Electronic sizing and counting of particles

In the following description it is assumed that the reader has at his disposal a Model B Coulter Counter which has been set up as an operational instrument by a company representative. Precise details on the operation of this instrument are given in A Practical Manual on the Use of the Coulter Counter in Marine Science by R. W. Sheldon and T. R. Parsons (Published in 1966 by Coulter Electronics Sales Company — Canada) and the subject is discussed further by Sheldon and Parsons (/. Fish. Res. Bd. Canada, 24 909, 1967). The following account is to some extent an abbreviated set of instructions obtained from the publication cited above. Only the essential operation of the instrument, the preparation of samples, and two basic types of particle distributions are dealt with here. For greater working details as well as for descriptions of accessmy apparatus, the reader is referred to the manual cited above. 

The apparatus specified in the following procedures is a Coulter Counter Model B. A detailed description of the electronics of this instrument is given in a manual provided by the manufacturer. The description given here is only intended to serve as an introduction to the use of the apparatus and to identify certain controls which have to be operated. 

The instrument consists of an electronic cabinet and a sampling stand. The former contains a digital counter, an oscilloscope screen, upper and lower threshold controls, aperture current and amplification switches, and a zero reset switch. The sampling stand consists of an aperture tube, a mercury manometer, a control stopcock, two electrodes, a vacuum pump, and a 35x microscope for viewing the aperture. 

The upper and lower thresholds act as electronic gates, above and below which no particles are counted. The controls may be operated in two ways and this is 

At each change of setting, the aperture current switch doubles the current, and the amplification switch doubles the instrument response. For the purpose of this text the combined effect of the aperture current and amplification settings has been expressed as a multiple and called the sensitivity. 

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