The International Organization for Standardization is a wide-reaching authority who comprehensively letters the benchmarks for safety and quality. Their standards range from child-resistant packaging to – you guessed it – hydraulic machinery. Two commonly expressed ISO Code standards in fluid power are for the use of hydraulic symbols (ISO 1219) and particle contamination of hydraulic fluid (ISO 4406 et al). Although there are various ISO measures for particle contamination, I’d like to describe the one related to the size of particles in hydraulic fluid.
ISO 4406 describes the size and quantity of particles within hydraulic fluid. ISO has identified the three sizes of particles most relevant to measure based on their ability to damage components of the hydraulic systems, such as pumps, valves and actuators. The three sizes of concern are 4 micron, 6 micron and 14 micron. For reference, there are 2450 microns in an inch, which is 2.54 cm. For additional reference, a grain of sand is at least 75 micron and a thin hair is usually around 40 microns wide. Even “large” particles in a hydraulic system, at 14 microns, are smaller than the eye can see.
The ISO code uses a three number system to describe the quantity of 4, 6 and 14 micron particles. Each number of the code represents a range of particles, rather than a specific number of them. An example is the best way to describe it:
In this example, 20 indicates the quantity of 4 micron particles, 17 describes the number of 6 micron particles and 16 denotes the sum of 14 micron particles. There are always more smaller particles than larger, mostly because smaller particles are harder to filter. However, each of these numbers does not represent the number of particles, but a scaled range spelled out in the ISO code. Each line number of the code doubles the range of particles. For example:
20 = anywhere from 5000 – 10,000 four micron particles
17 = anywhere from 640 – 1300 six micron particles
16 = anywhere from 320 – 640 fourteen micron particles
As you can see, there is a big difference between a 16 and 20 as far as how many particles are in the oil, which by the way, represents just 1 ml of fluid! Also, if you have oil with a 20/17/16 code, then you probably have an open bathtub instead of a proper reservoir, because that’s some extremely dirty oil. For a complete chart showing the particle contamination scale, as well as further detail on the subject, please have a look here.