(a) Scope and frequency. If you use a chiller to dry a sample upstream of a NOX measurement instrument, but you don't use an NO2-to-NO converter upstream of the chiller, you must perform this verification for chiller NO2 penetration. Perform this verification after initial installation and after major maintenance.

(b) Measurement principles. A chiller removes H2O, which can otherwise interfere with a NOX measurement. However, liquid H2O remaining in an improperly designed chiller can remove NO2 from the sample. If a chiller is used without an NO2-to-NO converter upstream, it could remove NO2 from the sample prior NOX measurement.

(c) System requirements. A chiller must allow for measuring at least 95% of the total NO2 at the maximum expected concentration of NO2.

(d) Procedure. Use the following procedure to verify chiller performance:

(1) Instrument setup. Follow the analyzer and chiller manufacturers' start-up and operating instructions. Adjust the analyzer and chiller as needed to optimize performance.

(2) Equipment setup and data collection.

(i) Zero and span the total NOX gas analyzer(s) as you would before emission testing.

(ii) Select an NO2 calibration gas, balance gas of dry air, that has an NO2 concentration within ±5% of the maximum NO2 concentration expected during testing.

(iii) Overflow this calibration gas at the gas sampling system's probe or overflow fitting. Allow for stabilization of the total NOX response, accounting only for transport delays and instrument response.

(iv) Calculate the mean of 30 seconds of recorded total NOX data and record this value as xNOXref.

(v) Stop flowing the NO2 calibration gas.

(vi) Next saturate the sampling system by overflowing a dewpoint generator's output, set at a dewpoint of 50 °C, to the gas sampling system's probe or overflow fitting. Sample the dewpoint generator's output through the sampling system and chiller for at least 10 minutes until the chiller is expected to be removing a constant rate of H2O.

(vii) Immediately switch back to overflowing the NO2 calibration gas used to establish xNOxref. Allow for stabilization of the total NOX response, accounting only for transport delays and instrument response. Calculate the mean of 30 seconds of recorded total NOX data and record this value as xNOxmeas.

(viii) Correct xNOxmeas to xNOxdry based upon the residual H2O vapor that passed through the chiller at the chiller's outlet temperature and pressure.

(3) Performance evaluation. If xNOxdry is less than 95% of xNOxref, repair or replace the chiller.

(e) Exceptions. The following exceptions apply:

(1) You may omit this verification if you can show by engineering analysis that for your NOX sampling system and your emission calculations procedures, the chiller always affects your brake-specific NOX emission results by less than 0.5% of the applicable NOX standard.

(2) You may use a chiller 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.

[73 FR 37312, June 30, 2008, as amended at 79 FR 23771, Apr. 28, 2014]