Technical Note 3

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LEAK RATE RANGES & DETECTION

The chart presents the full range of sealed package leak rates (cm3 atm sec-1) and capabilities of various test methods to measure those ranges. Leak rates range from 1 x 10-1 (E-1) to 1 x 10-12 (E-12) cm3 atm/sec-1. Ideally, viscous gas flow, nominally in the gross leak range, occurs when the mean free path of the gas is smaller than the cross-section dimension of the leak path. Molecular flow, nominally in the fine leak range, occurs when the mean free path of the gas is larger than the longest cross-section dimension of the leak path. There is a transitional range from viscous to molecular flow of gases that occurs imprecisely in the range of E-5 ⥦ E-6 cm3 atm sec-1.

In practice, leak rates are generally referred to as “gross” or “fine”, without the arbitrary distinctions “transition” and “ultrafine”.

The real physical behavior of gases moving through leak paths is complex and can be highly variable depending on gas species, leak size, tortuousness, physics and chemistry of leak path surfaces, temperature, internal and external differences in partial pressure, and other variables.

High Sensitivity Helium Leak Detection

This technique extends the dynamic range of helium leak testing to cover the full range of gross and fine leak detection using a highly sensitive quadruple mass spectrometer as the helium tracer gas detector, combined with specialized system fixturing and software for rapid testing cycle time.

It combines helium/oxygen dry gross leak and helium fine leak into one system with a single pressurization step, thereby joining fine and gross leak testing into one procedure. Extending helium leak testing capability into the E-12 range. Mil-Std 883 Method 1014 Test Conditions A1, A2.

Major factors in determining test cycle time and leak detection sensitivity are the length of time specimen packages must be “soaked” or “bombed” in tracer gas and the pressure level of the tracer gas being used. The smaller a leak path, the longer it takes to force tracer gas under pressure through the path into the interior headspace. Enough tracer gas must ingress the cavity to give enough escape time in the detection portion of the test to obtain a measurable result.

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