The DPD Tablets are used as follows: DPD #1: Is used to measure Free Available Chlorine (FAC). DPD #2 : Monochloramine. Use in the same sample after testing for FAC to determine monochloramine content. The difference in readings after adding the DPD #2 is the monochloramine content. DPD #3 : Dichloramine and Total Residual Chlorine. Use in the same sample after testing for FAC and monochloramine (i.e., after adding a DPD #1 and DPD #2) to determine dichloramine content. The difference in readings after adding the DPD #3 is the dichloramine content. You can also add a DPD #3 to the sample after adding a DPD #1 (Skip the DPD #2). The difference after adding the DPD #3 is the combined chlorine content. DPD #4 : Is used to measure Total Residual Chlorine.