Indigo Quick Response QAC Test strips (Quaternary Ammonium Compounds) cover the range of 0-400ppm & give accurate readings after 1 second. The 1500 & 10000ppm do this in 5 seconds and the residual 100ppm in 10 seconds. All Indigo QAC test strips are designed to work for most single, dual and 4-chain quat disinfectants.