2.2 Calibration and span gases

  2.2.1 Mixtures of gases having the following chemical compositions shall be available:

• .1 CO and purified nitrogen;

• .2 NO_x and purified nitrogen the amount of NO₂ contained in this calibration gas shall not exceed 5% of the NO content);

• .3 O₂ and purified nitrogen;

• .4 CO₂ and purified nitrogen; and

• .5 CH₄ and purified synthetic air or C₃H₈ and purified synthetic air.

• Note: Other gas combinations are allowed provided the gases do not react with one another.

  2.2.2 The true concentration of a calibration and span gas must be within ± 2% of the nominal value. All concentrations of calibration and span gases shall be given on a volume basis (volume per cent or volume ppm).

  2.2.3 The gases used for calibration and span may also be obtained by means of precision blending devices (gas dividers), diluting with purified N₂ or with purified synthetic air. The accuracy of the mixing device must be such that the concentration of the blended calibration gases is accurate to within ±2%. This accuracy implies that primary gases used for blending must be known to an accuracy of at least ±1%, traceable to national or international gas standards. The verification shall be performed at between 15 and 50% of full scale for each calibration incorporating a blending device. Optionally, the blending device may be checked with an instrument which by nature is linear, e.g., using NO gas with a CLD. The span value of the instrument shall be adjusted with the span gas directly connected to the instrument. The blending device shall be checked at the used settings and the nominal value shall be compared to the measured concentration of the instrument. This difference shall in each point be within ±1% of the nominal value. This linearity check of the gas divider shall not be performed with a gas analyser which was previously linearized with the same gas divider.

  2.2.4 Oxygen interference check gases shall contain propane or methane with 350 ppmC ± 75 ppmC hydrocarbon. The concentration shall be determined to calibration gas tolerances by chromatographic analysis of total hydrocarbons plus impurities or by dynamic blending. Nitrogen shall be the predominant diluent with the balance oxygen. Blends required are listed in table 5.