Palintest Photometer DPD 1 XF Test Tablets Palintest DPD High Range Photometer Tablets - Test tablets for photometers. DPD1 - To Test Free Chlorine Levels AP013 DPD1-XF (HR) Tablets x 250 - High Range 0-10ppm Free Chlorine Levels - Only to be used with High Range Photometers. DPD3 - To Test Combined levels AP033/1 DPD3-XT (HR) Tablets x 250 - High Range 0-10ppm Combined Levels - Only to be used with High Range Photometers. This Palintest Chlorine/10 test uses the DPD method developed by Dr A T Palin and now internationally recognised as the standard method of testing for chlorine and other disinfectant residuals. In the Palintest DPD method, the reagents are provided in tablet form for maximum convenience and simplicity of use. Free chlorine reacts with diethyl-p-phenylene diamine (DPD) in buffered solution to produce a pink coloration. The intensity of the colour is proportional to the free chlorine concentration. Subsequent addition of excess potassium iodide induces a furthe