NF X20-209

<p>This part of ISO 6145 specifies a dynamic method using permeation membranes for the preparation of calibration</p> <p>gas mixtures containing component mole fractions ranging from 10−9 and 10−6. A relative expanded uncertainty of</p> <p>2,5 % of the component mole fraction can be achieved using this method. In the mole fraction range considered, it</p> <p>is difficult to maintain some gas mixtures, for example in cylinders, in a stable state. It is therefore desirable to</p> <p>prepare the calibration gas immediately before use, and to transfer it by the shortest possible path to the place</p> <p>where it is to be used. This technique has been successfully applied in generating low content calibration gas</p> <p>mixtures of, for example, sulfur dioxide (SO2), nitrogen dioxide (NO2) and benzene (C6H6) in air.</p> <p>If the carrier gas flow is measured as a gas mass-flow, the preparation of calibration gas mixtures using permeation</p> <p>tubes is a dynamic-gravimetric method which gives contents in mole fractions.</p>