ORP (Oxidation-Reduction Potential)
Not how much sanitizer is in the water, but how fast it is actually working.
- Safe range
- 650 – 800 mV (The floor is the CDC Model Aquatic Health Code figure for venues using ORP for automated control; the upper end is a comfort and equipment limit rather than a disinfection one.)
- Ideal range
- 650 – 750 mV
- Measured in
- mV
What it is
Chlorine and bromine kill by oxidising — stripping electrons from the cell walls of whatever is in the water. ORP measures how strongly the water as a whole is able to do that, in millivolts, using an electronic probe. It is a measure of effect rather than of quantity, which is what makes it different from every other reading here.
Why it matters
Free chlorine tells you what is present; ORP tells you what it is achieving. The two can disagree, and where they disagree ORP is usually the one describing the water you are actually swimming in. A pool with a textbook chlorine reading and a poor ORP is a pool where something — heavy stabiliser, high pH, a big organic load — is holding the sanitizer back. This is why automated controllers dose on ORP rather than on a chlorine number.
How to measure it
ORP needs an electronic probe. There is no strip, no drop kit and no colour to match — this is the one pool reading that cannot be taken by eye.
Your own equipment's display
- How trustworthy
- Ballpark
- Cost
- Free
- Effort
- None
The number your chlorinator, controller or filtration system shows you. Free and always there, and worth reading, but it is a sensor with a service life sitting in the water it is measuring.
If you have an automated controller, it is already measuring this continuously and dosing against it. The number on its display is the reading.
Watch out: An ORP probe fouls and drifts, and a controller that trusts a dirty probe will happily overdose or underdose the pool for weeks. Clean and recalibrate it on the manufacturer's schedule, not when the water looks wrong.
Where to find it: Already installed.
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 handheld ORP meter gives you a spot reading without a controller, and is the practical way to sanity-check a controller you suspect.
Watch out: Store the probe wet, in its storage solution rather than plain water. An ORP probe allowed to dry out is usually finished, and it will keep producing plausible-looking numbers on its way there.
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.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • Take the reading in circulating water, away from a return jet and away from where chemicals were last added. A probe sitting in a plume of fresh sanitizer reads the plume.
- • Let the reading settle. ORP probes drift toward their value over tens of seconds and the first number shown is rarely the answer.
Further reading: CDC: Healthy Swimming
If it is too low
The sanitizer is not keeping up. Contact times lengthen sharply, which means organisms survive longer in the water than they should, even if the chlorine reading looks acceptable.
- • Check pH first. ORP is strongly pH-dependent, and a pool sitting high will show a poor ORP at a perfectly good chlorine level.
- • Then check stabiliser. Cyanuric acid holds chlorine back, and heavy stabiliser is the commonest reason a good chlorine reading produces a bad ORP.
- • Then add sanitizer. Doing this first is the usual mistake, and it works only when the two above are already right.
If it is too high
Over-sanitised and aggressive. The water is clean, but it is harder on eyes, skin, swimwear and equipment than it needs to be.
- • Stop dosing and let the level fall; on a controller, lower the setpoint rather than turning it off.
- • Check the probe against a calibration solution before acting — a drifting probe reads high as readily as low.
Common questions
Does ORP replace testing free chlorine?
No, and the two answer different questions. ORP tells you how effectively the water is sanitising; free chlorine tells you how much sanitizer is there to keep doing it. A pool can show good ORP on a residual that is nearly spent, and be fine now and unprotected by the evening.
Why is my ORP low when my chlorine is fine?
Almost always pH or stabiliser. High pH converts chlorine into a far less active form, and heavy cyanuric acid binds chlorine so tightly that a good measured level does comparatively little work. Both show up as a poor ORP against a reassuring chlorine reading — which is exactly the situation ORP exists to reveal.
Does ORP work in a biguanide pool?
No, and this is worth knowing before you buy a meter. Biguanide sanitises by rupturing cell walls rather than by oxidising, so there is no oxidation for a probe to detect. An ORP meter in a perfectly clean biguanide pool reads far below the range shown here, and acting on that reading would mean treating a pool that has nothing wrong with 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.