The reduction of vehicle exhaust particle emissions is a success story of European legislation. Various particle number (PN) counters and calibration procedures serve as tools to enforce PN emission limits during vehicle type approval (VTA) or periodical technical inspection (PTI) of in-use vehicles. Although all devices and procedures apply to the same PN-metric, they were developed for different purposes, by different stakeholder groups and for different target costs and technical scopes. Furthermore, their calibration procedures were independently defined by different stakeholder communities. This frequently leads to comparability and interpretation issues.
Systematic differences of stationary and mobile PN counters (PN-PEMS) are well-documented. New, low-cost PTI PN counters will aggravate this problem. Today, tools to directly compare different instruments are scarce. This is complicated by a dominance of proprietary, undisclosed or only partially disclosed manufacturer calibration procedures, often with unclear traceability.
This paper proposes a novel, unified calibration approach, which regards the PN counting system as a “black box” to quantify its performance as an “absolute” PN value and provides its measurement uncertainty independently of its design and the underlying legislation. This greatly simplifies accreditation, metrological traceability and improves the direct correlation of different PN measurements, bringing the quality of PN-measurements one step closer to that of gaseous pollutants. This paper proposes a novel standardized setup and optimized operating points for counting efficiency calibration. Emphasis is put on the selection of a universal aerosol and a fitting reference counter. A detailed three-step validation process demonstrates the aerosol’s suitability for all test-application. The foreseen unified procedure is used for a direct performance comparison of a commercial PMP-system and PN-PEMS, including an outlook for PN-PTI.