40 CFR § 1065.350 - H2O interference verification for CO2 NDIR analyzers.
(a) Scope and frequency. If you measure CO2 using an NDIR analyzer, verify the amount of H2O interference after initial analyzer installation and after major maintenance.
(b) Measurement principles. H2O can interfere with an NDIR analyzer's response to CO2.
If the NDIR 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. A CO2 NDIR analyzer must have an H2O 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 CO2 NDIR 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 zero gas that meets the specifications in § 1065.750 through distilled H2O in a sealed vessel or use a device that introduces distilled H2O as vapor into a controlled gas flow. 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, you must humidify your test gas to an H2O level at or above the level determined in § 1065.145(e)(2) for that dryer.
(3) Introduce the humidified 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 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 the wall temperatures in the transfer lines, fittings, and valves from the point where xH2O is measured to the analyzer 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) While the analyzer measures the sample's concentration, record 30 seconds of sampled data. Calculate the arithmetic mean of this data. The analyzer meets the interference verification if this value is within (0.0 ±0.4) mmol/mol.
(e) Exceptions. The following exceptions apply:
(1) You may omit this verification if you can show by engineering analysis that for your CO2 sampling system and your emission-calculation procedures, the H2O interference for your CO2 NDIR analyzer always affects your brake-specific emission results within ±0.5% of each of the applicable standards. This specification also applies for vehicle testing, except that it relates to emission results in g/mile or g/kilometer.
(2) You may use a CO2 NDIR 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.