Combined Chlorine
The spent chlorine that smells like chlorine. Worked out from your total and free readings rather than measured.
- Safe range
- 0 – 0.4 mg/L (CDC Model Aquatic Health Code 5.7.4.4.2, which requires action above this level rather than treating it as a health limit on the swimmer.)
- Measured in
- mg/L
What it is
Combined chlorine is chlorine that has already reacted — with sweat, urine, sunscreen, skin oils and whatever else came into the pool on a swimmer. Chemically these are chloramines. They still register on a chlorine test, which is why total chlorine counts them, but they have very little sanitising power left. WiseWaterGauge works this out for you as total chlorine minus free chlorine, on the same test result, and never across two.
Why it matters
This is the number behind the two complaints people actually have about pool water: the strong chemical smell and the stinging eyes. Both are usually read as too much chlorine and are almost always the opposite — they are the signature of chlorine that has been used up and not replaced. It is also the one chlorine figure with a published action level behind it.
How to measure it
You do not measure this one directly, and no kit reports it on its own. Measure total chlorine and free chlorine on one sample and the difference is the answer — which WiseWaterGauge calculates and judges for you when both are on the same test result.
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.
FAS-DPD is the honest way to get this number at home. The free chlorine count and the total chlorine count come off one sample minutes apart, and the drop counts subtract cleanly without a colour chart in the middle.
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.
Liquid reagent kit, colour match
- How trustworthy
- Ballpark
- Cost
- Low cost
- Effort
- A couple of minutes, at a sink or poolside
A measured sample in a plastic comparator block, a few drops of reagent, and a colour you match against printed standards beside the tube. The two-tube kit sold everywhere for pools is this.
A standard DPD kit run for both free and total chlorine. It works, and the limitation is arithmetic rather than chemistry: you are subtracting two colour-matched readings, so the error in each one lands in full on the difference.
Watch out: A small gap between two readings that each round to the nearest half is not a measurement. Treat a narrow gap as 'probably nothing' and act on a wide one.
Where to find it: Pool shops and online. Reagents expire, so buy replacement bottles rather than a whole new kit each season.
Digital photometer
- How trustworthy
- Dependable
- Cost
- Most expensive
- Effort
- A few minutes per parameter
The same reagents as a drop kit, read by an instrument instead of your eye. It removes the colour judgement and gives a repeatable number, which matters most for people who are colour blind or testing in poor light.
The right tool if this reading matters to you — an indoor pool, a heavily used one, or one where the smell keeps coming back. Electronic reads of both numbers remove the judgement call from a subtraction that magnifies it.
Where to find it: Pool trade suppliers and water-testing suppliers online. Reagent tablets or sachets are an ongoing cost.
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.
A strip with free and total pads will show you a wide gap and nothing finer. Useful as a prompt to run a real test, not as the test.
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.
Further reading: CDC: the Model Aquatic Health Code, the guidance US pool codes are written from; CDC: Healthy Swimming
If it is too low
Zero is the goal, so there is nothing below the band to be concerned about. A negative result means one of the two readings it came from was misread — free chlorine is part of total by definition — and WiseWaterGauge gives no verdict on a negative rather than badging your pool on the strength of a test error.
- • Nothing to do. This is where the pool should sit.
If it is too high
There are chloramines in the water. Expect the chlorine smell, irritated eyes and, indoors, air that feels heavy. Ordinary chlorine additions will not clear this; it takes enough chlorine at once to break the chloramines apart, which is what shocking means.
- • Shock the pool. Breakpoint chlorination needs a large enough dose at once to break the chloramines down — a series of small additions feeds them instead.
- • Check free chlorine at the same time. Combined chlorine builds up in the first place because there was not enough free chlorine to finish the job.
- • Indoors, run the ventilation hard while you do it. Chloramines leave the water and sit in the air above it, which is why an indoor pool smells worse than an outdoor one at the same reading.
- • Ask swimmers to shower first. Almost everything that makes chloramines arrives on people, and this is the only fix that stops it at the source.
Common questions
Why does my pool smell strongly of chlorine when the chlorine reading is low?
That smell is combined chlorine. It is chlorine that has already reacted and can no longer sanitise, and it is what a nose actually detects — properly chlorinated water has very little odour. A strong smell almost always means the pool needs more chlorine, not less.
Do I have to log combined chlorine myself?
No, and you cannot: it is calculated. Log total chlorine and free chlorine on the same test and this appears beside them, badged against the published action level, marked as a calculated value.
My combined chlorine came out negative. What does that mean?
That one of the two readings was wrong, because free chlorine is part of total chlorine by definition. The usual culprit is a bleached-out DPD test after shocking, which reads far too low for free chlorine. Retest both on a fresh sample, diluting it half and half with tap water if the colour flashes and fades.
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.