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Pool service glossary

The terms used across the pool library and the guides, in a sentence or two each.

Backwash: Running water backwards through a sand or DE filter to flush out what it has caught. Done when the filter pressure climbs 8–10 PSI over its clean baseline. More on Backwash →
Breakpoint chlorination (shocking): Adding enough chlorine at once to destroy combined chlorine: ten times the combined chlorine reading. A partial dose makes more chloramines than it destroys. More on Breakpoint chlorination →
Cal-hypo (calcium hypochlorite): Granular chlorine that also adds calcium. Granular cal-hypo added straight to a vinyl pool bleaches the liner, and on salt pools its calcium scales the cell. More on Cal-hypo →
Calcium hardness (CH): The calcium dissolved in the water. Target 200–400 ppm on plaster. Too low and the water etches plaster; too high and it scales. More on Calcium hardness →
Carbonate alkalinity: Total alkalinity minus about a third of the CYA reading. The LSI uses it because stabilizer shows up in a TA test without doing the same buffering work. More on Carbonate alkalinity →
Clean baseline: A filter's pressure reading right after a fresh clean, DE recharge or backwash. Every later reading is judged against it, so record it on the customer card. More on Clean baseline →
Combined chlorine (CC) (chloramines): Chlorine that has already reacted with sweat, urine and debris and no longer sanitizes. It causes the strong chlorine smell. Keep it at or below 0.5 ppm. More on Combined chlorine →
CPO (Certified Pool & Spa Operator): The pool operator certification from the Pool & Hot Tub Alliance (PHTA): a 16-hour course and an exam, valid for 5 years. More on CPO →
Cyanuric acid (CYA) (stabilizer, conditioner): Shields chlorine from sunlight so it lasts outdoors, but holds onto it too, so more CYA means you need more free chlorine. Target 30–50 ppm; salt pools 60–80. More on Cyanuric acid →
Etching: Water dissolving plaster and grout when the LSI runs negative. It does not reverse: the surface is gone. More on Etching →
FAS-DPD test: A drop-count test kit that measures free and combined chlorine accurately. Test strips are not accurate enough to log combined chlorine. More on FAS-DPD test →
FC–CYA ratio: The rule that free chlorine must be at least 7.5% of the CYA reading. At CYA 80 the minimum is 6 ppm, even though 5 ppm looks in range. More on FC–CYA ratio →
Free chlorine (FC): The chlorine that is still working to sanitize. Target 3–6 ppm, and never below 7.5% of the CYA reading. More on Free chlorine →
IPSSA (Independent Pool & Spa Service Association): An association of pool service technicians organized in local chapters, whose members cover each other's routes during illness or injury. More on IPSSA →
LSI (Langelier Saturation Index): One number that says whether water will scale, etch or leave surfaces alone, worked out from pH, temperature, calcium, alkalinity and TDS. Keep it between −0.3 and +0.3. More on LSI →
Muriatic acid: Hydrochloric acid, the usual way to lower pH and total alkalinity. Store it apart from chlorine, and add acid to water, never water to acid. More on Muriatic acid →
pH: How acidic or basic the water is. Target 7.4–7.6. It is the cheapest and fastest lever on the LSI. More on pH →
Phosphates: Algae food from fertilizer, leaves and some fill water. Under 500 ppb is fine; chase them only when algae keeps returning with correct chlorine. More on Phosphates →
PHTA (Pool & Hot Tub Alliance): The non-profit trade association for the pool and hot tub industry. It runs the CPO certification. More on PHTA →
ppm and ppb: Parts per million and parts per billion, the units for most pool readings. 500 ppb is 0.5 ppm.
PSI (filter pressure): The reading on the filter's pressure gauge. Service the filter at 8–10 PSI over its clean baseline; a reading below baseline with weak flow is a suction problem. More on PSI →
Route: A pool service business's list of recurring customers and the stops that serve them. Routes are bought and sold, priced as a multiple of monthly service billing. More on Route →
Route coverage: Another tech servicing your stops while you are sick or injured. IPSSA members cover each other's routes under their chapter's rules. More on Route coverage →
Route density: How close together a route's stops are. Denser routes spend less unpaid time driving, which is why buyers pay more for them. More on Route density →
Salt chlorine generator (SWG, salt cell): Equipment that makes chlorine from dissolved salt. Its output is capped, so it is too slow for shocking a pool. More on Salt chlorine generator →
Salt level: The salt in a pool with a salt chlorine generator, which the cell turns into chlorine. Use the range printed on the cell; running below it shortens the cell's life. More on Salt level →
Scaling: Calcium coming out of the water onto tile, equipment and salt cells, when the LSI runs high. A white line at the tile is the usual first sign. More on Scaling →
Soda ash: Sodium carbonate, used to raise pH. Baking soda raises total alkalinity instead. More on Soda ash →
Total alkalinity (TA): The water's buffer, which holds pH steady. Target 80–120 ppm; 80–100 on hard water so pH is easier to hold down. More on Total alkalinity →
Total chlorine: Free chlorine plus combined chlorine. It always reads better than the pool really is, so test free chlorine. More on Total chlorine →
Total dissolved solids (TDS): Everything dissolved in the water. Under 1,500 ppm on a regular pool; salt pools run 3,000–4,000 by design. More on Total dissolved solids →
Water temperature: An input, not a target. Warm water burns chlorine faster and scales faster; it is the one LSI input nobody can change. More on Water temperature →