CPI Sand, DE & Cartridge Filtration 2 — Questions and Answers
Question 1: What is the typical operating pressure range for a properly functioning sand filter?
- 2–5 PSI
- 8–15 PSI (Correct answer)
- 20–30 PSI
- 35–45 PSI
Correct answer: 8–15 PSI
Sand filters normally operate between 8–15 PSI at startup; a rise of 8–10 PSI above the clean baseline signals that backwashing is needed.
Question 2: Which sand filter component prevents sand from passing back into the pool during normal filtration?
- Multiport valve
- Lateral assembly (Correct answer)
- Diffuser
- Sight glass
Correct answer: Lateral assembly
Laterals are slotted tubes at the bottom of the sand bed whose narrow slots allow water through but retain sand particles.
Question 3: A pool inspector notices the sand filter sight glass remains cloudy after a full backwash cycle. The MOST likely cause is:
- Oversized pump
- Channeling through the sand bed (Correct answer)
- Clogged air relief valve
- Damaged multiport spider gasket
Correct answer: Channeling through the sand bed
Channeling creates bypass paths through the sand so water is never fully filtered, leaving effluent turbid even after backwashing.
Question 4: DE filter grids must be inspected for which defect that allows DE powder to pass into the pool?
- Calcium scale buildup
- Tears or holes in the fabric (Correct answer)
- Excessive grid spacing
- Corroded stainless rods
Correct answer: Tears or holes in the fabric
Torn or deteriorated grid fabric allows DE powder to bypass filtration and return to the pool, appearing as white cloudy material.
Question 5: What is the correct procedure after adding fresh DE powder to a DE filter following a backwash?
- Add powder directly into the skimmer with the pump running (Correct answer)
- Add powder into the pump strainer basket
- Pour powder into the return jets
- Dissolve powder in a bucket and add at the main drain
Correct answer: Add powder directly into the skimmer with the pump running
DE powder is slurried and poured into the skimmer with the pump running so it coats the grids evenly inside the filter.
Question 6: A cartridge filter element showing pleats that are collapsed or matted together indicates:
- Normal aging that does not affect performance
- Excessive flow rate or pump oversizing (Correct answer)
- Insufficient DE precoat
- Improper backwash direction
Correct answer: Excessive flow rate or pump oversizing
Collapsed pleats result from excessive flow velocity forcing the filter media together, dramatically reducing effective surface area and filtration efficiency.
Question 7: Which valve position on a sand filter multiport valve is used to waste backwash water directly to drain WITHOUT sending it through the filter?
- Filter
- Rinse
- Recirculate
- Waste (Correct answer)
Correct answer: Waste
The Waste position routes water from the pump past the filter and directly out the waste line, used for vacuuming heavy debris or lowering water level.
CPI Sand, DE & Cartridge Filtration 2
What is the typical operating pressure range for a properly functioning sand filter?