(a) The shelf life of a calibration gas may not be exceeded. The expiration date stated by the gas supplier must be recorded.

(b) Pure gases. The required purity of the gases is defined by the contamination limits specified in this subsection. The following gases must be available for operation:

(1) Purified nitrogen, also referred to as “zero-grade nitrogen” (Contamination ≤1 ppm C, ≤1 ppm CO, ≤400 ppm CO2, ≤0.1 ppm NO);

(2) Purified oxygen (Purity 99.5 percent vol O2);

(3) Hydrogen-helium mixture (40 ±2 percent hydrogen, balance helium) (Contamination ≤1 ppm C, ≤400 ppm CO);

(4) Purified synthetic air, also referred to as “zero air” or “zero gas” (Contamination ≤1 ppm C, ≤1 ppm CO, ≤400 ppm CO2, ≤0.1 ppm NO) (Oxygen content between 18-21 percent vol.).

(c) Calibration and span gases.

(1) Calibration gas values are to be derived from NIST “Standard Reference Materials” (SRM's) and are to be single blends as specified in this subsection.

(2) Mixtures of gases having the following chemical compositions must be available:

C3 H8 and purified synthetic air and/or C3 H8 and purified nitrogen;

CO and purified nitrogen;

NOX and purified nitrogen (the amount of NO2 contained in this calibration gas must not exceed five percent of the NO content);

CO2 and purified nitrogen.

(3) The true concentration of a span gas must be within ±two percent of the NIST gas standard. The true concentration of a calibration gas must be within ±one percent of the NIST gas standard. The use of precision blending devices (gas dividers) to obtain the required calibration gas concentrations is acceptable. Give all concentrations of calibration gas on a volume basis (volume percent or volume ppm).

(4) The gas concentrations used for calibration and span may also be obtained by means of a gas divider, diluting either with purified N2 or with purified synthetic air. The accuracy of the mixing device must be such that the concentration of the diluted gases may be determined to within ±two percent.

(d) Oxygen interference check gases must contain propane with 350 ppmC ±75 ppmC hydrocarbon. Determine the concentration value to calibration gas tolerances by chromatographic analysis of total hydrocarbons plus impurities or by dynamic blending. For gasoline fueled engines, oxygen contentration must be between 0 and 1 percent O2. Nitrogen must be the predominant diluent with the balance oxygen.

(e) Fuel for the hydrocarbon flame ionization detector (HC-FID) must be a blend of 40 ±two percent hydrogen with the balance being helium. The mixture must contain less than one ppm equivalent carbon response; 98 to 100 percent hydrogen fuel may be used with advance approval of the Administrator.

(f) Hydrocarbon analyzer burner air. The concentration of oxygen must be within one mole percent of the oxygen concentration of the burner air used in the latest oxygen interference check (percent O2 I), see §90.316(d). If the difference in oxygen concentration is greater than one mole percent, then the oxygen interference must be checked and, if necessary, the analyzer adjusted to meet the percent O2 I requirements. The burner air must contain less than two ppmC hydrocarbon.