ORP stands for Oxidation Reduction Potential, which is measured in milliVolts, and indicates the potential of the halogen (chlorine or bromine) to oxidize (changing the chemical structure of contaminants; or in plain terms, zapping the bugs).
In my CPO classes I use gasoline as the analogy, a gallon of gas is a gallon of gas, same volume but if you look at the octane, that can be 87 or 91, the higher the number the more “effective” the gasoline, but it is still a gallon…very much the same logic for ORP. The higher the ORP, the more effective your chlorine is at zapping those bugs.
The CDC states that drinking water is safe to consume at an ORP level of 650.
This is where ORP gets interesting, and can be confusing to some, and that is pH, the most important reading taken in a pool. As the pH goes up, the ORP value goes down, while the level – PPM – of chlorine does not change. What has happened here? Well, the rise in pH reduced the effectivity of the chlorine, not the volume. The inverse is also true, as the pH goes down, the ORP will go up.
I have seen many an operator, when seeing his chlorine spike up or down, due to pH change, immediately change the ORP value to stop/start chlorine production; this is kind of like changing your speedometer, which would be crazy right?
The correct procedure would be to first ascertain what caused your pH to change, correct that, and then change your Set Point on the ORP programming to create more or less chlorine.
So ORP is related to the effectivity of the chlorine, not the volume.