This recommended practice establishes test procedures and best practices for PM measurements under partial flow conditions. PFDS PM testing and certification is primarily used in heavy duty applications as described in 40 CFR Part 1065 because full flow sampling systems with a dilution tunnel and CVS can be very bulky for large heavy duty engines. 40 CFR Part 1066 also allows the use of partial flow systems for light duty applications in lieu of full flow systems. PFDS does not require the use of a CVS sampling system or a full flow dilution tunnel. This makes it easier and more convenient to perform PM measurement in smaller test cells that cannot fit a large CVS or a full flow dilution tunnel. This document describes the different components/specifications of LD and HD PFDS as well as the required quality checks necessary to ensure the integrity of the sampling system. It also addresses the many sampling options and calculations that are approved by the regulations. Correlation between partial and full flow systems will also be discussed.