40 CFR § 1065.272 - Nondispersive ultraviolet analyzer.
(a) Application. You may use a nondispersive ultraviolet (NDUV) analyzer to measure NOX concentration in raw or diluted exhaust for batch or continuous sampling. We generally accept an NDUV for NOX measurement, even though it measures only NO and NO2, since conventional engines and aftertreatment systems do not emit significant amounts of other NOX species. Measure other NOX species if required by the standard-setting part. Note that good engineering judgment may preclude you from using an NDUV analyzer if sampled exhaust from test engines contains oil (or other contaminants) in sufficiently high concentrations to interfere with proper operation.
(b) Component requirements. We recommend that you use an NDUV analyzer that meets the specifications in Table 1 of § 1065.205. Note that your NDUV-based system must meet the verifications in § 1065.372 and it must also meet the linearity verification in § 1065.307.
(c) NO2-to-NO converter. If your NDUV analyzer measures only NO, place upstream of the NDUV analyzer an internal or external NO2-to-NO converter that meets the verification in § 1065.378. Configure the converter with a bypass to facilitate this verification.
(d) Humidity effects. You must maintain NDUV temperature to prevent aqueous condensation, unless you use one of the following configurations: