Stabiliser (Cyanuric Acid)
Sunscreen for your chlorine. Too little and the sun destroys it; too much and it stops working.
- Safe range
- 30 – 100 mg/L
- Ideal range
- 30 – 50 mg/L
- Measured in
- mg/L
What it is
Cyanuric acid, usually sold as stabiliser or conditioner, binds loosely to free chlorine and shields it from ultraviolet light. Without it, an outdoor pool in direct sun can lose most of its free chlorine within hours.
Why it matters
CYA has a genuinely two-sided character, which is what makes it confusing. It protects chlorine from the sun, but it also holds chlorine back from reacting, so as CYA climbs, you need progressively more free chlorine to get the same sanitising effect. A pool with very high CYA can show a textbook chlorine reading and still be under-sanitised. This is often called chlorine lock.
How to measure it
The black dot tube, and be honest with yourself about how coarse it is. This is the least precise test in the kit, and its answer changes what all your chlorine is worth, so it deserves daylight and a slow pour.
Turbidity tube
- How trustworthy
- Ballpark
- Cost
- Low cost
- Effort
- A few minutes, outdoors in daylight
A cloudiness test rather than a colour test. Reagent turns the sample milky, and you pour it into a tube until a black dot at the bottom disappears. Where it vanishes is the reading.
Sample and reagent are mixed and left to turn milky, then poured slowly into a tube with a black dot in the bottom. The moment the dot disappears you stop, and the scale on the side is the reading.
Watch out: Read it outdoors in daylight with the sun behind you, never indoors under a bulb, and let the mixture reach room temperature first: cold water reads high. Pour slowly, because the answer is the moment you decide to stop.
Where to find it: Pool shops and online, sold either on its own or inside a full pool kit.
Test strips
- How trustworthy
- Ballpark
- Cost
- Low cost
- Effort
- Under a minute, wherever you drew the sample
A plastic strip with reagent pads on it. Dip it, hold it level, and match each pad against the chart on the bottle at the time the label tells you to. Most strips read several parameters at once.
Stabiliser strips are worth having for the coarse answer, particularly for telling none at all from plenty.
Where to find it: Pool shops, hardware stores, supermarkets and online, sold both per parameter and as multi-packs. Buy strips made for your kind of water: pool strips and drinking water strips read very different scales for the same chemistry.
Free test at a pool shop
- How trustworthy
- Dependable
- Cost
- Free
- Effort
- A trip, and a sample carried carefully
Take a sample in and they will run it, usually on a photometer, usually free, and print you a full panel. The catch is that the printout ends in a list of things to buy.
Worth using once a season as a check on your own tube, since this is the test most likely to disagree with itself twice in a row.
Where to find it: Most pool and spa retailers. Ask which parameters they cover before driving over.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • Nothing removes stabiliser except draining and dilution, so a reading that keeps climbing is a message about your tablets rather than about your testing.
- • Test before the season's first shock rather than after it, so the chlorine you plan is chosen against the stabiliser you actually have.
If it is too low
Chlorine burns off rapidly in sunlight, so an outdoor pool struggles to hold a level through a sunny day.
- • Add stabiliser directly, or use stabilised chlorine (dichlor or trichlor), which brings CYA with it.
- • Indoor pools generally do not need it; there is no UV to protect against.
If it is too high
Chlorine becomes progressively less effective at the same measured level. Very high readings can mean the pool is effectively under-sanitised despite testing fine.
- • Dilution is the only practical route: drain part of the pool and refill.
- • Switch to unstabilised chlorine (liquid chlorine or cal-hypo) so you stop adding more.
- • Watch for this creeping up if you use trichlor tablets continuously; every tablet adds CYA that never leaves on its own.
Common questions
Why is my chlorine not working even though the level looks right?
High cyanuric acid is the usual explanation. CYA holds chlorine in a bound, unavailable form, so a pool at 50 mg/L CYA and one at 150 mg/L need very different free chlorine levels to be equally sanitary. Test CYA before adding more chlorine.
Do indoor pools and hot tubs need stabiliser?
Generally no. Stabiliser exists to protect chlorine from sunlight, and indoors there is none to protect against. Adding it indoors gives you the downside (reduced chlorine effectiveness) with none of the benefit.
How do I lower cyanuric acid?
By diluting the water. There is no standard chemical that removes it, and it does not break down appreciably on its own, so it accumulates in any pool run on trichlor tablets until some water is replaced.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.