Fluid filters get rated based on how well they’re capable of removing particles of a certain size from the fluid in question. A 10-micron filter, for example, can filter out particles that are as small as 10 micrometers.
The most common filter ratings that are used are absolute and nominal ratings. These are different methods of indicating the effectiveness of a particular fluid filter.
Let’s take a close look at each of these rating methods, which can be helpful to know when undergoing hydraulic maintenance in Minnesota.
Absolute rating
A filter’s absolute rating is also known as its cutoff point. This rating provides the diameter of the largest spherical glass particle that would be able to pass through the filter. These diameters are measured in micrometers, which measure at a millionth of a meter.
Filter media that have an exact, consistent pore size are also going to have an exact absolute rating, because the absolute rating is the medium’s pore opening size. It should not be confused with the largest particle that passes through a filter—the absolute rating just indicates the size of the largest glass bead that passes through the filter under non-pulsating, low-pressure conditions.
It is important to note that filter media that have exactly consistent pore sizes do not exist in practice. Pore size is determined by the form of the filter element, but it is not necessarily consistent with actual open areas. Depending on the shape of the particle, it might be possible for it to pass through a smaller hole in the media than might have otherwise been expected based on at least one of the particle’s dimensions. For example, consider how a cylinder might be capable of passing through a small hole if put through lengthwise, but not widthwise. This type of passage is influenced by the shape and size of the opening and the fluid depth over which the filtering process is occurring.
Nominal rating
A nominal rating is a measure of a given filter’s ability to prevent a certain minimum percentage of particles greater than the nominal rating’s micron size from passing through. Particles are measured by weight for each specific contaminant in a nominal rating. The nominal rating also indicates a degree of filtration, which gives an idea of how efficient a particular filter is. For example, a nominal rating might be “95 percent of 10 micron,” in which the filter is capable of removing about 95 percent of all 10 micron and larger particles passing through a fluid.
The nominal rating method is not used as frequently as the absolute rating method, because there is enough variance in testing conditions such as contaminant concentration and operating pressure in testing settings that the rating might not be very consistent, which means it cannot be relied upon to the same degree that an absolute rating can.
For more information about these different types of rating systems and how they measure the effectiveness of fluid filters, contact M & M Hydraulic Company today to learn more about hydraulic maintenance in Minnesota.