DPD tablets (all manufacturers!) only work up to approx. 10 mg/l chlorine in the sample water. Values above 10mg/l cause the DPD reagent to bleach out again, up to colourless. If you consider that 10mg/l in a small pool with only 5,000 liters means just 50g, i.e. 2 1/2 times the weight of a letter, it can quickly happen, especially for inexperienced pool owners, that this value is exceeded. Often the DPD tablet is blamed and as 'proof' a bucket of water (5 litres) is mixed with a lot of chlorine, then measured and the same effect is found. At 5 litres, the limit value is then only 0.05 grams, i.e. approximately one grain of chlorine. Even tap water cannot be used for comparison, because German tap water does not contain chlorine. Fortunately, it is easy to detect overchlorinated water. Instead of filling the cuvette completely, the user should only add a few drops of water to the cuvette and dissolve the DPD tablet in it. If these few drops turn red, the tablet is working properly. If the solution decolorizes when the cuvette is filled, the water is highly chlorinated. In this case the water sample must be diluted before measurement.