JRC releases updated assessment on PEMS measurement uncertainty

26 February 2021

The EU Joint Research Center (JRC) has updated its assessment of portable emissions measurement systems (PEMS) measurement uncertainty suggesting that the RDE conformity factors (CF) for NOx and PN can be further lowered. The NOx margin can be reduced to 0.23 (CF=1.23) and the PN margin to 0.34 (CF=1.34) [5057]. The previous estimate of NOx margin was 0.32 and the current PN margin used in the legislation is 0.5.

The improvements in NOx margin arise from lower exhaust mass flow meter uncertainty during urban driving (7.5% versus 10%) and a change to the framework for assessment of (i.e., method used to estimate) NOx margin that takes into account the fact that zero drift is random. The framework changes also reflect PEMS standard development activities being carried out under CEN.

Further reductions in the permissible tolerance for zero drift from 5 ppm to 3 ppm and changes to the method for taking into account the zero drift in each individual test, as is already done in the heavy duty legislation, can further decrease the NOx margin in the future to 10% (CF=1.10). The PEMS systems tested all manufacturers have zero drift within 3 ppm for at least >94% of the tests. This is an improvement from the past and reflects better maintenance and/or laboratory procedures. This step would require changes to the exhaust flow meter specification, zero drift requirements and laboratory validation requirements in the applicable regulation (EU 2017/1151).

The improvements in PN margin arise from modifications to the framework to assess PN margin to account for particle losses and size uncertainties. Further reductions of the PN margin would require further refinement of the technical and calibration specifications for the PN-PEMS and the reference PMP systems and would require the involvement of the instrument manufacturers and the PMP working group.

The PEMS used in the study were manufactured by AVL, Horiba and Sensors.

Source: JRC