Quote Originally Posted by pmeloy View Post
What I'm interested in here is using pH/ORP in conjunction with CYA. Because CYA sequesters part of the HOCL residual in a form that an ORP probe doesn't react to it would be vital to keep CYA levels constant. if anyone has attempted it I'd like to hear how they made out.
The ORP sensor is measuring a voltage that will be related to the hypochlorous acid (HOCl) level. When there is CYA in the water, this active chlorine level will be very low. Roughly speaking near a pH of 7.5 the active chlorine level will be equivalent to FC/CYA compared to a pool with no CYA. So when the FC is roughly 10% of the CYA level, the active chlorine level will be around 0.1 ppm. ORP sensors tend to not do very well measuring such low active chlorine levels, even though such levels are more than enough to prevent algae growth and to kill fecal bacteria with a 99.9% reduction in around 1 minute.

As for the actual ORP level at such low active chlorine levels, this depends on the sensor since ORP is not actually absolute in spite of what the ORP manufacturers claim. Looking at their own tables shows not only differences in absolute levels, but in the slopes of ORP mV increase per doubling of FC (with no CYA). For example, With 3 ppm FC and 30 ppm CYA and a pH of 7.5 and at a temp of 80ºF, the HOCl is 0.042 ppm FC (equivalent to an FC of 0.087 ppm with n CYA) gives an ORP for Chemtrol of 689 mV, for Oakton of 647 mV, for Aquarius of 572 mV and for Sensorex (if you believe their tables) of 406 mV.