# 1065.378 NO-to-NO converter conversion verification.
(a) *Scope and frequency.* If you use an analyzer that measures only NO to determine NO<sub>X</sub>, you must use an NO<sub>2</sub>-to-NO converter upstream of the analyzer. Perform this verification after installing the converter, after major maintenance and within 35 days before an emission test. This verification must be repeated at this frequency to verify that the catalytic activity of the NO<sub>2</sub>-to-NO converter has not deteriorated.
(b) *Measurement principles.* An NO<sub>2</sub>-to-NO converter allows an analyzer that measures only NO to determine total NO<sub>X</sub> by converting the NO<sub>2</sub> in exhaust to NO.
(c) *System requirements.* An NO<sub>2</sub>-to-NO converter must allow for measuring at least 95% of the total NO<sub>2</sub> at the maximum expected concentration of NO<sub>2</sub>.
(d) *Procedure.* Use the following procedure to verify the performance of a NO<sub>2</sub>-to-NO converter:
(1) *Instrument setup.* Follow the analyzer and NO<sub>2</sub>-to-NO converter manufacturers' start-up and operating instructions. Adjust the analyzer and converter as needed to optimize performance.
(2) *Equipment setup.* Connect an ozonator's inlet to a zero-air or oxygen source and connect its outlet to one port of a three-way tee fitting. Connect an NO span gas to another port, and connect the NO<sub>2</sub>-to-NO converter inlet to the last port.
(3) *Adjustments and data collection.* Perform this check as follows:
(i) Set ozonator air off, turn ozonator power off, and set the analyzer to NO mode. Allow for stabilization, accounting only for transport delays and instrument response.
(ii) Use an NO concentration that is representative of the peak total NO<sub>X</sub> concentration expected during testing. The NO<sub>2</sub> content of the gas mixture shall be less than 5% of the NO concentration. Record the concentration of NO by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as *x*<sub>NOref</sub>.
(iii) Turn on the ozonator O<sub>2</sub> supply and adjust the O<sub>2</sub> flow rate so the NO indicated by the analyzer is about 10 percent less than *x*<sub>NOref</sub>. Record the concentration of NO by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as *x*<sub>NO + O2mix</sub>.
(iv) Switch the ozonator on and adjust the ozone generation rate so the NO measured by the analyzer is 20 percent of *x*<sub>NOref</sub> or a value which would simulate the maximum concentration of NO<sub>2</sub> expected during testing, while maintaining at least 10 percent unreacted NO. This ensures that the ozonator is generating NO<sub>2</sub> at the maximum concentration expected during testing. Record the concentration of NO by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as *x*<sub>NOmeas</sub>.
(v) Switch the NO<sub>X</sub> analyzer to NO<sub>X</sub> mode and measure total NO<sub>X</sub>. Record the concentration of NO<sub>X</sub> by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as *x*<sub>NOxmeas</sub>.
(vi) Switch off the ozonator but maintain gas flow through the system. The NO<sub>X</sub> analyzer will indicate the NO<sub>X</sub> in the NO + O<sub>2</sub> mixture. Record the concentration of NO<sub>X</sub> by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as *x*<sub>NOx + O2mix</sub>.
(vii) Turn off the ozonator O<sub>2</sub> supply. The NO<sub>X</sub> analyzer will indicate the NO<sub>X</sub> in the original NO-in-N<sub>2</sub> mixture. Record the concentration of NO<sub>X</sub> by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as *x*<sub>NOxref</sub>. This value should be no more than 5 percent above the *x*<sub>NOref</sub> value.
(4) *Performance evaluation.* Calculate the efficiency of the NO<sub>X</sub> converter by substituting the concentrations obtained into the following equation:
(5) If the result is less than 95%, repair or replace the NO<sub>2</sub>-to-NO converter.
(e) *Exceptions.* The following exceptions apply:
(1) You may omit this verification if you can show by engineering analysis that for your NO<sub>X</sub> sampling system and your emission calculations procedures, the converter always affects your brake-specific NO<sub>X</sub> emission results by less than 0.5% of the applicable NO<sub>X</sub> standard.
(2) You may use a converter that you determine does not meet this verification, as long as you try to correct the problem and the measurement deficiency does not adversely affect your ability to show that engines comply with all applicable emission standards.
(3) You may request to verify converter conversion efficiency using an NO<sub>2</sub> concentration whose value is representative of the peak total NO<sub>2</sub> concentration expected during testing, in place of the procedure in paragraph (d) of this section, with our approval.
[70 FR 40516, July 13, 2005, as amended at 73 FR 37313, June 30, 2008; 73 FR 59330, Oct. 8, 2008; 76 FR 57447, Sept. 15, 2011; 89 FR 29802, Apr. 22, 2024]