In some situations, for instance when working with clean water, injectables or lubricating oils, it may be desirable to know the number of particles in a given volume.

LLS measurements give a particle size distribution by volume (which is generally equivalent to a distribution by mass). This can be converted mathematically to a distribution by number but the result may be unsatisfactory as errors will be high at the fine end. In high purity fluids there may not be enough particles to generate a satisfactory response to LLS.

In these cases it is better to use a technique which directly senses and measures individual particles, generically known as ‘zone sensing’. The classic technique is the ‘Coulter Counter’, not to be confused with the Coulter LS instrument that we use for LLS. In the Coulter Counter, particles in dilute suspension in an electrolyte are drawn through a small orifice while the electrical resistance across the orifice is monitored. Whenever a particle passes through the orifice the resistance increases due to the displacement of electrolyte. The size of the change correlates with the volume of the particle, so it is possible to both count and size the individual particles.

At Microns to Measure we use a similar technique based on optics (PMS APSS-200). The fluid is drawn through a small cell illuminated by a laser and the obscuration of the light beam caused by the particles is measured. The method is sensitive to particles in the size range 2 – 125 microns. The results are normally presented as numbers of particles per millilitre (counts/ml).