CPI Sand, DE & Cartridge Filtration 3 — Questions and Answers
Question 1: What is the primary reason a sand filter requires a 'Rinse' cycle after backwashing?
- To lubricate the multiport valve
- To settle the sand bed and flush loosened particles to waste (Correct answer)
- To prime the pump
- To recharge the sand with clarifier
Correct answer: To settle the sand bed and flush loosened particles to waste
The Rinse cycle briefly returns flow in the normal filter direction to re-settle disturbed sand and send any remaining loosened debris out the waste port before returning to Filter mode.
Question 2: When inspecting a DE filter, the inspector should verify that the manual air relief valve:
- Is permanently sealed to prevent DE bypass
- Opens to release trapped air when the system is started (Correct answer)
- Is located below the waterline
- Is connected directly to the return line
Correct answer: Opens to release trapped air when the system is started
The air relief valve bleeds trapped air from the filter tank on startup; a stuck-closed valve can cause air locks that reduce flow and filtration efficiency.
Question 3: A pool cartridge filter has a dirty pressure reading 25 PSI above its clean baseline. The inspector should note this as:
- Normal operating condition requiring no action
- Minor issue — increase pump speed
- A sign the cartridge needs cleaning or replacement (Correct answer)
- Evidence of a broken multiport valve
Correct answer: A sign the cartridge needs cleaning or replacement
A pressure rise of 8–10 PSI or more above the clean baseline is the standard trigger to clean or replace a cartridge filter element.
Question 4: Which chemical should NEVER be used to clean a DE filter grid or cartridge filter element because it destroys the filter media?
- Muriatic acid (Correct answer)
- Sodium bicarbonate
- Filter cleaner/degreaser
- Diatomaceous earth
Correct answer: Muriatic acid
Muriatic acid can damage or dissolve the polyester or polypropylene fabric of filter grids and cartridge elements, causing media failure.
Question 5: In a sand filter, what term describes the condition where oil, sunscreen, and organic matter bind the sand granules into a solid mass?
- Calcification
- Mudballing (Correct answer)
- Channeling
- Backpressure lock
Correct answer: Mudballing
Mudballing occurs when oils and organics cement sand particles together into clumps, drastically reducing the media's filtering surface area.
Question 6: What is the standard DE dosage used when recharging a DE filter after a complete cleaning?
- 1 lb per 5,000 gallons of pool water
- Manufacturer-specified amount based on filter square footage (Correct answer)
- 1 cup per filter grid
- The same weight as the filter tank capacity in gallons
Correct answer: Manufacturer-specified amount based on filter square footage
DE dosage is determined by the filter manufacturer based on total grid surface area (typically 1 lb per 10 sq ft), not pool volume.
Question 7: An inspector observes white powder returning through the pool return jets immediately after a DE filter is recharged. The MOST probable cause is:
- Too much DE added to the skimmer
- Torn or failed grid fabric allowing DE bypass (Correct answer)
- Air in the pump basket
- Incorrect multiport valve position
Correct answer: Torn or failed grid fabric allowing DE bypass
DE returning to the pool immediately after recharging almost always indicates a torn grid or manifold crack that allows the powder to pass through the filter.
CPI Sand, DE & Cartridge Filtration 3
What is the primary reason a sand filter requires a 'Rinse' cycle after backwashing?