40 CFR § 1065.357 - CO2 interference verification for H2O FTIR analyzers.
(a) Scope and frequency. If you measure H2O using an FTIR analyzer, verify the amount of CO2 interference after initial analyzer installation and after major maintenance.
(b) Measurement principles. CO2 can interfere with an FTIR analyzer's response to H2O. If the FTIR 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. An H2O FTIR analyzer must have a CO2 interference that is within (0.0 ± 0.4) mmol/mol, though we strongly recommend a lower interference that is within (0.0 ± 0.2) mmol/mol.
(d) Procedure. Perform the interference verification as follows:
(1) Start, operate, zero, and span the H2O FTIR analyzer as you would before an emission test.
(2) Use a CO2 span gas that meets the specifications of § 1065.750 and a concentration that is approximately the maximum CO2 concentration expected during emission testing.
(3) Introduce the CO2 test gas into the sample system.
(4) Allow time for the analyzer response to stabilize. Stabilization time may include time to purge the transfer line and to account for analyzer response.
(5) Operate the analyzer to get a reading for H2O concentration and record results for 30 seconds. Calculate the arithmetic mean of these data.
(6) The analyzer meets the interference verification if the result of paragraph (d)(5) of this section meets the tolerance in paragraph (c) of this section.
(e) Exceptions. The following exceptions apply:
(1) You may omit this verification for CO2 for engines operating on fuels other than carbon-containing fuels.
(2) You may omit this verification if you can show by engineering analysis that for your H2O sampling system and your emission-calculation procedures, the CO2 interference for your H2O FTIR analyzer always affects your brake-specific emission results within ±0.5% of each of the applicable standards in this chapter. This specification also applies for vehicle testing, except that it relates to emission results in g/mile or g/kilometer.
(3) You may use an H2O FTIR analyzer 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.