The ISO Code is a system for representing particle concentrations in oil. The test is commonly referred to as the Particle Count. Without the use of the ISO Code, a confusing series of numbers would have to be examined to determine how clean an oil is.

In oil laboratories, automatic particle counters determine the ISO code. Shown below is a particle count report for a hydraulic oil.

The number of particles are counted in seven different size ranges (see the sample Particle Count test results below). The left column shows the size in microns. The right column is the number of particles counted in the sample that are larger than each micron size. In the sample below, the number of particles found above 5 microns in size was 244.

The ISO Code in this example is 15/12. These two numbers (15 and 12) identify a range of particles counted in the size above 5 microns and 15 microns, respectively. The higher the ISO Code, the dirtier the oil. However, different oils can have different levels of "clean" and "dirty." The chart below shows the ranges for four types of oil: