Appendix 17. Essential water quality measurements


Carry out these measurements in daylight (but not in direct sunlight, nor wearing sunglasses).

17.1 Turbidity measurement with a turbidity tube

– Rinse the tube 3 times with the water to be tested.

– Fill the tube completely up to the 5 TU (turbidity unit) level.

– Wait a few seconds for the water to stabilise and the air bubbles to disappear.

– With the tube in vertical position, look down over the tube and try to see the circle (or cross, depending on the model) at the bottom of the tube:
• If the circle is visible, the water turbidity is below 5 TU.
• If the circle is not visible, empty out some water, and if it is still not visible empty out some more and repeat again until the circle is visible. For example, if the level is just above the 20 TU mark, the water turbidity is below 20 TU. If the level is just under the 20 TU mark, the water turbidity is higher than 20 TU.

17.2 Measuring free residual chlorine (FRC) and pH with a ‘pool tester’

– Remove the lid of the pool tester and rinse the pool tester (including the lid) 3 times with the water to be tested.

– Fill the 3 compartments up to the top with the water to be tested. Do not dip the pool tester into the water to be tested. If the water is from a tap, leave the tap running a few seconds before taking the sample.

– Add one phenol red tablet into the left hand compartment (to measure the pH).

– Add one white DPD1 tablet into the right hand compartment (to measure the FRC, “Cl”).
 Do not touch the tablets as this will modify the results.
Only use whole intact tablets. Discard tablets that break or crumble while removing from packaging.
Do not use tablets that have changed colour (dull grey instead of white for DPD1 and brown instead of bright orange for phenol red).
‘DPD1’ and ‘Phenol red’ should be marked in green on the packaging. There are other tablets that are labelled in black writing: do not use these tablets.

– Replace the lid, close tightly making sure the arrows are pointing towards you.

– Shake the tester until the tablets have completely dissolved (approx. 20 seconds).

– Read the results comparing the colour in the sample compartments (outer compartments) with the colour of the reference compartment (central compartment). Read the results within 60 seconds of the tablets completely dissolving: after this the results are no longer reliable.