JRC presented the background to and current status of the exercise. Golden VPR, PNC and (graphite, spark) Aerosol Generator were being circulated to participating laboratories. Each lab was measuring gas dilution factors and PCRFs at 30, 50 and 100nm aerosol sizes using the Golden PNC and Aerosol generator and their own equipment. As an option labs were asked to check PNC linearity and measure PCRF at 15nm aerosol size.

Eight of ten labs have now completed testing and the following aerosols have been used; diesel exhaust, CAST, mini-CAST, NaCl, Palladium. This includes testing with polydisperse aerosol at Empa and with & without neutraliser at Horiba. The Round Robin is expected to conclude by the end of February 2012. Participating laboratories were reminded to submit their results to JRC as soon as possible, including raw measurement data, a schematic of the equipment set-up and a 3-4 page description of the measurements taken.

Horiba presented their experience from the Round Robin. They noted that they initially tried PCRF measurement with polydisperse aerosol, but had experienced significant problems with this and not pursued it. Using a 23nm D₅₀ PNC they found substantial variation in 30nm PCRF across three different aerosol generators (NaCl nebuliser, mini-CAST and graphite spark), with only the (Golden) graphite spark generator giving plausible results. The implausible PCRF results for 30nm particles may have been due to the combination of the low VPR downstream particle concentrations, being on the steep part of the PNC cut-off curve and some material dependency in the PNC cut-off.

Matter Engineering noted that the mini-CAST generator might be producing significant concentrations of volatile particles which were not fully removed by thermal conditioning before the VPR inlet. Professor Kittelson commented that some generators produced very highly charged aerosol, although Horiba noted that, in this case, the results with and without a neutraliser were similar. Using a 2nm D₅₀ PNC all aerosol generators gave similar PCRF results (+/-5% CoV), again the neutraliser appeared to have no impact on the measurements. TSI commented that particle losses for the Golden VPR design were not very size specific reducing the effect of the neutraliser.

Horiba showed data on the (size classified) aerosol stability over time for the different aerosol generators. The NaCl nebuliser generator produced very stable concentrations at all three aerosol diameters used for VPR calibration. The miniCAST generator was fairly stable at 30 and 100nm, but showed substantial variation at 50nm. The spark generator concentrations varied by around +/-1000cm^-3, from mean concentrations in the 5000-9000 range (depending on aerosol diameter). TSI noted that it was best to keep the DMA inlet concentration in the 10⁶-10⁷ cm^-3 range.

AVL also presented their VPR calibration experience using thermally treated CAST aerosol, a neutraliser and a 10nm D₅₀ PNC. They found PCRF_av repeatability to be +/-6% (2σ) for normal PCRF settings, this included measurements taken on units after 11 months in service. They did however note that repeatability became significantly poorer at PCRF settings above 2000. This was attributed to downstream aerosol concentrations at high PCRF settings being very low. The chairman noted that the PMP roadmap calibration work in 2007 had also seen poorer calibration repeatability at high PCRF settings for this reason. Scania noted that they had seen poorer repeatability at PCRF settings of 2000, AVL suggested that if PRCF was outside of +/-10% at PCRFs of up to 5000 then this was indicative of a problem with the unit. Horiba noted that their system operated at PCRFs up to 3000, for raw exhaust measurement they used an additional, separately calibrated diluter.