The resulting output voltage is proportional to mass flow, which provides the measurement of the leakage rate. Measuring the pressure of the test part twice also doubles the opportunity for errors, which will produce equivalent or greater errors in the subsequent leak-rate calculations. Mass-flow systems are particularly well suited to the rapid measurement of small leaks and leaks in cavities with volumes of 20 L or more. In light of the improvements that have been made, the mass-flow leak sensor can now be considered a precision gage. These helium-based methods, which use mass spectrometers as detectors, offer several advantages, although their higher cost usually restricts them to specialized applications that require those advantages.
Helium testing can detect slower leak rates for two reasons.
First, the helium mass spectrometer is much more sensitive than air-leak sensors. In situations where helium-based leak testing can detect pores small enough to block the passage of microbes, this technique can be used to help audit packaging processes or to production test package components such as formed plastic trays or covers. The chamber is then evacuated, inviting the helium to pass through any leakage points in the test part into the surrounding vacuum. A mass spectrometer then samples the vacuum chamber and ionizes any helium present. Helium techniques are also particularly useful for testing sealed enclosures for leakage, because the air inside the enclosure cannot be pressurized or measured directly by traditional means.