FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsEasy
A hydraulic filter element is marked as rated at 15 microns absolute. What does this rating indicate?
- AThe thickness of the filter media measured in thousandths of an inch
- BThe smallest particle size the filter is guaranteed to trap
- CThe maximum system pressure the filter element can safely withstand
- DThe flow rate capacity of the filter housing in gallons per minute
Show answer & explanationAnswer & explanation
Correct answer: B. The smallest particle size the filter is guaranteed to trap
Micron ratings describe filtration fineness — the absolute rating is the largest particle size that will pass through the filter, meaning particles at or above that size are reliably trapped. It has nothing to do with pressure rating, flow capacity, or media thickness.
Why the other options are wrong
- A. Media thickness is a physical dimension unrelated to the micron rating.
- C. Pressure rating is a separate structural specification of the filter housing.
- D. Flow capacity is specified separately in gpm, independent of micron rating.
Micronic Filter Rating
A specification stating the size, in microns, of the smallest particles a hydraulic filter element is designed to trap.
- 1 micron = 0.000039 inch
- Absolute rating = largest particle guaranteed to be stopped
- Finer ratings increase filtration but may increase pressure drop
Memory trick: Micron = the mesh size guarding against grit.