Fluids are kept clean in a system principally by devices such as filters and strainers. Magnetic

Figure 9-38.-Magnetic plugs.

plugs (fig. 9-38) also are used in some strainers to trap iron and steel particles carried by fluid. Studies have indicated that even particles as small as 1 to 5 microns have a degrading effect, causing failures and hastening deterioration in many cases.

There will always be controversy over the exact definitions of filters and strainers. In the past, many such devices were named filters but technically classed as strainers. To minimize the controversy, the National Fluid Power Association gives us these definitions:

FILTER - A device whose primary function is the retention, by some porous medium, of insoluble contaminants from a fluid.

STRAINER - A coarse filter.

To put it simply, whether the device is a filter or a strainer, its function is to trap contaminants from fluid flowing through it. The term porous medium simply refers to a screen or filtering material that allows fluid flow through it but stops various other materials.

MESH AND MICRON RATINGS

Filters, which may be made of many materials other than wire screen, are rated by MICRON size. A micron is 1-millionth of a meter or 39-millionths of an inch. For comparison, a grain of salt is about 70 microns across. The smallest particle visible to the naked eye is about 40 microns. figure 9-39 shows the relationship of

Figure 9-39.-Relationship of micron sizes.

Figure 9-40.-Inlet line filter.

Figure 9-41.-Inlet strainer.

the various micron sizes with mesh and standard sieve sizes.

A simple screen or a wire strainer is rated for filtering fineness by a MESH number or its near equivalent, STANDARD SIEVE number. The higher the mesh or sieve number, the finer the screen.

When a filter is specified as so many microns, it usually refers to the filter's NOMINAL rating. A filter nominally rated at 10 microns, for example, would trap most particles 10 microns in size or larger. The filter's ABSOLUTE rating, however, would be a somewhat higher size, perhaps 25 microns. The absolute rating is the size of the largest opening or pore in the filter. Absolute rating is an important factor only when it is mandatory that no particles above a given size be allowed to circulate in the system.