If you press "fCl" again after displaying the measurement result, the displayed value may increase continuously. This is because the reagent (DPD N°1) reacts with all substances that have an oxidative character, including atmospheric oxygen, for example. In most cases, however, it is the case that there is a high content of combined chlorine, which is gradually transferred over time to react with the DPD 1 tablet. Free chlorine values should therefore be measured without fail within 1 minute after complete dissolution of the DPD 1 tablet (time until the result is displayed). Results displayed after this time may not be meaningful.