Pool water terms: free chlorine, combined chlorine, ORP, cyanuric acid and more

Free chlorine is chlorine still free to disinfect the water. Combined chlorine has already reacted with organic and inorganic matter and no longer disinfects. ORP is a millivolt reading of the water's oxidizing strength, which the Turkish pool regulation neither defines nor limits. Cyanuric acid is a chlorine stabiliser, allowed in outdoor pools only.

The person the hotel holds responsible for the pool signs the pool log, and the sheet in front of them has columns headed free chlorine, combined chlorine, ORP and cyanuric acid. Nobody ever taught them what those four words measure, or which of them the regulation actually limits. This glossary fixes that: each pool water term gets a plain definition, the regulation's own wording where it exists, and a line on where in the hotel you meet it. After reading, you can read a lab report and a logbook page and know which numbers carry legal weight.

What do free chlorine and combined chlorine actually mean?#

Free chlorine is the chlorine still able to disinfect the water. Combined chlorine is chlorine that has already reacted with organic and inorganic substances and formed compounds, so it no longer disinfects. The Ministry of Health's Regulation on the Health Principles for Swimming Pools defines both in Article 4 and sets a limit for each in Annex 1.

Article 4 words it plainly. Free chlorine is chlorine that has not formed compounds, used to disinfect the water. Combined chlorine is chlorine that has reacted with organic and inorganic substances in the water and formed compounds. That difference is the whole story of a chlorine reading: one number is working, the other is spent.

mg/L means milligrams per litre, and suppliers often write ppm, parts per million, for the same thing in water. Read the unit on the report before comparing it with the regulation. Reports often print a total chlorine column too. It is the sum of the two, so a healthy free figure with a rising combined figure means the water is carrying a load that the free chlorine is still working through.

The free chlorine limit changes with the disinfection method, which is the detail most often missed. Where chlorine is the disinfectant, Annex 1 sets 1 to 1.5 mg/L in indoor pools and 1 to 3 mg/L in outdoor pools. Where ozone, UV, chlorine dioxide or other systems are used, the range is 0.3 to 0.6 mg/L. Know which method your pool runs before you compare a reading with a number.

Where the hotel meets these terms: the daily log, the dosing controller display and the laboratory sheet, all of which the responsible manager signs. For the daily routine of measuring and correcting, see hotel pool water care: measuring and correcting pH and chlorine each day.

What is ORP in a pool, and why is it not in the regulation?#

ORP, oxidation-reduction potential, is a millivolt reading of how strongly the water can oxidise, which operators use as a running proxy for disinfecting power. The pool regulation defines free chlorine, pH and other terms but does not define ORP, and sets no ORP limit.

A millivolt is a thousandth of a volt. A probe in the pipe produces a small voltage that rises as the water's oxidizing strength rises, and the controller shows it as a number. Its value to an operator is that it is continuous: it moves within minutes when chlorine is consumed, long before anyone takes a manual sample.

Its limit as a compliance number is that it shifts with pH and with cyanuric acid, so the same free chlorine can produce different readings on two pools, or on one pool across a week. The enforceable limits in Annex 1 sit on free chlorine, combined chlorine, pH, total alkalinity and cyanuric acid. When a controller reading and a manual colour-change test disagree, re-test and check the probe and the sample point before writing anything down.

Where the hotel meets it: the dosing controller in the plant room, sometimes a building management screen, occasionally a column in the pool log that nobody explained.

How do pH, total alkalinity and cyanuric acid affect each other?#

pH measures how acidic or basic the water is, total alkalinity measures how strongly the water resists a change in pH, and cyanuric acid holds chlorine against sunlight in outdoor pools. The regulation sets a range for each, and the three decide how a chlorine reading behaves.

Article 4 defines pH as the unit that describes the water's degree of acidity or basicity, and Annex 1 sets the range at 6.5 to 7.8. Article 4 defines alkalinity separately, as the water's capacity to neutralize acids, which is its buffering capacity and a different measure from pH. The limit is the practical part: total alkalinity 30 to 180 mg/L expressed as CaCO3, meaning calcium carbonate, the unit chemists use to state alkalinity.

