Strips for a quick check, liquid DPD kits for routine operation, and a photometer where results must be accurate and recorded.

Test strips are convenient and inexpensive. They are sensitive to how long they are dipped, how they are held, humidity in the tub and reagent age, and the colour matching is coarse.

Liquid reagent kits with a comparator block are the practical standard for domestic and smaller commercial pools. DPD chemistry distinguishes free and combined chlorine, which is essential information.

Photometers read the developed colour electronically and eliminate subjective matching, which matters when different staff test the same pool and when readings are recorded for a regulator.

Commercial and public pools generally need the accuracy and the auditable record, so photometers dominate there.

Whatever the method, consistency between operators matters as much as absolute accuracy for spotting trends.

DPD distinguishes free from combined chlorine; OTO measures only total chlorine, which hides whether the chlorine is actually working.

OTO is the older yellow-colour method. It is cheap and stable but gives a single total chlorine figure.

A pool can show acceptable total chlorine while almost all of it is combined - spent, smelly and barely sanitising. OTO cannot reveal that.

DPD develops pink and, with separate reagents, allows free chlorine to be read first and total afterwards, with combined calculated as the difference.

Because combined chlorine is the number that triggers shock treatment, DPD is the method any pool with bathers needs.

One caution: at very high chlorine concentrations DPD can bleach out and read low or zero, which is misleading in the opposite direction. Diluting the sample with known chlorine-free water resolves it.

Away from returns and skimmers, at elbow depth, in a clean rinsed container, and tested immediately.

Sampling next to a return jet gives freshly dosed water rather than pool water, and next to a skimmer gives surface debris. Both produce misleading readings.

Take the sample from a representative point, well away from inlets, at around elbow depth rather than from the surface film.

The container must be clean and rinsed with pool water. Residue from a previous test or from a detergent affects the chemistry.

Test immediately. Chlorine readings in particular fall as the sample stands, especially in sunlight.

Fingers should not touch inside the cell or the tablet - skin contamination affects results.

For a large pool, sampling at more than one point occasionally reveals circulation problems that a single sampling point never shows.

Typically a season once opened, less in heat and humidity - and degraded reagents read low, which causes over-dosing.

Liquid reagents and tablets both degrade with age, heat, moisture and light. DPD reagents are particularly affected.

A degraded reagent under-reports chlorine. The operator adds more chlorine, the test still reads low, and the pool ends up substantially over-chlorinated.

Date reagents when opened, replace them at the start of each season regardless of apparent remaining quantity, and store them cool, dry and dark.

Keep tablet foils sealed until use and never handle tablets directly.

A simple check is to test a sample against a second kit or a fresh reagent set periodically. Where results are recorded for compliance, that verification is usually expected.

Free and combined chlorine and pH frequently - several times a day on a commercial pool - with alkalinity, calcium hardness and cyanuric acid at longer intervals.

Chlorine and pH change fastest and matter most, and are tested most often. Public pool standards typically require testing several times daily with records kept.

Total alkalinity is checked weekly or so, since it moves more slowly but governs pH stability.

Calcium hardness and cyanuric acid change slowly and are typically checked monthly or at longer intervals, and after any significant water replacement.

Domestic pools need less frequency but the same parameters; the chemistry does not differ.

Frequency should rise with bather load, temperature and after any incident, rain, or chemical intervention.

The applicable standard sets the requirement for regulated pools - follow the local one rather than a general schedule.

Yes - on the manufacturer's schedule, with the supplied standards, and the record kept where compliance requires it.

Photometers measure light through a developed sample and drift over time as optics age and the cell wears.

Calibration standards or check tablets are supplied for verifying the instrument, and the interval is set by the manufacturer.

Cells and vials must be clean and unscratched. Scratches and fingerprints scatter light and skew results, and a scratched cell is a common cause of drifting readings.

Batteries matter more than expected - low power can affect the light source and produce inconsistent readings before the instrument warns of it.

Where readings feed compliance records, calibration records are part of the documentation an inspector will expect.

Keep a liquid comparator kit as a back-up. An instrument failure should not stop pool testing.

Different methods, different reagent age, different sampling points, or contamination - and the discrepancy is worth resolving rather than averaging.

Strips and DPD kits measure differently and will not agree precisely; that is expected.

Reagent age explains most genuine discrepancies, with the older set reading low.

Sampling from different points can genuinely reflect different water, which is useful information about circulation rather than an error.

Contamination of cells, fingers on tablets or an unrinsed container all shift results.

Very high chlorine bleaching a DPD test to a false low reading is a specific trap worth knowing.

Resolve by testing a fresh sample with fresh reagents in a clean cell. Averaging two results, one of which is wrong, produces a number that describes nothing.