Free residual chlorine and Total residual chlorine analysers measure the chlorine in water that is ‘residual’ (left over) after disinfection has happened. i.e. if you add chlorine to water it disinfects, and if there is any leftover, it is ‘residual’ and therefore available to disinfect more. This residual chlorine can be measured and used to give confidence that disinfection is complete. Pi has three products for measuring residual chlorine; HaloSense (that uses electrochemical sensors), DPDSense (that uses online DPD technology), and Chloribrid® (that uses a hybrid of both). Free chlorine refers to HOCl and OCl^–, and total chlorine refers to free chlorine plus chloramines. Both free and total chlorine are effective disinfectants.