Exhaust flow measurements are becoming increasingly important to ensure accurate emissions measurement on both a continuous and averaged (i.e. bag) basis. When performing continuous mass measurements of hydrocarbons, nitrous oxide, carbon monoxide, carbon dioxide and other pollutant/greenhouse gases, the exhaust flow measurement is critical to the task of providing an accurate result. In addition, the acceptance of the Bag Mini-Diluter (BMD) for low-level emissions measurements has amplified the need for an accurate exhaust flow measurement. With the BMD, there is a direct correlation between fuel economy/emissions accuracy and exhaust flow measurement accuracy.
To meet these demands, several new exhaust meters have emerged in the last few years that encompass promising technology and unique solutions to the traditional problems that tend to complicate exhaust flow measurements. These problems, including high temperature operation, water condensation, and exhaust pulsations at low flows, have been addressed with various innovative approaches. Regardless of the measurement approach, however, it is necessary that calibration systems be developed to verify the accuracy of these meters throughout their range of operation.
This paper details the suggested requirements for an exhaust flow calibration system and provides an overview of two systems that we developed to ensure the accuracy of our exhaust flow meters. The primary calibration system uses Laminar Flow Elements (LFEs) and an automated flow cart to determine the accuracy and drift characteristics of an exhaust flow meter. This system also includes added features that may be used to evaluate an exhaust flow meter's performance under special operating conditions. The second calibration system was designed to be simpler and more portable while using the same LFE transfer standard.