Particle Size and Concentration in Oils


If you go to the Beckman Coulter website,, look for the type of product in which you are interested – say Coulter Counter – and then click on ‘Literature’ in the right-hand column, you will open up a whole world of information. For example, I’ve just come across some application notes for use of a Beckman Coulter Multisizer 3 Coulter Counter in determining the size and concentration of particles in oils.

Why is this important? Well the particle content needs to be known in order to classify oils and check their compliance with the International Standard ISO 4406. This defines the Solid Contaminant Code for Hydraulic Fluids and Lubricants.

Particle content is classified in relation to the number of particles per ml in the following size ranges: greater or equal to 4 μm; greater or equal to 6 μm; greater or equal to 14 μm;

As you will probably know from other posts on this site, a Coulter Counter determines the size distribution and concentration of particles in a sample – in this case a sample of oil – using the Coulter Principle. You may also hear this principle referred to as the Electrical Sensing Zone or ESZ.

For the analysis to work, a known volume of oil first needs to be dissolved in a solution of a suitable organic electrolyte. In some cases the oil under test will not dissolve in the electrolyte. This can be overcome by finding a solvent in which it will dissolve and then dispersing a sample from that solution into the electrolyte.

The method involves running a blank first and subtracting from the sample. Results are expressed in terms of the numbers of particle per ml for each specified range of particle size.

Full practical instructions for the method are given in the application notes available as a download from the Beckman Coulter website.