New particulate sensing technologies are currently being readied for production to meet the on-board diagnostic (OBD) regulations associated with diagnosing diesel particulate filter (DPF) efficiency. The threshold levels for diagnosis have been tightened starting in 2013, requiring a new approach beyond the current techniques which often rely on differential pressure sensing across the filter. A new sensor has been developed to directly detect the particles passing through the DPF and estimate the cumulative particle flow. Using this information, an estimate can be made of the filter's efficiency and an associated diagnosis of its ability to meet emissions requirements. In this paper we will discuss the sensor's operating principle, accuracy and repeatability. Experimentally-observed variability of exhaust particulate matter will be compared to simulated deposition of soot1 in a modeling environment.1The authors acknowledge that “soot” is an imprecise colloquial term relating to chimney deposits. Soot is used for brevity in this paper and is intended to be interchangeable, unless otherwise noted, with “particulate matter,” since the sensor in discussion has the potential to collect the full range of particulate matter in the exhaust, as opposed to only a purely carbonaceous fraction .