Carbonate Alkalinity
Total alkalinity with the stabiliser's contribution taken back out. Not a verdict of its own — the figure the saturation index is supposed to use.
- Measured in
- mg/L
No band, and deliberately none. The published alkalinity range is written for total alkalinity as measured, and applying it to a corrected figure would be an inference nobody published. The verdict on alkalinity lives on the Total Alkalinity guide; this row exists to be an input.
What it is
An alkalinity test measures everything in the water that resists a change in pH, and in a stabilised pool part of that is the cyanuric acid rather than the carbonate. The two are not interchangeable for every purpose: it is the carbonate part that decides whether water scales or etches, so the saturation index is meant to be worked out from carbonate alkalinity rather than from the total the kit printed. The correction is a subtraction — a share of the cyanuric acid reading, which the operator handbooks put at a third of it.
Why it matters
In a pool with a lot of stabiliser the difference is large enough to change the answer. Total alkalinity comfortably in range can hide a carbonate figure well below it, and a saturation index calculated from the uncorrected number will call that water balanced when it is quietly aggressive. This is the correction that explains a plaster pool etching while every reading on the sheet looks fine.
How to measure it
You do not measure this one — there is no kit for it. Measure total alkalinity and cyanuric acid, then subtract a third of the cyanuric acid reading from the alkalinity. WiseWaterGauge records both inputs; today the subtraction is yours or your pool shop's to do.
Drop-count titration kit
- How trustworthy
- Dependable
- Cost
- Low cost
- Effort
- A few minutes, and it rewards being unhurried
Add reagent one drop at a time until the sample changes colour, and count the drops. The count is the answer, so there is no colour to judge by eye and no lighting to get wrong.
Take the two inputs properly: the alkalinity drop count and the cyanuric acid turbidity test. A correction is only as good as the two numbers behind it, and cyanuric acid is the one most often read from a strip that cannot resolve it.
Where to find it: Pool shops and online, usually as part of a multi-test kit. Aquarium shops sell the same chemistry in smaller bottles.
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.
The stabiliser half of the pair, and the one worth doing carefully. The correction subtracts a third of this reading, so an error here arrives at a third of its size in the answer.
Watch out: Read the tube in daylight with your back to the sun, and read it at the point the black dot disappears rather than after it has been gone for a while. Over-reading stabiliser over-corrects the alkalinity.
Where to find it: Pool shops and online, sold either on its own or inside a full pool kit.
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.
Many pool shop printouts do this arithmetic for you and label it adjusted or carbonate alkalinity, usually beside a saturation index worked out from it.
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.
- • Both inputs from one sample. The correction is a subtraction between two readings, so taking them on different days puts the difference between two pools into the answer.
Further reading: CDC: the Model Aquatic Health Code, the guidance US pool codes are written from
If it is too low
A low carbonate figure behind an acceptable total alkalinity means most of what the test saw was stabiliser. The water has less genuine buffering than the sheet suggests, and the fix is the one on the alkalinity guide rather than anything specific to this number.
- • Treat it as an alkalinity problem, not a separate one: add bicarbonate and retest total alkalinity.
- • Check cyanuric acid at the same time. A large correction is itself the finding — it says the stabiliser is high enough to be distorting more than this one number.
If it is too high
Carbonate alkalinity close to the total means there is little stabiliser in the water, which is the ordinary state of an indoor pool or a spa. Nothing to correct here; judge it on the Total Alkalinity guide.
- • Nothing specific to this figure. Act on total alkalinity, which is where the published range and the dosing advice both live.
Common questions
Do I need to log this?
No, and you cannot log it directly: it is derived from total alkalinity and cyanuric acid, both of which you already record. It is here so that the number the saturation index needs has a name and an explanation.
Why a third of the cyanuric acid reading?
It is the share of a cyanuric acid reading that an alkalinity test picks up, and it is the figure the industry operator handbooks give, with a worked example. It is an approximation rather than exact chemistry — good enough to correct an index, not a precise account of what is in the water.
My pool has no stabiliser. Does this apply?
No. With no cyanuric acid in the water there is nothing to subtract, and carbonate alkalinity is simply total alkalinity. That is the normal case for an indoor pool and for most spas.
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.