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.

JRC presented their experience of calibrating the Golden PNC used in the VPR Round Robin exercise. They used a tandem DMA set-up in order to enable double charging of particles for the purpose of extending electrometer calibration range to sub 1000cm^-3 concentrations. With double charging and a fivefold increase in electrometer flow rate they extended electrometer measurements down to a concentration of 300cm^-3. However they found discrepancies between simulated and measured post DMA concentrations of 10-14% suggesting uncertainty into these measurements.

JRC also calibrated against a reference PNC using spark generated graphite aerosol and evaporation-condensation generated emery oil aerosol. They found lower counting efficiencies for graphite particles even above the D₉₀ diameter and that the difference relative to emery oil measurements increased as the PNC evaporator temperature delta was reduced. VW and TSI expressed surprise at this, not having seen counting efficiency above the D₉₀ diameter to be aerosol material dependent in their own measurements. Horiba queried whether this might be a concentration effect, JRC responded that that had seen the same effect at different aerosol concentrations. TSI queried whether the experiment had been repeated with the positions of the two neutralisers reversed, JRC indicated that they had not, but had seen the same effect with a range of different neutralisers. JRC suggested that the graphite aerosol generator might be producing agglomerates, which TSI agreed could have an influence.

AVL presented their experience from calibration of over 40 PNCs. They found some non-linearity of PNC response, but only +/-3%. In line with other investigations they also found different cut-off counting efficiency for CAST compared to emery oil particles. Most significantly they found the counting efficiency of a significant number of PNCs had degraded after 1 year, although some of these had been used for raw exhaust measurement, rather than solely for regulatory measurement. They however found that counting efficiency was restored when the PNC wick has changed. TSI noted that they now recommend changing the wick every 6 months as part of routine maintenance. VW commented that they had not seen this to be necessary in their experience.

Scania presented information on problems they had experienced with 4 PNCs failing calibrations on both slope and cut-off performance. All had lost a significant proportion of their counting efficiency. Scania are now instituting routine PNC wick replacement and using purer butanol in an attempt to address this issue. They are also conducting comparisons between measurement PNCs sampling the same CAST or ambient aerosol.

Reasons for PNC counting efficiency deterioration were discussed. TSI commented that this was more prevalent in heavy duty testing and was probably due to reaction of acidic components in the exhaust gas and butanol contaminating the PNC wick. JRC were examining whether use of a Catalytic Stripper instead of a VPR would help prevent this. Ricardo expressed doubts about this if organic acids were the problem. Professor Kittelson noted that it could be related to sulphuric acid formation, which would be consistent with the problem being more prevalent in heavy duty engine testing. AVL noted that they had found presence of butyl esters in their analyses and also that it would be worth investigating urea effects as Scania’s experience was with post SCR exhaust.