Measuring Fuel Cleanliness

Diesel is Dispensed Dirty 

Diesel Truck
Based on the needs of the modern fuel system, typical diesel is often dispensed in a state that is unacceptable for use in equipment. Fuel 
is not any dirtier today than it ever was, but engines have evolved to require a higher quality diesel. Modern fuel systems are extremely sensitive to even the smallest of microscopic contaminants. Far too small to be seen with the naked eye, these tiny dirt particles are measured in microns. 

A micron is defined as one millionth of a meter. The human eye can see objects down to about 40 microns. Microscopic particles of 5 microns 
or less can lead to catastrophic damage of high pressure common rail fuel systems. The old “clear and bright” rule of visual inspection is clearly no longer an adequate measure of contamination in diesel fuel. 

To put this in perspective, here are the sizes
(in microns), of some​ familiar objects:
Micron Chart
Grain of salt         100 microns
Diameter of a human hair ​80 microns
Lower limit of visibility​ ​        40 microns
White blood cell ​ 25 microns
Fog ​ ​10 microns
​Single cell of bacteria         2 microns​
ISO 4406 codes are the standard for measuring fuel cleanliness today. Originally intended for measuring lube cleanliness, ISO 
codes express the size (in microns) and quantity of contaminants 
per volume of liquid. Each ISO code is a series of 3 numbers, each representing a different minimum particle size (i.e. 4, 6, and 14 microns) and corresponding to a quantity code on the cleanliness chart. 
”ISOThe chart to the left shows how to read each code in regard to particle count. For example: Fuel that meets ISO 22/21/18, the typical cleanliness when delivered, can have between 2,000,000 and 4,000,000 particles 4 microns or larger in size per 100 ml. Particle counts double from one code to the next. Fuel at 22/x/x is twice as dirty as one rated 21/x/x. It has double the number of particles 4 microns and larger. 

Typical cleanliness of diesel fuels at the time of delivery is ISO 22/21/18, meaning that normal, in-specification diesel typically contains millions of dirt particles per 100 ml sample. Diesel industry standards do not specify a minimum ISO cleanliness level. 

Compare ISO 22/21/18 to the minimum cleanliness levels required by equipment manufacturers, and you can see the dilemma. According to manufacturers, it is the end-user’s responsibility to ensure that their fuel is fit for use. Damage caused by hard particulate or free water is considered to be caused by inadequate fuel quality and not a “factory defect.” Therefore, it is best to get fuel as clean and dry as possible before putting it into your equipment. The inside of a modern engine must be a virtual "clean room" in order for parts to meet full service life.