ISO 13319-1:2021 pdf download – Determination of particle size distribution — Electrical sensing zone method — Part 1: Aperture/orifice tube method.
6.2 Size limits The lower size limit of the electrical sensing zone method is generally considered to be restricted only by thermal and electronic noise. It is normally stated to be about 0,6 µm but, under favourable conditions, 0,4 µm is possible. There is no theoretical upper size limit, and for particles having a density similar to that of the electrolyte solution, the largest aperture available (normally 2 000 µm) may be used. The practical upper size limit is about 1 200 µm, limited by particle density. The size range for a single aperture is related to the aperture diameter, D. The response has been found to depend linearly in volume on D, within about 5 % under optimum conditions, over a range from 0,015 D to 0,8 D (i.e. 1,5 µm to 80 µm for a 100 µm aperture) although the aperture may become prone to blockage at particle sizes below the maximum size where the particles are non-spherical. In practice, the lower limitation is due to thermal and electronic noise and the upper limitation is due to non-spherical particles passing through the aperture. This restricts the operating range to be within 2 % to 60 % of the aperture size. This size range can be extended by using two or more apertures (see Annex F). In practice, this procedure can be avoided by the careful selection of the diameter of one aperture, to achieve an acceptable range. Sedimentation of particles becomes important when the particles are large and have a high density (for example, 100 µm quartz particles have a sedimentation rate in water of about 1 cm/s). Large apertures are available, up to 2 000 µm. In such applications, the viscosity and the density of the electrolyte solution should be increased, for example, by addition of glycerol or sucrose, in order to prevent particle sedimentation and to increase the possibility of keeping the particles in homogeneous suspension.
6.3 Effect of coincident particle passage Ideal data would result if all particles traversed the aperture singly and, thus, would produce single pulses. However, the opportunity exists, especially at increased concentrations, that two or more particles arrive in the sensing zone more or less together, which would result in a complex pulse. Several possibilities exist, i.e (a) two particles pass the sensing zone at the same time, leading to a pulse height equal to the sum of both pulse heights, and to a loss of counts; (b) two particles pass the sensing zone at slightly different times but within the same measurement period of the larger particle, leading to the same pulse height for the larger particle but a distorted pulse shape, and to a loss of counts; (c) two particles, which are individually too small for measurement but have together sufficient volume, pass the sensing zone at the same time, leading to an extra pulse of measurable height, and to an increase of counts. This occurrence is named coincidence. Its effects will distort the size distribution obtained but can be minimized by using low particle concentrations.