Good day,
Does ORP probe work in a saline system and track the Chlorine level properly?
Thank you
Does ORP probe work in a saline system and track the Chlorine level properly?
Thank you
ORP in a Saline
- Thread starter Pasargard
- Start date
Depends on your CYA/stabilizer level. If higher than about 30 ppm, ORP is useless.
CYA binds up HOCL. This bound HOCL still reads as free chlorine on a FAS/DPD test, but does not increase ORP. So CYA reduces the ORP reading you will get at a particular FC level. The more CYA, the more bound HOCL and the lower the ORP.
FYI - ORP is also reduced by any H2 gas in the water quite substantially. H2 is generated by SWGs and depending on how well the H2 leaves your water via aeration, the impact on ORP will range from substantial to very little.
Got it, thank you Scottt! great information!
I have a 700 gal hot tub with 5000 salinity , Alk 90 ppm and PH 7.5. I try to keep the FC at 3.5 ppm but the ORP is not tracking the Chlorine, not sure what should I do. Thanks again
pH is also a huge factor in ORP. As the pH falls, ORP rises. I don't really think there's a hope of running an ORP control scheme without tight pH control (+/- .1 or .2). I'm current running pH control at 7.6 and my daily ph excursion is about 7.62 at night, dropping to 7.45 during direct sunlight.
This is only my second full season with my pool, but in my SWG pool, the H2 production makes it impossible to use a scheme of controlling FC by trying to reach a particular ORP setpoint. The slope of ORP is negative when the SWG kicks on, especially if the SWG has been off for several hours and all the H2 in the water has dissipated.
I just did a partial water replacement about a month ago and brought my CYA to somewhere sub 20. Based on Taylor's new method of measuring CYA sub-20, it looks like I'm running about 5-10ppm. I'm managing this low CYA level by occasionally adding a trichlor puck to the pool to bump CYA by about 1.5ppm at a time.
I did this to experiment with operating in a low CYA regime to see if ORP control of the SWG became feasible. I understand all the downsides of running low-CYA, but its an experiment. It did not. While my ORP level now rides at a higher average and is almost always over 650mV with less CYA and that makes me feel better as higher ORP is a "cleaner" water state, I still cannot use the method of controlling the SWG to reach an ORP setpoint. I still have to use time based SWG operation (currently 12 minutes on 5 off) to keep an appropriate FC level.
The next experiment I'm contemplating is the addition of boric acid to establish 50-70ppm borate in the water to see if that makes any positive change.
I've seen some people switch to liquid chlorine dispensing for ORP control as it doesn't have the H2 problem. The ORP doesn't plunge when LC is dosed. This makes ORP a much closer proxy to FC.
ORP however is NOT useless in an SWG pool. One a scheme of dosing by time is established, monitoring ORP will tell you if your water sanitation has somehow been changed radically. I manage a remote pool and only physically see it once a week. ORP lets me know that everything is OK during that week.
I assume someday with the advent of economical FC probes, manufacturers will eventually make a residential system that will measure FC and dose to an FC setpoint rather than ORP. I've seen commercial systems like this advertised, some of which actually dose liquid chlorine or SWG to maintain an FC setpoint and a non-chlorine oxidizer like MPS to an ORP setpoint. That would be some cool tech to have on a pool.
Point of reference - my water is currently 4000ppm salt, 7.62pH, 50 TA, 500 CH. My 14:00 FC is about 2.8-3.2 PPM.
Great information, thanks for sharing your experience! This is my first saline system, there is no issue with Chlorine in pool but the I always have many issues with hot tub as it’s smaller, I had to change the hot tub from Chlorine PPM probe to ORP, there was an issue circulation system and not sufficient flow in sample jar, so I had to change it to ORP.
Could you please advise how H2 is being generated in SWG, how does H2 affect Cl2 and what happens to it?
I have worked at many water & steam cycle plants, ORP is most common factor for measuring oxidation and corrosion and I have worked with Free Chlorine meter as well. Its probe has a copper blade.
There's a Wikipedia page on the chemistry of SWG here that shows how the H2 is generated. Its created by the reaction of the sodium ions created by electrolysis of the salt water with H2O. The info I have read on the effect of H2 on ORP comes mainly from here. That's really the sum of my understanding. I don't see where the H2 affects the Cl2. The H2 just lowers the ORP of the water.
With your background it sounds like you probably know more that I do! I'm just an amateur trying to understand the complex chemistry in my pool.
With your background it sounds like you probably know more that I do! I'm just an amateur trying to understand the complex chemistry in my pool.
