Frequently Asked Questions: ORP & Chlorine Chemistry
What is the relationship between ORP (mV) and Free Chlorine (ppm)?
Free chlorine measures the concentration of chemical sanitiser in parts per million (ppm), whereas Oxidation-Reduction Potential (ORP), measured in millivolts (mV), indicates the sanitising power and oxidation rate of the water. ORP reflects how effectively hypochlorous acid (HOCl) can destroy bacteria, organics, and viruses in real time.
How does pH affect ORP and chlorine effectiveness?
When chlorine dissolves in water, it forms active hypochlorous acid (HOCl) and less reactive hypochlorite ions (OCl⁻). At lower pH levels (around 7.20), approximately 66% of free chlorine exists as powerful HOCl, yielding higher ORP readings. At higher pH levels (above 7.80), HOCl drops below 30%, causing a significant decrease in ORP even when free chlorine ppm remains high.
What is an acceptable ORP reading for commercial swimming pools?
According to the World Health Organization (WHO) and PWTAG (Pool Water Treatment Advisory Group) guidelines, a minimum ORP of 650 mV to 750 mV ensures rapid inactivation of common pathogens such as E. coli in under one second. Most commercial automatic dosing controllers are set between 700 mV and 750 mV.
Why does ORP drop despite having adequate free chlorine?
ORP readings can be suppressed by elevated cyanuric acid (stabiliser/CYA) levels, heavy bather load and organics, high Total Dissolved Solids (TDS), scaling on the platinum sensor tip, or an alkaline pH drift. In outdoor pools with high CYA, free chlorine may read 2.0 ppm while ORP remains sluggish below 650 mV.