# 1065.659 Removed water correction.
(a) If you remove water upstream of a concentration measurement, *x*, correct for the removed water. Perform this correction based on the amount of water at the concentration measurement, *x*<sub>H2O[emission]meas</sub>, and at the flow meter, *x*<sub>H2Oexh</sub>, whose flow is used to determine the mass emission rate or total mass over a test interval. For continuous analyzers downstream of a sample dryer for transient and ramped-modal cycles, you must apply this correction on a continuous basis over the test interval, even if you use one of the options in § 1065.145(e)(2) that results in a constant value for *x*<sub>H2O[emission]meas</sub> because *x*<sub>H2Oexh</sub> varies over the test interval. For batch analyzers, determine the flow-weighted average based on the continuous *x*<sub>H2Oexh</sub> values determined as described in paragraph (c) of this section. For batch analyzers, you may determine the flow-weighted average *x*<sub>H2Oexh</sub> based on a single value of *x*<sub>H2Oexh</sub> determined as described in paragraphs (c)(2) and (3) of this section, using flow-weighted average or batch concentration inputs.
(b) Determine the amount of water remaining downstream of a sample dryer and at the concentration measurement using one of the methods described in § 1065.145(e)(2). If you use a sample dryer upstream of an analyzer and if the calculated amount of water remaining downstream of the sample dryer and at the concentration measurement, *x*<sub>H2O[emission]meas</sub>, is higher than the amount of water at the flow meter, *x*<sub>H2Oexh</sub>, set *x*<sub>H2O[emission]meas</sub> equal to *x*<sub>H2Oexh</sub>. If you use a sample dryer upstream of storage media, you must be able to demonstrate that the sample dryer is removing water continuously (*i.e., x*<sub>H2Oexh</sub> is higher than *x*<sub>H2O[emission]meas</sub> throughout the test interval).
(c) For a concentration measurement where you did not remove water, you may set *x*<sub>H2O[emission]meas</sub> equal to *x*<sub>H2Oexh</sub>. You may determine the amount of water at the flow meter, *x*<sub>H2Oexh</sub>, using any of the following methods:
(1) Measure the dewpoint and absolute pressure and calculate the amount of water as described in § 1065.645.
(2) If the measurement comes from raw exhaust, you may determine the amount of water based on intake-air humidity, plus a chemical balance of fuel, DEF, intake air, and exhaust as described in § 1065.655.
(3) If the measurement comes from diluted exhaust, you may determine the amount of water based on intake-air humidity, dilution air humidity, and a chemical balance of fuel, DEF, intake air, and exhaust as described in § 1065.655.
(d) Perform a removed water correction to the concentration measurement using the following equation:
[73 FR 37335, June 30, 2008, as amended at 76 FR 57462, Sept. 15, 2011; 79 FR 23804, Apr. 28, 2014; 86 FR 34566, June 29, 2021]