The circles in Figure 7-7 indicate relative sizes of contamination particles. Not all contamination is nice round marble like pieces as the two overlaid examples show. Particle size is measured by enclosing it in a circle until it touches at two or more places. The 150-micron (150-µ) particle may only be 20 to 25 µ thick but is considered 150 µ because of its length.

Most pump manufacturers specify at least 10-µ clean fluid to protect their pumps from premature failure.

ISO has set up cleanliness level standards for hydraulic and lubrication fluids. The system most used for hydraulics is based on the ISO 4406 standard. This standard covers the number of particles of a given size that can be present in a fluid sample in three different micron ranges. It is designated ISO Code XXX/XXX/XXX, where the numbers relate to the minimum and maximum number of particles of a given size that can be present in a 100 milliliter sample. The first number indicates how many particles of 2-µ size can be present; the second number is for 5-µ particles; and the third number is for 15-µ particles. It might be written ISO Code 18/16/13. The numerals always descend in value from left to right. This code would mean that there could be between 1300 and 2500 particles of 2-µ size, 320 to 640 particles of 5-µ size, and 40 to 80 particles of 25 µ.

Figure 7-7 indicates that, even with good eyesight, people cannot see a particle smaller than 40µ. Thus it is impossible to look at a sample of fluid and determine whether it is clean. Of course, it is possible to tell it is contaminated when large particles are plainly visible in the sample.

The ISO 4406 chart shows the range number and the number of particles that it represents. From this chart it is easy to set up or pick out any ISO Code cleanliness level.

Some typical fluid cleanliness level ISO Code’s are shown in the following chart:

Beta ratios

Another rating applied to hydraulic filters is the Beta ratio (also known as the Filtration ratio). It is a measure of the particle-capture efficiency of a filter element. The ISO 4572 Multipass Test passes fluid through the circuit shown in Figure 7-7 to check for contaminant retention. A measured amount of contaminant is injected upstream of the filter. Laser particle counters record the number of particles into and out of the filter. When 100,000 particles are measured upstream of a 10-µ filter and 10,000 downstream, it would have a Beta ratio of 10 (100,000/10,000 = 10).

A Beta Ratio number is of no use alone, but it is required to find the filter’s efficiency rating. Efficiency of a filter element is what counts when comparing one filter to another. The higher the efficiency, the fewer contaminants will pass through it. Efficiency coupled with the volume of contaminant retention can make a more expensive filter cost less due to its longer useful life.

Efficiency is calculated by the formula:

Efficiency10 = (1–1/10) X 100
Efficiency10 = (0.9) X 100
Efficiency10 = 90%

Always make sure the filter meets or exceeds the desired cleanliness level of the system it is protecting.

Filter locations

Figure 7-8 shows most of the locations where filters might be found in any hydraulic circuit. Note that all of these filters are seldom found in a single circuit but some circuits might have filters in two places. Two other types of filtration -- off-line and filter fill -- also are shown in Figure 7-8. Any of these filters could be the dual, change-on-the-run type when required. Dual filters are more expensive but can reduce downtime.