## Conversion of helium leak rate to air leak rate

The above formula with the relative masses of air and helium

$\frac{q_{He}}{q_{Air}} = \frac{\sqrt{29}}{\sqrt{4}} = 2.7$

$\textup {helium leak rate} \, q_A = 1 \times 10^{-8} \, mbar \cdot l/s$

$\textup {relative molecular mass of helium} = 4$

$\textup {air leak rate} = \, ?$

$\textup {relative molecular mass of air} = 29$

$q_B = \frac{q_A \cdot \sqrt{M_A}}{\sqrt{M_B}} = \frac{1 \cdot 10^{-8} \cdot \sqrt{4}}{\sqrt{29}} = 3,7 \cdot 10^{-9} \, mbar \cdot l/s$

at equal conditions of pressure and temperature

The air leak rate for molecular flow is a factor of 2.7 smaller than the helium leak rate. Data sheets of helium leak detectors often mention the equivalent air leakrate as well as the smallest detectable helium leak rate. This can be misinterpreted since the factor is only valid for molecular flow and not for the total range of measurement. Modern helium leak detectors can measure leakrates millions of times larger than in molecular flow. In conclusion, a helium leakdetector cannot measure air leakrates.