hydraulic filter (fig. 10-31) is used in some hydraulic systems. This filter uses one or a combination of the contamination indicators previously described.

Under normal conditions the fluid enters the inlet of the filter (view (A) of fig. 10-31), passes through the filter element, and leaves the filter through the outlet. As the fluid passes through the filter element, impurities are deposited on the outside of the element. As the deposits accumulate, they cause a differential pressure to build up between the inlet and outlet of the filter. The pressure is sensed across the contamination indicator switch; on this particular filter, the switch closes at 70 ÷ 10 psi, actuating a warning device (light, horns, etc.). The equipment should be stopped and the filter serviced, cleaned, or replaced. An important fact for you to remember is that cold hydraulic fluid can produce a false pressure indication. To prevent needless changing of filters, fluid should beat operating temperature for a true

Figure 10-31.-Full-flow, porous metal, bypass electrical-indicating hydraulic filter.