The precision of the size determination of resistive particle counting systems was studied by: (i) measuring the apparent size of a single particle cycled repeatedly through the sensing electrodes: (ii) by measuring the apparent size distribution of a population of particles of known and limited size range. (iii) by moving individual particles through a large-scale model of the electrode assembly, and (iv) by measuring the size of algal cells microscopically and (subsequently) with a resistive counting system. Three systematic errors were indicated (i) An ‘edge error’ results when a particle passes through the orifice near the perimeter of the orifice (∼ 15% overestimation in volume relative to the median estimate): the volume of the same particle passing through the orifice along the central axis is underestimated (∼-15%) in volume. (ii) A ‘shape factor’ error associated with non-spherical particles is apparent when an elongate particle passes through the orifice sideways. the volume is overestimated by ∼20% in excess of its equivalent spherical diameter. The same particle is underestimated in volume by ∼25% if it passes through “end on” (iii) A ‘particle resistance error’ is noted if particles have a markedly different specific resistance from the particles used to calibrate the system. This error is most extreme in the case of dead or detrital particles. In consideration of these several sources of variance, the maximum number of useful channels in which a spectrum of live non-spherical plankton may he expressed is ∼17 when data are expressed in the conventional plot of log2 of particle diameter.
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Boyd et al. (1995) studied this question.