Reference

What the letters on your test mean

The abbreviations on a test kit and the terms poolcntrl uses, in a sentence or two each. The same definitions the app shows when you tap a term.

Free chlorine (FC): Free chlorine is the chlorine that hasn't been used up yet and is still available to kill germs and algae. It's the single most important number for keeping your water safe to swim in. More on this reading →
Total chlorine (TC): Total chlorine is all the chlorine in the water — the free chlorine still working plus the chlorine that has already been used up. On its own it sets no target; we use it to work out the difference. More on this reading →
Combined chlorine (CC): Combined chlorine is the chlorine that has already reacted with sweat, sunscreen, and other contaminants — total chlorine minus free chlorine. It is what makes a pool smell harsh and sting eyes, and you want it under 0.5 ppm. More on this reading →
pH: pH measures how acidic or basic the water is, on a scale where 7 is neutral. It decides how the water feels on skin and eyes and how well your chlorine actually works. More on this reading →
Alkalinity (TA): Alkalinity — total alkalinity, or TA on your test kit — is the buffer in the water that absorbs acids and bases before they can swing your pH. Keep it in range and pH stays where you put it. More on this reading →
Stabilizer (CYA): Stabilizer — often labeled cyanuric acid, conditioner, or CYA — shields chlorine from being destroyed by sunlight. It's why your chlorine target is set against your stabilizer rather than a fixed number. More on this reading →
Water hardness (CH): Water hardness — calcium hardness, or CH on your test kit — is how much dissolved calcium the water carries. Too little and the water pulls calcium out of plaster and grout; too much leaves chalky scale. More on this reading →
Salt: Salt level is how much dissolved salt is in the water. It only matters on a saltwater pool, where a generator cell turns that salt into chlorine — so a saltwater pool is still a chlorine pool. More on this reading →
Clarifier: A clarifier is a polymer that clumps particles too fine for your filter into pieces it can catch. It helps only once the cause of the cloudiness is already fixed, needs a clean filter running long hours, and never replaces chlorine.
Flocculant: A flocculant binds everything in the water into a blanket that settles on the floor, which you then vacuum out to waste rather than through the filter. It needs a multiport valve, costs you several inches of water, and pulls pH down.
Overnight chlorine loss test: The overnight chlorine loss test is how you tell a green pool is finished. Record the chlorine at dusk, run the pump overnight, and read it again before sunrise; if it has barely moved, nothing living is left in the water eating it. More on this reading →
Available chlorine: Available chlorine is the percentage of a product that is actually chlorine able to sanitize. It is why a pound of one granular product does not do the same work as a pound of another, and why the app asks for the strength on your label.
Stabilized chlorine: Stabilized chlorine means the chlorine is chemically attached to cyanuric acid — trichlor tabs and dichlor granules. Every dose raises your stabilizer as well as your chlorine, and stabilizer only leaves with the water. More on this reading →
Salt chlorine generator: A salt chlorine generator is a cell in the return line that makes chlorine from lightly salted water as the pump runs. A salt pool is still a chlorine pool; it makes its own instead of buying it, and its pH tends to climb. More on this reading →
FAS-DPD: FAS-DPD is a drop-count chlorine test: you count reagent drops until a pink sample turns clear, so there is no colour to judge. It is the method that stays accurate at high chlorine, where a colour-matched DPD test can read falsely low. More on this reading →
Non-chlorine shock (MPS): Non-chlorine shock, or MPS, is an oxidizer that burns off sweat and sunscreen so chlorine does not have to. It sanitizes nothing and kills no algae, and it reads as combined chlorine on most test kits for a few days afterwards. More on this reading →
Tab feeder: A tab feeder is a canister plumbed into the return line, or mounted beside it, that meters water over chlorine tabs at an adjustable rate. It holds a steadier chlorine level than a floating dispenser does.
Shock: Shocking a pool means raising chlorine well above its normal level for a short time, to burn off what a normal level cannot keep up with. It is a one-off correction, not a routine: a pool that needs it weekly has a different problem underneath. More on this reading →
Clearing: Clearing is holding chlorine high for several days running until algae is gone and the water passes an overnight test. It is a process, not a single dose: it usually takes three to seven days, and the level looks alarming on purpose. More on this reading →

Keep learning

How a water test worksA five-step, illustrated walkthrough from scooping a sample to getting your plan.How to add chemicalsThe safe, simple sequence for adding anything to your pool — animated step by step.Cloudy, green and what to do about itWhy pools go cloudy or green, what really clears them, clarifier versus flocculant, and how to read a pool store’s printout.What each measurement meansPlain-language explanations of every reading — what it measures and why it matters.What each chemical doesWhat every product does, how it’s added, and why the method matters.Choosing your chemicalsLiquid or powder, tabs or a salt cell, drops or strips — the real options for each job and the honest trade-offs of each.After you add chemicalsThe safety and comfort steps that follow any addition — and the reasoning behind them.Types of poolsPlaster, fiberglass, vinyl, tile, chlorine, saltwater — how each changes your routine.Pool equipment explainedPumps, filters, chlorinators, heaters, timers — what each part does for you.Closing & opening your poolThe two guided checklists that bookend the season — each step with its why.Protecting your pool from a freezeWhat to do when a freeze is forecast — four steps, ranked by the damage each one prevents.Where our numbers come fromThe methodology behind every target and dose — the ratios, the caps, the rounding, and why AI reads but chemistry decides.