Helium leak testing is a vital step in assuring product reliability for anything that must be packaged in a sealed container. Examples abound from blister packaging for pharmaceuticals to aluminum wheel rims for automobiles. In order to quantify detected leaks, mass spectrometer based helium leak detectors must be calibrated with helium flow transfer standards. Best results are obtained when the leak detector is calibrated with a stable transfer standard having a quantified uncertainty. Helium permeation leak artifacts are the most popular transfer standards used for this purpose. Though permeation elements are most often made of glass, Teflon is sometimes used for the leak element in applications where mechanical shock may damage a glass element. Teflon -based helium leak artifacts are sold by several different manufacturers, often with a calibration certificate. The National Institute of Standards and Technology (NIST) calibrates helium permeation leak artifacts as a function of temperature, and has found some anomalous behavior coming from Teflon -based artifacts in comparison with glass-based artifacts. These anomalies include hysteretic effects which are much larger than the uncertainty of the measured leak rate, and sudden discontinuities (jumps up or down) in the measured leak rate as a function of temperature. This behavior has been attributed to the well documented phase transitions that occur in the Teflon polymers near room temperature (19 oC and 30 oC). Data on two commercially available Teflon -based helium permeation leaks will be presented.
Citation: NCSL International Measure
Pub Type: Journals
flow standards, helium, leak artifact, permeation, Teflon