With no moving parts or electrical switches to fail, cartridge elements do not have a defined termination point. Instead the fine interstices of the media matrix gradually plug up over time. In a typical spa, the culprit that plugs the media is perspiration and body oils combined with soaps, chemicals and very fine particulate. In a swimming pool, the loading is primarily debris along with sun tan lotions and organic matter such as algae.
Assuming the filter is properly maintained and correctly sized to the pump, determining when the cartridge is exhausted depends primarily on three factors:
1. Shorter cycle time between cleanings?
2. Low water flow rate and high differential pressure?
3. Catastrophic failure such as a tear in the media or center core collapse.
All three are dependent upon proper pool or spa water chemistry and following a routine maintenance schedule. Homeowners should be reminded that filter elements are plastic and should be handled and maintained accordingly.