I agree with Jason as ORP is only useful for process control and not absolute measurement and even for process control it can get interference. In practical ORP situations, one tries to keep such interferences to a minimum, such as not using non-chlorine shock in such a pool. What I describe below would have you at least try and get a baseline ORP level that means something by using a chlorine standard.
You would first need to "calibrate" your ORP sensor to a standard chlorine level. So let's say you measure it when having an FC that is around 10% of the CYA level or roughly 0.1 ppm FC with no CYA which would kill pathogens reasonably quickly (around 1 minute for a 99% kill of most heterotrophic bacteria) and would prevent algae growth as well. Different sensors would report different ORP according to field data for the Oakton and published charts for the others:
Oakton: 655 mV
Chemtrol: 695 mV
Aquarius: 585 mV
Sensorex: 430 mV
Then you could compare your ozone ORP reading in the middle of your pool against that standard. However, ORP measurements for controlling ozone systems are done in the side stream or at the outlet to the pool and not in the bulk pool water so I'm not really sure what you are trying to accomplish here.
The current EPA limit for ground level ozone is 75 ppb and they are about to lower it more to somewhere between 60 and 70 ppb. For swimming pools and spas, ozone is only allowed as a supplemental oxidation system off-line and a small residual that is usually < 0.1 ppm enters the pool and is intentionally a low amount that does not last. Even Del-Ozone, who is hardly unbiased, says "Ozone's role is actually as the primary sanitizer, but maintaining an ozone residual in the pool that is high enough to ensure continuous in-pool protection from bather-to-bather cross contamination can be expensive; and present the risk of ozone off-gas."
Del-Ozone goes on to equate ozone levels with ORP where they say that "The ORP reading can range between 750 mV and 900 mV at the point of introduction into the main return line before entering the pool. An ORP of 800 mV is ~0.2 PPM dissolved ozone which equates to ~40 PPM Cl in terms of oxidation and efficacy." For commercial recreation/lap pools, they apply 1.6 ppm over 24 hours to the volume of the pool (that is not a residual, but an equivalent amount injected as water circulates through the ozonator). The time for the CT value of ozone in the side stream where it is injected is 4 minutes in their commercial system. You have to take some of what they say with a grain of salt since they claim that "ozone's reaction with chlorine is minimal" and we know that isn't true as we've seen chlorine demand double in spas when ozone is used and there is no bather load (when there is bather load, chlorine demand is cut in half as ozone can oxidize some of the bather waste).
Interestingly, their 40 ppm chlorine equivalent to 800 mV doesn't fly with any sensors that I know of. 40 ppm FC (with no CYA) would be closer to 900 mV for both Oakton and Chemtrol and even higher for the other sensors.
For something like E.coli, ozone has a CT value that is about half that of chlorine, but whereas 0.1 ppm FC with no CYA gives the ORP levels I listed above in the 650-700 mV range for Oakton and Chemtrol, ozone would give an ORP of around 770-780 mV. The Ozone CT value for E.coli is about half that of chlorine so the net result is that the ORP of ozone is higher by around 50 mV than it's true chlorine equivalent in terms of disinfection, but there are so many assumptions here that I wouldn't put a lot of faith in this.
So roughly speaking if you wanted equivalent killing power with ozone compared to chlorine, you would take your ORP reading with 0.1 ppm FC with no CYA equivalent of chlorine (or an FC that is around 10% of the CYA level) and add 50 mV to that and see if ozone alone with no chlorine reaches that level in the middle of the pool away from the returns. If it did, then you might have equivalent sanitation, but would possibly be at the risk of outgassing too much ozone.
Rather than go through this mess with ORP, why don't you just measure the ozone level directly using a test kit such as the
CHEMetrics K-7402 (there are also test strips that others make, but I'm not sure I'd trust those).
I really don't understand why you want to try and flood your pool with ozone levels that even the ozonator industry doesn't attempt due both to cost and safety. You might as well just irradiate your entire pool area with UV and ignore the cancerous effects on your skin. That's basically what you are trying to do by exposing yourself to high ozone concentrations, most especially if they outgas.
is an ORP of 730 when the ozone is blasting away and there is .5 ppm of FC equal to 730 with just ozone equal to 730 with just chlorine (assuming the other variables, temp/ph etc...are constant)?
