pH
How acidic or basic the water is. It decides how well your chlorine works and how the water treats your equipment.
- Safe range
- 7.2 – 7.8 pH
- Ideal range
- 7.4 – 7.6 pH
- Measured in
- pH
What it is
pH measures how acidic or basic the water is on a scale where 7 is neutral. It affects swimmer comfort, how effectively chlorine can sanitise, and how the water interacts with pool surfaces and equipment. The scale is logarithmic, so pH 8.0 is ten times more basic than pH 7.0; small-looking movements are larger than they appear.
Why it matters
pH is the multiplier on everything else. At pH 7.2 a given amount of free chlorine is dramatically more effective than the same amount at pH 8.0, where much of it converts to a far weaker form. This is why a pool with 'enough' chlorine can still go green: the chlorine is present but chemically hobbled.
How to measure it
The phenol red test in the standard two-tube kit is enough for almost everybody, and it is usually sitting in the same kit as your chlorine test. A digital pen is better only if you will actually calibrate it.
Liquid reagent kit, colour match
- How trustworthy
- Dependable
- 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.
Phenol red: a few drops into a measured sample, and the yellow through red to purple gradient is matched against the block moulded into the tester. It is the other half of the kit most pool owners already own.
Watch out: Straight after shocking, high chlorine bleaches phenol red and can wash the reading out or throw it purple. Wait until chlorine is back in its normal range before believing a pH result.
Where to find it: Pool shops and online. Reagents expire, so buy replacement bottles rather than a whole new kit each season.
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.
The pH pad on a multi-strip is coarse, but it is honest about direction, which is often all a mid-week check needs.
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.
Digital meter or pen
- How trustworthy
- Dependable
- Cost
- Mid range
- Effort
- Seconds to read, minutes to calibrate
A probe on a handheld body that reads the water electrically and prints a number. No colour to judge, which removes the commonest source of error and introduces a new one: a meter that has drifted looks exactly as confident as one that has not.
A pH pen removes the colour judgement and reads to a decimal place, which is worth having if you are trying to hold a narrow band rather than stay inside a wide one.
Watch out: A pen is only as good as its last calibration and it drifts whether or not you use it. Buy the buffer sachets at the same time, calibrate monthly, and store the probe wet: a probe left to dry out is usually finished.
Where to find it: Online and in pool and aquarium shops. Buy the calibration solution at the same time; a meter without it is a guess with a decimal point.
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.
Reads the same phenol red chemistry electronically, at the cost of the instrument and its tablets.
Where to find it: Pool trade suppliers and water-testing suppliers online. Reagent tablets or sachets are an ongoing cost.
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.
Comes free with the rest of the panel, and pH is the number their photometer is least likely to argue with yours about.
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.
- • Test a fresh sample rather than one that has been standing in a bottle. pH moves as water is agitated and gives up dissolved gas.
- • Test pH after an alkalinity correction has circulated, not during it. A reading taken while soda ash is still dissolving describes one part of the pool.
If it is too low
Can irritate eyes and skin, and corrode metal fittings, ladders and heaters. Often caused by heavy rain or by acidic sanitisers (like trichlor tablets) used steadily over time.
- • Add soda ash (sodium carbonate) to raise pH, or sodium bicarbonate if alkalinity is also low.
- • Check total alkalinity. Low alkalinity is usually the reason pH will not stay put.
- • Retest after a full circulation cycle; pH adjustments need time to distribute.
If it is too high
Reduces how effectively chlorine sanitises and can cloud the water or cause scale buildup. Often caused by soda ash overdosing, or fresh plaster or concrete in a new pool.
- • Add a pH reducer: muriatic acid or sodium bisulphate.
- • Adjust in small steps and retest. Overshooting downward is easy and unpleasant to correct.
- • New plaster raises pH for weeks. If the pool is freshly finished, expect to correct repeatedly.
Common questions
Why is pool pH different from drinking water pH?
They are measuring the same property for different reasons. Pool pH is held in a narrow band because that is where chlorine works well and the water is comfortable on eyes and skin. Drinking water is judged over a much wider range, because there the concern is corrosion of pipes and taste, not sanitiser efficiency. Each page prints its own band at the top.
What happens if pool pH is too high?
Chlorine loses much of its sanitising power, so the pool can test as adequately chlorinated and still grow algae. High pH also encourages scale on surfaces and inside heaters, and can make the water look cloudy.
Should I fix pH or chlorine first?
Alkalinity first, then pH, then chlorine. Alkalinity is what holds pH steady, and pH is what decides whether your chlorine can work. Adding chlorine to a pool with badly wrong pH wastes it.
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.