Droplet spectra measurements with the FSSP-100. Part II: Coincidence effects

At high droplet number concentration, measurements of the droplet number concentration, of the droplet spectrum, and of the liquid water content with the FSSP-100 are significantly affected by the coincidences of particles in the probe's detection beam and by count losses during the dead time of the probe's electronics. Accurate statistical procedures have been proposed in the literature for the correction of the coincidence and dead-time losses in the estimation of the droplet number concentration. There are, however, no techniques available for correction of the spectral distortion, hence of the derived liquid water content. The model of probe functioning described in Part I of this series is now used on a stochastic mode in order to simulate all possible events of coincidences and dead-time losses. Observed features, such as variations of the depth of field and beam edge acceptance ratios with the droplet concentration, are correctly reproduced by the simulator. The three most currently used techniques for the correction of the droplet concentration are then evaluated with the simulator. The distortion of the spectral shape is also examined. The simulator shows that the measured droplet mean volume diameter increases with the droplet number concentration, by up to 40% at a concentration of 1500 cm(-3). An empirical procedure is finally proposed for the correction of the spectral distortion and of the derived liquid water content.