40 CFR § 1065.320 - Fuel-flow calibration.
(b) You may also develop a procedure based on a chemical balance of carbon or oxygen in engine exhaust.
(c) You may remove system components for off-site calibration. When installing a flow meter with an off-site calibration, we recommend that you consider the effects of the tubing configuration upstream and downstream of the flow meter. We recommend specifying calibration reference quantities that are NIST-traceable within 0.5% uncertainty.