40 CFR § 1065.225 - Intake-air flow meter.
(a)Application. You may use an intake-air flow meter in combination with a chemical balance of fuel, inlet air, and exhaust to calculate raw exhaust flow as described in § 1065.655(f) and (g), as follows:
(1) Use the actual value of calculated raw exhaust in the following cases:
(i) For multiplying raw exhaust flow rate with continuously sampled concentrations.
(ii) For multiplying total raw exhaust flow with batch-sampled concentrations.
(iv) For calculating the dilution air flow for background correction as described in § 1065.667.
(2) In the following cases, you may use an intake-air flow meter signal that does not give the actual value of raw exhaust, as long as it is linearly proportional to the exhaust flow rate's actual calculated value:
(i) For feedback control of a proportional sampling system, such as a partial-flow dilution system.
(ii) For multiplying with continuously sampled gas concentrations, if the same signal is used in a chemical-balance calculation to determine work from brake-specific fuel consumption and fuel consumed.
(b)Component requirements. We recommend that you use an intake-air flow meter that meets the specifications in Table 1 of § 1065.205. This may include a laminar flow element, an ultrasonic flow meter, a subsonic venturi, a thermal-mass meter, an averaging Pitot tube, or a hot-wire anemometer. Note that your overall system for measuring intake-air flow must meet the linearity verification in § 1065.307 and the calibration in § 1065.325.
(c)Flow conditioning. For any type of intake-air flow meter, condition the flow as needed to prevent wakes, eddies, circulating flows, or flow pulsations from affecting the accuracy or repeatability of the meter. You may accomplish this by using a sufficient length of straight tubing (such as a length equal to at least 10 pipe diameters) or by using specially designed tubing bends, orifice plates or straightening fins to establish a predictable velocity profile upstream of the meter.