The counting efficiency of the TSI CPC 7610 operated with FC-43 as working fluid was investigated for three temperature differences in the pressure range 60-1000 hPa. At the standard temperature difference of 17 K, the CPC threshold diameter decreases from 20 to 11.5 nm and the maximum counting efficiency increases from similar to 30% to similar to 100% as the operating pressure is reduced from 1000 hPa down to 60 hPa. Calibration with NaCl and H2SO4 particles indicates that the counting efficiency depends on particle material. Comparing the results with butanol curves shows that butanol yields above 200 hPa more stable counting efficiency curves with lower threshold diameters and higher maximum counting efficiencies. However, below 200 hPa, FC-43 shows the better performance and can be used at least down to 60 hPa. Computational fluid dynamics modelling was carried out to explain some of the observed differences. (C) 2005 Elsevier Ltd. All rights reserved.