(a) The efficiency of the converter used for the conversion of NO2 to NO is tested as given in paragraphs (a)(1) through (a)(8) of this section (see Figure 2 in appendix B to this subpart).
(1) Using the test setup as shown in Figure 2 in appendix B to this subpart (see also §91.318 of this chapter) and the procedures described in paragraphs (a)(2) through (a)(8) of this section, test the efficiency of converters by means of an ozonator.
(2) Calibrate the HCLD in the most common operating range following the manufacturer's specifications using zero and span gas (the NO content of which must amount to about 80 percent of the operating range and the NO2 concentration of the gas mixture less than 5 percent of the NO concentration). The NOX analyzer must be in the NO mode so that the span gas does not pass through the converter. Record the indicated concentration.
(3) Calculate the efficiency of the NOX converter as described in §91.318(b).
(4) Via a T-fitting, add oxygen continuously to the gas flow until the concentration indicated is about 20 percent less than the indicated calibration concentration given in paragraph (a)(2) of this section. Record the indicated concentration as “c”. The ozonator is kept deactivated throughout the process.
(5) Activate the ozonator to generate enough ozone to bring the NO concentration down to about 20 percent (minimum 10 percent) of the calibration concentration given in paragraph (a)(2) of this section. Record the indicated concentration as “d”.
Note: If, with the analyzer in the most common range the NOX converter cannot give a reduction from 80 percent to 20 percent, then use the highest range which will give the reduction.
(6) Switch the NO analyzer to the NOX mode, which means that the gas mixture (consisting of NO, NO2, O2 and N2) now passes through the converter. Record the indicated concentration as “a”.
(7) Deactivate the ozonator. The mixture of gases described in paragraph (a)(6) of this section passes through the converter into the detector. Record the indicated concentration as “b”.
(8) Switched to NO mode with the ozonator deactivated, the flow of oxygen or synthetic air is also shut off. The NOX reading of the analyzer may not deviate by more than ±5 percent of the theoretical value of the figure given in paragraph (a)(2) of this section.
(b) The efficiency of the converter must be tested prior to each calibration of the NOX analyzer.
(c) The efficiency of the converter may not be less than 90 percent.
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