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In June 2005, the United States Environmental Protection Agency (USEPA) signed a final rule implementing in-use testing requirements for heavy-duty diesel (HDD) engines. The program requires manufacturers to measure gaseous and particulate matter (PM) exhaust emissions from diesel engines using portable emission measurement systems (PEMS) installed on vehicles. In order to measure PM emissions, a cooperative contract between Sensors, Inc. and the USEPA was commenced to develop a Proportional Particulate Mass Device (PPMD). It is comprised of three key elements: a micro-proportional sampling system (MPS) incorporating an exhaust flow-meter (EFM) and an eight element quartz-crystal microbalance (QCM) to measure particulate mass. Evaluation of the exhaust flow meter (EFM) at third-party facilities have shown that the mass emission data and flow data produced with the EFM correlates extremely well with traditional (non-portable) constant volume sampling (CVS) certification systems.