40 CFR § 1065.375 - Interference verification for N2O analyzers.
(a) Scope and frequency. See § 1065.275 to determine whether you need to verify the amount of interference after initial analyzer installation and after major maintenance.
(b) Measurement principles. Interference gases can positively interfere with certain analyzers by causing a response similar to N2O. If the analyzer uses compensation algorithms that utilize measurements of other gases to meet this interference verification, simultaneously conduct these other measurements to test the compensation algorithms during the analyzer interference verification.
(c) System requirements. Analyzers must have combined interference that is within (0.0 ±1.0) µmol/mol. We strongly recommend a lower interference that is within (0.0 ±0.5) µmol/mol.
(d) Procedure. Perform the interference verification as follows:
(1) Start, operate, zero, and span the N2O analyzer as you would before an emission test. If the sample is passed through a dryer during emission testing, you may run this verification test with the dryer if it meets the requirements of § 1065.342. Operate the dryer at the same conditions as you will for an emission test. You may also run this verification test without the sample dryer.
(2) Create a humidified test gas by bubbling a multi component span gas that incorporates the target interference species and meets the specifications in § 1065.750 through distilled H2O in a sealed vessel. If the sample is not passed through a dryer during emission testing, control the vessel temperature to generate an H2O level at least as high as the maximum expected during emission testing. If the sample is passed through a dryer during emission testing, control the vessel temperature to generate an H2O level at least as high as the level determined in § 1065.145(e)(2) for that dryer. Use interference span gas concentrations that are at least as high as the maximum expected during testing.
(3) Introduce the humidified interference test gas into the sample system. You may introduce it downstream of any sample dryer, if one is used during testing.
(4) If the sample is not passed through a dryer during this verification test, measure the H2O mole fraction, xH2O, of the humidified interference test gas as close as possible to the inlet of the analyzer. For example, measure dewpoint, Tdew, and absolute pressure, ptotal, to calculate xH2O. Verify that the H2O content meets the requirement in paragraph (d)(2) of this section. If the sample is passed through a dryer during this verification test, you must verify that the H2O content of the humidified test gas downstream of the vessel meets the requirement in paragraph (d)(2) of this section based on either direct measurement of the H2O content (e.g., dewpoint and pressure) or an estimate based on the vessel pressure and temperature. Use good engineering judgment to estimate the H2O content. For example, you may use previous direct measurements of H2O content to verify the vessel's level of saturation.
(5) If a sample dryer is not used in this verification test, use good engineering judgment to prevent condensation in the transfer lines, fittings, or valves from the point where xH2O is measured to the analyzer. We recommend that you design your system so that the wall temperatures in the transfer lines, fittings, and valves from the point where xH2O is measured to the analyzer are at least 5 ºC above the local sample gas dewpoint.
(6) Allow time for the analyzer response to stabilize. Stabilization time may include time to purge the transfer line and to account for analyzer response.
(7) While the analyzer measures the sample's concentration, record its output for 30 seconds. Calculate the arithmetic mean of this data. When performed with all the gases simultaneously, this is the combined interference.
(9) You may also run interference procedures separately for individual interference gases. If the interference gas levels used are higher than the maximum levels expected during testing, you may scale down each observed interference value (the arithmetic mean of 30 second data described in paragraph (d)(7) of this section) by multiplying the observed interference by the ratio of the maximum expected concentration value to the actual value used during this procedure. You may run separate interference concentrations of H2O (down to 0.025 mol/mol H2O content) that are lower than the maximum levels expected during testing, but you must scale up the observed H2O interference by multiplying the observed interference by the ratio of the maximum expected H2O concentration value to the actual value used during this procedure. The sum of the scaled interference values must meet the tolerance for combined interference as specified in paragraph (c) of this section.