SWCG controlled by ORP sensor

[VinceL]

I have the Hayward Sense & Dispense system. My SWCG is automatically controlled by the ORP reading. I had been keeping the ORP target level at 680. When I tested the water yesterday, FC was 1.5 (CC = 0). I wanted to get it a little higher so I bumped up the ORP target to 710. The SWCG ran all afternoon and was never able to get the ORP above 695. It was sunny and 89° today, water temp was 79°and our pool has no shade. There was only person who used the pool today. Late this afternoon, I tested the water and the FC was up to 4 (CC=0). So, the SWCG was definitely making chlorine. My CYA is at 40. With the heat and sunshine, it seems like my SWCG (cell less than 2 years old) was working at its limit to make chlorine. Does that seem right?

I turned off the pump for a few hours to let the chlorine drop. I started it back up and the ORP was at 680, and i lowered the ORP target to 690. The SWCG had it up to 690 in about 15 minutes.

 

[JasonLion]

I don't recommend using ORP with a SWG in a residential pool. Problems like the one you describe are very common and there is no real solution. Simple percentage based control is much more reliable, and less expensive for residential pool usage.

A number of things can change the ORP reading that have nothing at all to do with the FC level. For example the ORP reading tends to fall by 20 or 30 when there is sunlight shining directly on the water. More significantly, ORP goes down when there is hydrogen gas dissolved in the water. SWGs add hydrogen, some of which dissolves. Depending on several factors the lowering of ORP from the hydrogen gas sometimes overwhelmed the increase in ORP from the rising FC level, resulting in the cell running constantly.

It is very plausible that your FC level went up from 1 to 4. SWGs are designed to add chlorine slowly and steadily, and are not so good at rapid increases. Also, it was compensating for the daytime chlorine loss that entire time. So it was doing rather well to raise FC by 3.

 

[VinceL]

Thanks for the quick response and explanation.

Last summer I didn't have this problem, or maybe I did and didn't realize it. I know I didn't have the ORP target as high as I had it set today. So, given your comments about the sunlight and hydrogen lowering the ORP, maybe I got lucky and the SWCG didn't get stuck in that vicious cycle of lowering the ORP with hydrogen while raising it with chlorine. Kind of like running your heat and air conditioning at the same time.

I guess I need to do some experimenting to figure out a good % setting for the SWCG.