of leak uses metal powder compressed into a tube. They are usually offered without reservoir. They are used for routine check of the sensitivity of leak detectors but they are not stable enough to be Ised as calibrated leaks. Their suitability depends on how well controlled the storage and operating conditions are, and on the required uncertainty.