How to test your pool water properly

What to test and how often

Combined chlorine is not measured directly: it is total minus free. When it rises, you get the familiar "pool smell" (see why the pool smells of chlorine).

The three testing methods

Test strips

Dip the strip, wait the number of seconds stated on the pack and compare the colours with the chart. Strips are quick and give several parameters at once, but their accuracy is only indicative. Moisture spoils them easily: close the lid straight away, do not touch the pads with wet fingers and watch the expiry date.

DPD drops or tablets

The classic test kit with tubes: phenol red for pH and DPD for chlorine, usually DPD 1 for free and DPD 3 for total. It is more reliable than strips, provided you read the colour in daylight with a white background behind the tube, not under a lamp or in bright sun.

Photometer

A digital instrument that "reads" the colour of the reagent and gives a number, with no subjective judgement. It suits people who want accuracy or who manage a pool at a rental property. It needs a clean cuvette without fingerprints, zeroing with a sample without reagent, and reagents that are in date. Electronic pH pens, like the sensors of automatic pH and chlorine control, also need regular calibration.

How to take a proper sample

1. Test after the pump has been running for a good while and the water is mixed, not straight after adding chemicals.
2. Rinse the tube with pool water.
3. Push it under upside down, opening facing downwards, to about elbow depth, and turn it over there to fill it.
4. Take the sample away from the return inlets, the skimmer and the floating tablet dispenser.
5. Test immediately, because chlorine in the sample is lost quickly in the sun.
6. Note the date, time and readings in a notebook or on your phone.

A testing log shows trends that a single reading hides. If, for example, the pH rises steadily every week despite corrections, it is worth checking the total alkalinity.

Mistakes that skew the result

• Very high chlorine: after a shock chlorination, DPD can fade or not change colour at all and show falsely low chlorine. The same high chlorine can also give a falsely high pH.
• Spoiled reagents: out of date, or left in the sun and in the heat of the plant room.
• Wrong sample volume: water above or below the line on the tube changes the shade.
• Reading too early or too late: some reagents keep changing colour; read at the time stated in the instructions.
• A new correction without a new test: after every correction, test again once the water has completed a full circulation cycle.

At the start of the season, or when something does not add up, take a sample in a clean, closed bottle to a shop with a photometer for a full check that includes hardness, stabilizer and, if the water comes from a borehole, metals.

Questions

See also