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 H2 O interference after initial analyzer installation and after major maintenance.
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 H2 O 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.
(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 water in a sealed vessel. If the sample is not passed through a dryer during emission testing, control the vessel temperature to generate an H2 O 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 H2 O level at least as high as 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 water mole fraction, x
H2O, of the humidified test gas, as close as possible to the inlet of the analyzer. For example, measure dewpoint, T
dew, and absolute pressure, p
total, to calculate x
H2O. Verify that the water 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 water 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 water content (e.g., dewpoint and pressure) or an estimate based on the vessel pressure and temperature. Use good engineering judgment to estimate the water content. For example, you may use previous direct measurements of water 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 x
H2O 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 x
H2O 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 ±0.4) mmol/mol.