The Valterra Blue Devil No.1 OTO Chlorine Level Indicator is a 32-ounce reagent refill designed for use with Blue Devil Test Kits to measure chlorine levels in swimming pools and spas. Regular monitoring of chlorine is essential for maintaining water quality and ensuring a safe swimming environment. To use, follow your test kit's instructions by adding the OTO solution to your water sample and comparing the resulting color to the provided chart to determine chlorine concentration. This solution is compatible with existing Blue Devil Test Kits.