Pool equipment and test kits are full of abbreviations. Here is what each one means, in plain terms.
Water chemistry
ppm (parts per million)
The unit almost every pool reading uses. One ppm is one milligram per litre — so 3 ppm of chlorine means three milligrams of chlorine in every litre of water. Because it is a ratio, it reads the same whether the pool is measured in gallons or litres.
pH
How acidic or alkaline the water is, on a scale where 7 is neutral. Pools are kept at 7.4 to 7.6. Below that, water corrodes metal and etches plaster; above it, chlorine becomes much less effective and calcium starts to scale.
Free chlorine (FC)
The chlorine still available to sanitise. This is the number that matters — the one that has to stay in range at all times.
Combined chlorine (CC)
Chlorine that has already reacted with sweat, sunscreen or urine and is now spent. It causes the sharp “chlorine smell” and irritates eyes. Anything above 0.5 ppm means the pool needs shocking.
Total chlorine (TC)
Free chlorine plus combined chlorine. Total minus free gives you the combined figure, which is the useful one.
Total alkalinity (TA)
The water’s resistance to pH change — pH’s shock absorber. Kept at 80 to 120 ppm. Too low and pH swings wildly; too high and pH will not stay down.
Calcium hardness (CH)
How much dissolved calcium is in the water, kept at 200 to 400 ppm on a plaster pool. Too little and the water dissolves calcium out of the plaster and grout; too much and it forms scale.
Cyanuric acid (CYA)
Also called stabiliser or conditioner. It shields chlorine from sunlight, which an outdoor pool needs, but it also holds chlorine back. Kept at 30 to 60 ppm.
Chloramines
The technical name for combined chlorine. Chloramines smell strongly, sting eyes, and no longer sanitise anything.
Breakpoint chlorination
Raising free chlorine high enough to destroy chloramines completely — roughly ten times the combined chlorine reading. It is what clears the chlorine smell.
Pool shock
Raising free chlorine far above its normal level to kill algae or burn off chloramines. The level needed depends on cyanuric acid, not on the size of the bag.
TDS (total dissolved solids)
Everything dissolved in the water added together — salt, calcium, cyanuric acid, the lot. Very high TDS means the water has aged and needs partly replacing rather than treating.
Langelier Saturation Index (LSI)
A single figure combining pH, temperature, calcium hardness and alkalinity to say whether water will scale or corrode. Negative is corrosive, positive is scale-forming, near zero is balanced.
Borates
An optional second buffer, usually added as boric acid, that holds pH steady around 7.5. Useful on pools whose pH climbs constantly. Toxic to dogs that drink pool water.
Sequestrant
A product that holds dissolved metals in solution so they do not stain surfaces. Needs re-dosing monthly because it breaks down in chlorinated water.
Chemicals
Liquid chlorine
Sodium hypochlorite, usually 10 to 12.5%. Adds nothing but chlorine and a little salt, which is why it is the standard choice for regular dosing. Loses strength within weeks, faster in heat.
Cal-hypo
Calcium hypochlorite, usually 65 to 73% — a granular chlorine that also adds calcium. Fine occasionally; a problem if it is your only chlorine source in a hard-water area.
Dichlor
A stabilised granular chlorine, around 56%, that adds cyanuric acid with every dose — roughly 0.9 ppm of CYA per 1 ppm of chlorine.
Trichlor
Stabilised chlorine tablets, around 90%. Convenient, but strongly acidic and adds cyanuric acid steadily. Pools chlorinated only by tablets usually end the season with CYA far too high.
Soda ash
Sodium carbonate — the product used to raise pH.
Baking soda
Sodium bicarbonate — the product used to raise total alkalinity. Grocery baking soda and branded alkalinity increaser are the same chemical.
Muriatic acid
Hydrochloric acid, usually 31.45%, used to lower pH and alkalinity. Corrosive: eye protection, gloves, and never near chlorine.
Equipment and flow
GPM (gallons per minute)
The unit pump flow is measured in. Required flow is pool volume divided by the turnover time in minutes.
LPM (litres per minute)
The metric equivalent. One GPM is about 3.785 LPM.
Turnover rate
How long the pump takes to push the whole pool through the filter once. Eight hours is the usual residential target.
Head pressure
The resistance the pump has to work against — pipe length, bends, filter condition, heaters and salt cells all add to it. It is why a pump rarely delivers its rated flow.
Cavitation
Vapour bubbles forming and collapsing inside the pump because water cannot reach the impeller fast enough. It sounds like gravel rattling, and it damages the impeller. Usually caused by blocked or undersized suction plumbing.
Variable-speed pump
A pump whose motor speed can be adjusted. Because power draw follows roughly the cube of speed while flow follows speed directly, running slower for longer moves the same water for a fraction of the electricity.
Skimmer
The opening at the water line that draws surface water, and the debris on it, into the filtration system before it can sink.
Salt chlorinator
A cell in the plumbing that makes chlorine from dissolved salt by electrolysis. The pool is still a chlorine pool — it just produces its own.
Backwash
Reversing flow through a sand or DE filter to flush trapped dirt to waste. Done on pressure — 8 to 10 psi above the clean reading — not on a schedule.
DE (diatomaceous earth)
A fine powder coating the grids of a DE filter. It filters the finest particles of any common pool filter, and has to be replaced after every backwash.
BTU
The unit heater output is measured in. One BTU raises one pound of water by one degree Fahrenheit, and a US gallon of water weighs 8.34 lbs.
COP (coefficient of performance)
How much heat a heat pump moves for each unit of electricity it uses. A COP of 5 means five units of heat per unit of power — which is why heat pumps appear to be 500% efficient.
Put the numbers to work
Knowing what a reading means is half of it. These tools turn the readings into amounts.
Water Health Check
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