The element should not be serviced on the basis of visual observation because this will always lead to over-servicing. Element damage, due to excessive handling, 2. Improper installation of element, 3. Contaminated from dust, 4. Increased service cost, time, and material.
Restriction is the resistance to airflow through the air cleaner system into the engine.
Restriction is best recorded by a water manometer, an air cleaner service indicator or a dial gauge calibrated in inches of water. Since some users will not have a water manometer of dial gauge available, the use of permanently mounted service indicators should be considered. The indicator should be considered. The indicator can be mounted in an area where the operator can monitor the condition of the element constantly. Accurate restrictions can only be measured at maximum airflow.