This monitor is based on the standard HJ586-2010 "WaterQualityDetermination of Free Chlorine and Total Oxy-gen-N,N-Diethyl-1,4-Phenylenediamine (DPD) Spectro-photometric Method." Under conditions of pH 6.2 to 6.5.firee chlorine reacts directly with DPD to produce a redcompound,and its absorbance is measured at a specificwavelength.