A calibration apparatus for pressure leak with the lower limit of $2\times 10^{-10}\ \mathrm{Pam}^{3}/\mathrm{s}$

A calibration apparatus with the lower limit of calibration of $\pmb{2\times 10^{-10}}\mathbf{Pam}^{\pmb{3}}/\mathbf{s}$ is developed to solve the calibration problem of leak with tiny leak rate. The concentration of the indicator gas in the gas mixture in the accumulating chamber was increased by using accumulation method. The standard gas mixtures were obtained by direct measurement and inflation pressure attenuation method, respectively. The ‘fractionation’ effect in the gas mixture was eliminated and the partial pressure of the leak indicator gas in the mass analysis chamber was increased by using dynamic injection of molecular flow method and pumping orifice with constant flow conductance. Using the quadrupole mass spectrometer (QMS) as a comparator, the leak rate of a pressure leak can be calibrated by measuring the partial pressure of the leak indicator gas in the accumulation gas through the leak and that in the standard gas mixture in the mass analysis chamber. The impact of temperature on the measurement results is reduced by using high precise two-stage semiconductor water bath method to limit the temperature within $\pmb{296+0.02}\mathbf{K}$ . The experimental results show that the calibration of the apparatus is in the range from $\pmb{7\times 10^{-6}}\mathbf{Pam}^{\pmb{3}}/\mathbf{s}$ to $\pmb{2\times 10^{-10}} \mathbf{Pam}^{\pmb{3}}/\mathbf{s}$ with the combined standard uncertainty between 2.0% and 7.5%.