ORP /SWG System, Free Chlorine, zero correlation between them

[HarrisonHRG]

Hi All, Got a very interesting issue here.

My Zodiac EXO IQ LS with Dual link (both PH and ORP controllers) seems to struggle with chlorine levels. Let me give you some examples.

All Free Chlorine or Hardness testing is using a Photometer from a reputable brand (PalinTest)

Test 1:
FC: 0.55PPM
ORP: 790Mv
Alk: 120
Salt PPM: 3950
TC: 0.01
PH: (controlled by Unit) - 7.2

I put setpoint of ORP controller to 800Mv to increase that FC measurement.

Test 2:
FC: 6.55 PPM
ORP:780Mv
All the rest the same

Pool temp consistent 31 Deg.

I have calibrated the OPR Sensor with the 470mv fluid supplied.

The pool is indoors, no CYA ever added, spends 90% of its time with the cover on...


Any ideas on how to fault-find or diagnose this, the ORP sensor tends to be in a world of its own but does tend to change occasionally. What evidence do I need to gather to go back to manufacturer and claim its at fault, could it be something else at fault?

 

[ajw22]

ORP control does not work well with a SWG. We never recommend it.

Best to work with the manufacturer and see what satisfaction you can get.

Manual SWG control setting an output % with manual FC tetsing should not be very burdensome.

 

[HarrisonHRG]

So my issue is that with manual % and FC testing isnt easy for me due to the fluctuations in useage.

Sometimes itll go a week with the cover on and other times itll be used 0600-2100 all day by various people, so i was keen to get the ORP sensor fitted to give some indication as to the available chlorine and hopefully moderate the FC without it going sky high or going to 0…

Strangely, when someone uses the pool, the ORP jumps up to a higher number, its now 840 just 6 hours after turning the cell off with a swimmer in it.

Any mythical reason behind that?

 

[ajw22]

Any mythical reason behind that?

The thing to understand about ORP is that the voltage detected by the probe is affected by every chemical species in the water. Some of those chemical levels change slowly with time while others can change on an hourly or daily basis. Even certain chloramines can affect ORP levels. So it’s not unsurprising to see diurnal changes in ORP. Have you done an OCLT to determine if something isn’t growing in your water? Overnight loss of chlorine should be less than 1ppm and if it is more than that, then you’re battling two problems at once.

The question to answer is this - what does the ORP signal even mean in swimming pool water?

Sure, the ORP correlates to the FC level, but it is not exact. There have been many studies that have shown ORP probes showing a signal level of >750 mV but the solution only has minimal amounts of FC in it. Then other solutions that show ORP signals less than 700mV with high levels of FC. These variations happen between different probe manufacturers and even within the same line of ORP probes. So, in order for you to specifically know what your system is even doing, you have to map out the correlation of FC to ORP signal over a short period of time (how they vary within a given day) as well as the long term trend. Then you might actually have the ability to know with some certainty what your pool water is doing. While it is generally accepted that an water sample with an ORP signal >700mV is considered “sanitizing” that is only true under very carefully controlled circumstances and, in particular, when a strong oxidizer buffer like CYA is not present.

 

[HarrisonHRG]

Thanks for the info, I’ve been closely monitoring the ORP this evening.

Is there a correlation to the aeration of the water and the ORP, no swimmers and cover closed the ORP remains around 770-780 and it doesn’t seem to matter on the FC content.

But had someone swimming in there tonight and during the swim it’s gone from 770-840! SWG turned off too.

I am not loosing more than 1ppm overnight, in fact 1ppm will remain for 3 days roughly, it’s just the ORP doesn’t seem to vary much as I’d expected, and when the salt cell runs it tends to “depress” the reading.
Could it be due to aeration? My pool has no fountains or aerating factors in it at the moment, and that’s the only reason I can think it increases with a swimmer present?

 

[ajw22]

Enjoy diving into the engineering and chemistry of ORP sensors. They are poor technology for residential pools.

 

[HarrisonHRG]

Enjoy diving into the engineering and chemistry of ORP sensors. They are poor technology for residential pools.

Thanks for the information anyhow.

Surprised that a major company supply an ORP Setup for controlling pools if they are truly useless.

Still waiting for the day a reasonable priced consistent FC measuring device is available and can control chlorine output from a SWG! The dream that’ll be.

Thanks for the info, I’ll go into experimentation mode and see what I find works…

 

[Donldson]

Surprised that a major company supply an ORP Setup for controlling pools if they are truly useless.

You are going to find that in the residential pool supply industry there are many products that are not particularly useful but sold (and in fact pushed strongly by salespeople) anyway. Some like UV and Ozone systems take something that's useful in a commercial setting and build it down to a price so low that it barely functions and sell it to pool owners without the bather load to benefit anyway. Still others are outright shams at any level. It's an unfortunate symptom of any product where most people who purchase it but do not fully understand how it works, similar to motor vehicles. It is, however, one reason TFP exists: to educate the pool owner and help them avoid such pitfalls when we can.

 

[HarrisonHRG]

You are going to find that in the residential pool supply industry there are many products that are not particularly useful but sold (and in fact pushed strongly by salespeople) anyway. Some like UV and Ozone systems take something that's useful in a commercial setting and build it down to a price so low that it barely functions and sell it to pool owners without the bather load to benefit anyway. Still others are outright shams at any level. It's an unfortunate symptom of any product where most people who purchase it but do not fully understand how it works, similar to motor vehicles. It is, however, one reason TFP exists: to educate the pool owner and help them avoid such pitfalls when we can.

Thanks for the information,

As per usual I suspect, this was sold by a company, thinking it’s all self sufficient as a non-pool experienced person, I trusted their opinion.

Then once I have problems I started digging and end up coming across information like you all have provided for example.

kicking myself I didn’t do the research originally before forking out ££££’s on equipment…

Hey ho, I’ll do some trial and error and report back if anything mega interesting occurs!