ORP or redox is the oxidation-reduction potential of water and is expressed in millivolts (mV).
It is an indirect measure of the concentration of free chlorine . There is sufficient free active chlorine in your water at a redox value between 600 and 800 mV, provided your pH is between 7.0 and 7.6.The ORP and pH value can be measured with the Hanna pocket tester HI-98121
Caution! The correlation between ORP and free chlorine depends on the quality of the water. Consequently, it is advisable to regularly check the free chlorine in mg/l with a photometer such as the Hanna pocket HI-701 or the professional HI-83326