Section 86.514-78 - Analytical gases(a)Analyzer gases.(1) Gases for the CO and CO2 analyzers shall be single blends of CO and CO2 respectively using nitrogen as the diluent.(2) Gases for the THC analyzer shall be:(i) Single blends of propane using air as the diluent; and(ii) Optionally, for response factor determination, single blends of methanol using air as the diluent.(3) Gases for the NOX analyzer shall be single blends of NO named as NOX with a maximum NO2 concentration of 5 percent of the nominal value using nitrogen as the diluent.(5) The allowable zero gas (air or nitrogen) impurity concentrations shall not exceed 1 ppm equivalent carbon response, 1 ppm carbon monoxide, 0.04 percent (400 ppm) carbon dioxide, and 0.1 ppm nitric oxide.(6) "Zero grade air" includes artificial "air" consisting of a blend of nitrogen and oxygen with oxygen concentrations between 18 and 21 mole percent.(7) The use of proportioning and precision blending devices to obtain the required analyzer gas concentrations is allowable provided their use has been approved in advance by the Administrator.(b) Calibration gases (not including methanol) shall be known to within 2 percent of true values.(c) Methanol in air gases used for response factor determination shall: (1) Be traceable to within ±2 percent of NIST (formerly NBS) gas standards, or other gas standards which have been approved by the Administrator; and(2) Remain within ±2 percent of the labeled concentration. Demonstration of stability shall be based on a quarterly measurement procedure with a precision of ±2 percent (two standard deviations), or other method approved by the Administrator. The measurement procedure may incorporate multiple measurements. If the true concentration of the gas changes by more than two percent, but less than ten percent, the gas may be relabeled with the new concentration.42 FR 1137, Jan. 5, 1977, as amended at 60 FR 34354, June 30, 1995