Alkalinity is the buffer. Low alkalinity lets pH swing during the day; high alkalinity makes pH hard to move, so a correction that should work appears to do nothing. Cyanuric acid is not defined in the regulation. Annex 1 caps it at 100 mg/L and says it is checked only where stabilised chlorine is used.

Article 14(3) says stabilised chlorine compounds may be used only in outdoor pools. An indoor pool should therefore have no cyanuric acid story to tell, and a cyanuric acid figure on an indoor report is a question worth asking.

Which pool water chemicals appear on the delivery note?#

The regulation sorts them into two groups: disinfectants, which remove or destroy disease-causing microorganisms, and pool water chemicals, which adjust pH, settle suspended solids, prevent mould, yeast and algae, and balance other parameters.

Article 4 defines disinfectants as organic and inorganic solid, liquid and gaseous substances used to remove or destroy bacteria, viruses and fungi in the water. It defines pool water chemicals as substances used to adjust pH, settle suspended solids, prevent mould, yeast and algae, and balance some chemical parameters. That single sentence covers acid, flocculant and algaecide.

Chlorine arrives in stabilised and unstabilised forms. Stabilised compounds carry cyanuric acid and, under Article 14(3), belong to outdoor pools only. There is a second rulebook for pools running hydrogen peroxide with biguanide: Annex 1 note 2 accepts hydrogen peroxide in the 40 to 80 range, pH up to 8.2, ammonium up to 1.5 and total alkalinity up to 220, and free chlorine is not measured at all.

The regulation sets limits in the water, never quantities to pour. A dose depends on volume, current readings and the product label, which is why this glossary deliberately carries no doses. Chlorine and acid are stored and handled apart: mixing them releases gas, and the plant room layout should make that impossible rather than merely unlikely.

What does the plant room equipment do?#

Every item either moves water or moves a chemical. Skimmers and balance tanks handle surface water and surge, dosing pumps add chemical, filter media trap solids, and backwash cleans the filter so it can trap again.

Circulation time is defined in Article 4 as the time needed for the total pool volume to pass once through the filtration system. It explains why a correction made now shows up at the sample point later, not immediately. A skimmer pulls surface water and floating debris toward the filter. A balance tank holds the water that bathers displace so the pool level stays steady and the filter keeps a constant flow.

A dosing pump delivers chemical at a controlled rate. The controller decides how much, the pump delivers it, and the two are separate things to check when a reading will not move. Filter media, usually sand or another granular medium, trap particles, and the pressure gauge is the cheapest instrument in the room: rising pressure means the filter is loaded.

Backwash reverses the flow to flush trapped solids out, and a rinse step settles the media before the pool is put back on line. Check with your local water and sewerage authority before you change where backwash water goes. Shock treatment is an operating action, not a term the regulation defines: a large corrective dose to clear combined chlorine or respond to a contamination event. Record it as an action with a time and a name.

If the water is already cloudy or green, the diagnosis order is in finding the cause of cloudy or green pool water.

What do the words on the lab report and in the logbook mean?#

A laboratory report describes one sample taken at one moment. The logbook shows whether the pool was stable between samples. Reading both needs a named person who understands every column on the page.

Colony counts are reported as colony-forming units per millilitre, written cfu/ml: the number of bacterial colonies that grew from the sample under the laboratory's conditions. It describes what grew from that sample, not what any guest experienced. The sample point and the sampling time belong on the report: a sample from the balance tank and a sample from the deep end are not the same measurement, and neither is a sample taken before the morning correction.

Turbidity is the clarity measure, the cloudiness the eye catches at the steps. Where a report prints a turbidity figure, ask which method produced it before comparing it with anything else. Three roles are easy to confuse: the responsible manager signs and answers for the pool; the pool water operator does the rounds and the corrections; the lifeguard watches people, not chemistry.

Good practice is one page per pool per day: the reading, the action taken, the time and the initials. A page with readings and no actions is a page that was filled in, not kept.

What to do next#

Take one recent pool report and one week of the logbook, and match every column against the definitions and limits in Article 4 and Annex 1 of the Ministry of Health regulation on the health principles for swimming pools (opens in a new tab). Where a term has no limit, such as ORP, mark it as an operating signal; where a figure sits outside a range, write the action and the name beside it. That single exercise turns a signature into a decision, and the pool health regulation as a checklist shows how the rest of the regulation fits around it.