FC and ORP... who wins?

[kpischel]

Hi all... I have an easytouch automation system with intellichem. A SWCG is attached. I've been balancing my pool as replastered late last year and just got the SWCG up an running last week... I've been following the prescribed TFP chemical levels for a SWCG pool... so CYA is around 70. This is causing ORP to be funky... and my free chlorine level is now really high (13; had been 6) as presumably the system appears to be trying to get the ORP higher.

Is ORP key in that it measures the ability to oxidize... Does it trump the FC level? My target FC is 6 (is this measure affected by CYA levels?)... If FC is key I could tweak ORP to drive FC @ 6... If ORP is key, I'd need do something else... like reduce CYA to 30-40 or bypass the intellichem for the SWCG and just let the SWCG drive FC to 6 based on run time and %.

The intellichem holds the ph spot on... nice not having to monkey with that.

Any thoughts greatly appreciated!

 

[ping]

Here is a previous thread that explains your situation and your options, Why are CYA levels lower when using ORP?.

 

[kpischel]

Here is a previous thread that explains your situation and your options, Why are CYA levels lower when using ORP?.

Thanks. I reviewed your note and the related link... my preference is to disengage the ORP control of the intellichlor (SWCG) leaving intact the automated acid feed. I could then just set the chlorine generation to whatever generates what I need (5-6) based on run time and %. I think I've read in other threads where this can be done with pentair automation... that said, just got off the line with pentair and they say it can't be done... So, I'm hoping someone will read this who has pursued the same goal and will be willing to share with me how it was done. Thanks.

 

[kpischel]

It appears what others have done is ignored the ORP reading and just adjusted the run time and % to drive adequate FC... Is it possible to find the relative ORP level that corresponds to the correct level of FC and set that as the target or is the ORP relativity simply unreliable?

 

[JoyfulNoise]

This is the problem with ORP control systems and why TFP does NOT recommend them for residential, outdoor pools. The ORP is nothing more than a voltage measurement of the redox potential of the water. Unfortunately, the redox potential is affected or influenced by multiple parameters (pH, TA, etc). It also affected by sunlight exposure as UV light generates powerful hydroxyl radicals in the water which can shift the ORP voltage. You can see this clearly when ORP output is plotted as a function of time - daylight hours show a higher ORP while night time hours show a lower ORP. If that were not bad enough, stabilizer, which is critical for outdoor pool care, fouls the membranes used in ORP probes. You either have to use very low stabilizer levels (and deal with excess chlorine demand from UV extinction) OR you have to clean and recalibrate your ORP probe on a weekly basis. These two reasons alone make ORP effectively useless as a control point for FC generation. The main use of ORP control is in large indoor commercial pools (or water parks) that absolutely need chemical automation systems. In those cases, CYA is not used and is therefore not an issue AND most municipal health departments require that FC/CC and pH be tested several times per day with a drop-based chemical test, in other words, ORP is not allowed to be used as a proxy value for water quality.

You can certainly try to measure your ORP voltage and correlate it with FC measurements from a proper test kit (DPD-FAS test), but I think you will find it to be very scattered. You may find an ORP value that corresponds to the right FC level and then simply use that as a way of keeping tabs on your pool water. I'm not sure what benefit it will really provide as you still have to measure your pool's chemical levels on a routine basis and you could just as easily get a "ball park" readout of your water's health by doing a quick and simple pH & OTO (yellow chlorine indicator) test.

 

[kpischel]

This is the problem with ORP control systems and why TFP does NOT recommend them for residential, outdoor pools. The ORP is nothing more than a voltage measurement of the redox potential of the water. Unfortunately, the redox potential is affected or influenced by multiple parameters (pH, TA, etc). It also affected by sunlight exposure as UV light generates powerful hydroxyl radicals in the water which can shift the ORP voltage. You can see this clearly when ORP output is plotted as a function of time - daylight hours show a higher ORP while night time hours show a lower ORP. If that were not bad enough, stabilizer, which is critical for outdoor pool care, fouls the membranes used in ORP probes. You either have to use very low stabilizer levels (and deal with excess chlorine demand from UV extinction) OR you have to clean and recalibrate your ORP probe on a weekly basis. These two reasons alone make ORP effectively useless as a control point for FC generation. The main use of ORP control is in large indoor commercial pools (or water parks) that absolutely need chemical automation systems. In those cases, CYA is not used and is therefore not an issue AND most municipal health departments require that FC/CC and pH be tested several times per day with a drop-based chemical test, in other words, ORP is not allowed to be used as a proxy value for water quality.

You can certainly try to measure your ORP voltage and correlate it with FC measurements from a proper test kit (DPD-FAS test), but I think you will find it to be very scattered. You may find an ORP value that corresponds to the right FC level and then simply use that as a way of keeping tabs on your pool water. I'm not sure what benefit it will really provide as you still have to measure your pool's chemical levels on a routine basis and you could just as easily get a "ball park" readout of your water's health by doing a quick and simple pH & OTO (yellow chlorine indicator) test.

Thanks for your response... I guess I'm trying to let the system manage the chemistry as much as possible... and hope if I set the target ORP at the end of the day to a corresponding FC level of 6 or so it will trigger the SWCG to recover to the level needed to keep FC @ 6 or so throughout the day... end of day seems to be lowest... maybe I misunderstood your point about how sunlight effects ORP... you said daylight hours have higher ORP while nighttime hours have lower ORP... my experience has been the exact opposite with ORP rising at night and falling precipitously as soon as the sun hits the water...

 

[JoyfulNoise]

I see, the scale on the output is technically wrong. ORP is typically a negative voltage reading (-600mV to -750mV). Hence my statement that its higher (-600mV is larger than -700mV). But I see your unit reports it as a magnitude (absolute value) which is fine too. Either way, you can see the diurnal/nocturnal variation on the signal. That's partly from UV light hitting the water and reacting with the oxidizers in the water.

You can certainly try what you suggest and see how it goes. My gut feeling says your water is either going to be over-chlorinated or under chlorinated and you will be chasing the "ghost in the machine". That being said, I'm happy to be proven wrong.

Honestly speaking, as a fellow SWG owner, I think these ORP control systems are somewhat silly and not at all necessary. Using just a simple duty cycle percentage output, my SWG maintain a perfectly consistent FC output with minimal min-to-max variation every day. I adjust the output maybe 3 or 4 times per year as the seasons change and temperatures go up or down. But in the short term, day-to-day operations, the SWG is completely consistent in FC production and requires little to no attention other than see if it's on or off. This is why I think ORP control is silly - there's is really no need to have hourly control over your FC, it's just not necessary for a low bather load, residential pool.

To each his own....


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006

 

[Patrick_B]

I don't know that I would say TFP is against ORP control, just that we try to make people aware that it can be pretty tricky to nail down consistent FC levels with it. A lot of factors contribute to ORP readings other than Oxidizer levels.