(a) Scope and frequency. This section describes how to perform interference verification for certain analyzers as described in § 1065.277. Perform interference verification after initial analyzer installation and after major maintenance.

(b) Measurement principles. Certain compounds can positively interfere with analyzers by causing a response similar to NH3. If the analyzer uses compensation algorithms that utilize measurements of other gases to meet this interference verification, a correct result depends on simultaneously conducting 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 ±2.0) µmol/mol.

(d) Procedure. Perform the interference verification as follows:

(1) Start, operate, zero, and span the NH3 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) Except as specified in paragraph (d)(9) of this section, select a multi-component span gas meeting the specification of § 1065.750 that incorporates the all the appropriate interference species. Use a humidity generator that meets the requirements in § 1065.750(a)(6) to humidify the span gas. If the sample does not pass through a dryer during emission testing, humidify your test gas to an H2O level at or above the maximum expected during emission testing. If the sample passes through a dryer during emission testing, humidify your test gas to an H2O level at or above 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 upstream or downstream of any sample dryer, if one is used during testing.

(4) If the sample does not pass 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 analyzer inlet. You may 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 passes through a dryer during this verification test, either measure dewpoint, Tdew, and absolute pressure, ptotal, to calculate xH2O or use good engineering judgment to estimate the value of xH2O based on the vessel pressure and temperature. For example, you may use previous direct measurements of H2O content at certain vessel pressures and temperatures to estimate xH2O.

(5) If the verification procedure does not include a sample dryer, use good engineering judgment to prevent condensation in the transfer lines, fittings, or valves between the point of xH2O measurement and the analyzer. We recommend that you design your system so that the wall temperatures in those transfer lines, fittings, and valves are at least 5 °C 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) Operate the analyzer to measures the sample's NH3 concentration and record results for 30 seconds. Calculate the arithmetic mean of these data to determine the interference value. When performed with all the interference species simultaneously, this is the combined interference.

(8) The analyzer meets the interference verification if the result of paragraph (d)(7) of this section meets the tolerance in paragraph (c) of this section.

(9) You may instead perform interference verification procedures separately for individual interference species. The interference verification specified in paragraph (c) of this section applies based on the sum of the interference values from separate interference species. If the concentration of any interference species used is higher than the maximum levels expected during testing, you may scale down each observed interference value by multiplying the observed interference value by the ratio of the maximum expected concentration value to the concentration in the span gas. You may run separate H2O interference concentrations (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 value by multiplying the observed interference value by the ratio of the maximum expected H2O concentration value to the concentration in the span gas. The sum of the scaled interference values must meet the tolerance for combined interference as specified in paragraph (c) of this section.

[89 FR 29801, Apr. 22, 2024]