If chlorine is the biocide in use at your pool (rather than bromine), there should always be enough free chlorine in the pool to minimise the risk of infection. The recommended range is 1.00 – 2.00mg/l for most types of pool. Pools using stabilised chlorine need a bit more 2.50 – 5.00mg/l , but spa pools, because of their increased risk of legionella contamination, have a much higher recommended range of 3.00 – 5.00mg/l. Free chlorine is measured with the DPD1 tablet test.
Much of the chemical pollution in swimming pools is in the firm of ammonia, which is a decomposition by-product of urea (which comes from sweat and urine etc. from bathers). This ammonia reacts with chlorine to form what are known as ‘chloramines’, also known as combined chlorine. You can work out how much has accumulated in the pool water by subtracting the free chlorine reading from the total chlorine reading, which is measured with the DPD3 tablet test.
Free Chlorine + Combined Chlorine = Total Chlorine
Combined chlorine is not effective as a disinfectant and is actually more of a pollutant. It needs to be removed from the pool by a combination of dilution (adding fresh water to the pool) and filtration. The build-up of combined chlorine can be minimised in the first place by the practice of pre-swim showering.
Combined chlorine levels should be kept as low as possible, aim for no more than half of the free chlorine level and certainly no more than 1.00mg/l (whichever is lower).