The lifetime and maintenance interval of the air filter should be measured gas flow rate that flows through the filter and gas flow amount that the engine needs

When the flow rate is bigger than flow amount, the filter will work normally:
When the flow rate equals to the flow amount, the filter should be maintained;
When the flow rate is lower than flow amount, the filter should stop using, or the working condition will be worse and worse and even stop working, which need to replace new filters

In real working, when the element of the air filter is blocked and it can not meet the air flow amount that the engine needs, the working condition of the engine will be abnormal, For example: 

1.Boom stifling, accerlation slow( the air inflow is not sufficient and cause the cylinder pressure not enough)

2. Working pressure(because of the strong mixed air and cause the insuficient burning)
3. The water temperature is relatively high( when goes to the exhausting procedure it is still burning)

4.When accerlating, the exhausting smoke is heavy

When there are the above phenomenons, it can judge the air filter is blocked, it should dismantle the filter element and maintain or replace the filter. When maintaining the air filter element, it should take care of the color change of the inside and outside of the filter. After cleaning the surface dust of the filter element, if the outside of the filter paper is clear while the inside is bright,  this filter element can be used continually; if the color of outside filter paper has lost its true colorsm and the inside is dark, it needs to change filter!