ORP stands for Oxygen Reduction Potential, also known as “Redox.” Most sanitizers, including chlorine (“salt water” pools are actually chlorine pools), bromine and ozone, use oxidation to kill bacteria and break down organic compounds which your filter can then remove. PPM, Parts Per Million, is a measure of the number of Parts of chemical sanitizer there are per One Million Parts of water.
However, the effectiveness of those Parts Per Million is not given by a simple PPM reading. It is very possible to have an “acceptable” PPM level, say 2.5ppm, but have the sanitizer be dangerously ineffective. If the pH is too high, or the CYA levels are too high, your pool may have acceptable PPM levels, but in fact be dangerously low in its ability to kill harmful bacteria.
ORP cannot, in fact, “measure PPM.” However, we can estimate “effective” PPM, which is given as a number on the EPOS main screen. This number is based on a calculation of the pool’s ORP level, the pH number and the water temperature. Based on these three numbers, the “effective” PPM can be estimated.
For example, if your “actual” PPM of free chlorine is 3.0PPM but your pH is at 7.8, your “effective” PPM may be closer to 1.0PPM. Higher pH causes ORP to drop so your sanitizer is less effective at higher pH numbers and more effective at lower numbers. At a pH of 8.0, your oxidization-based sanitizer is 10x less effective than it is at a pH of 7.0. Why not just keep the pH at 7.0? Because of swimmer comfort and other factors such as preservation of pool equipment. pH that is too low can result in destruction of plaster, stainless steel and seals.
The World Health Organization (WHO) has established a minimum ORP reading of 650mV (ePool’s “Red Line”) as a standard for sanitized water. ORP readings below this minimum indicate dangerously low sanitizer effectiveness and could result in the harmful growth of microorganisms in your pool.
Think of it like this: Your pool or spa is a battlefield. Bacteria, virus, organic matter, etc. are the enemy. PPM is the number of soldiers you have on the battlefield. ORP is an accurate measurement of the morale of your soldiers – how willing they are to kill.