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.