The chemiluminescent oxides of nitrogen analyzer shall receive the following initial and periodic calibration.

(a) Prior to introduction into service and at least monthly thereafter the chemiluminescent oxides of nitrogen analyzer must be checked for NO2 to NO converter efficiency. Figure B78-9 is a reference for paragraphs (a) (1) through (11) of this section.

(1) Follow the manufacturer's instructions for instrument startup and operation. Adjust the analyzer to optimize performance.

(2) Zero the oxides of nitrogen analyzer with zero-grade air or zero-grade nitrogen.

(3) Connect the outlet of the NOX generator to the sample inlet of the oxides of nitrogen analyzer which has been set to the most common operating range.

(4) Introduce into the NOX generator analyzer-system an NO in nitrogen (N2) mixture with an NO concentration equal to approximately 80 percent of the most common operating range. The NO2 content of the gas mixture shall be less than 5 percent of the NO concentration.

(5) With the oxides of nitrogen analyzer in the NO mode, record the concentration of NO indicated by the analyzer.

(6) Turn on the NOX generator O2 (or air) supply and adjust the O2 (or air) flow rate so that the NO indicated by the analyzer is about 10 percent less than indicated in step (5). Record the concentration of NO in this NO + O2 mixture.

(7) Switch the NOX generator to the generation mode and adjust the generation rate so that the NO measured on the analyzer is 20 percent of that measured in step (5). There must be at least 10 percent unreacted NO at this point. Record the concentration of residual NO.

(8) Switch the oxides of nitrogen analyzer to the NOX mode and measure total NOX. Record this value.

(9) Switch off the NOX generation but maintain gas flow through the system. The oxides of nitrogen analyzer will indicate the NOX in the NO + O2 mixture. Record this value.

(10) Turn off the NOX generator O2 (or air) supply. The analyzer will now indicate the NOX in the original NO in N2 mixture. This value should be no more than 5 percent above the value indicated in step (4).

(11) Calculate the efficiency of the NOX converted by substituting the concentrations obtained into the following equation:

Percent Efficiency = [1 + (a − b)/(c − d)] × 100 where: a = concentration obtained in step (8). b = concentration obtained in step (9). c = concentration obtained in step (6). d = concentration obtained in step (7). If converter efficiency is not greater than 90 percent corrective action will be required.

(b) Initial and periodic calibration. Prior to its introduction into service and monthly thereafter the chemiluminescent oxides of nitrogen analyzer shall be calibrated on all normally used instrument ranges. Use the same flow rate as when analyzing samples. Proceed as follows:

(1) Adjust analyzer to optimize performance.

(2) Zero the oxides of nitrogen analyzer with zero-grade air or zero-grade nitrogen.

(3) Calibrate on each normally used operating range with NO in N2 calibration gases having nominal concentrations of 15, 30, 45, 60, 75, and 90 percent of that range. For each range calibrated, if the deviation from a least-squares best-fit straight line is 2 percent or less of the value at each data point, concentration values may be calculated by use of a single calibration factor for that range. If the deviation exceeds 2 percent at any point, the best-fit non-linear equation which represents the data to within 2 percent of each test point shall be used to determine concentration.

(c) When testing methanol-fueled vehicles, it may be necessary to clean the analyzer frequently to prevent interference with NOX measurements (see EPA/600/S3-88/040).

[42 FR 32954, June 28, 1977, as amended at 42 FR 45652, Sept. 12, 1977; 52 FR 47869, Dec. 16, 1987; 58 FR 58422, Nov. 1, 1993; 60 FR 34347, June 30, 1995]