LSI, CSI, APPs, Watergram…

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C

CFB

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Jan 7, 2018
13
Victoria
I am new to the forum and enjoy reading the useful information. I mostly keep (or at least try to!) my pool water within the ranges given on the forum.

My pool is SWG, values are (from Taylor's Kit):
pH 7.6
CH 230
ALK (total) 70
CYA 45 [I keep this less than 50 ppm because I have an ORP probe]
Temp 70
Salt 3870

Using the PoolMath APP and the above values I get CSI +0.18.

Using the Taylor Watergram I get LSI -0.18. I understand that the Watergram does not take into account salt. The standard constant in the LSI equation goes from 12.10 to 12.41 to consider salt (using the relevant Table) so the correction seems to be -0.3 (negative because the factor is subtracted). This makes LSI even more negative at a value around -0.48.

Using the Orenda APP (available online, takes into account salt and CYA) the same values above give LSI -0.13.

These differences appear to be quite large (assuming I have calculated correctly and inserted values correctly in the APPs/equations) and so it would be good to understand what is different, if this is indeed the case.
 
CSI and LSI are calculated differently. Honestly, CSI is the better value to use because it uses actual thermodynamic models of calcium carbonate saturation/precipitation while the LSI was derived from a more phenomenological basis with correction factors to make the data fit. There are historical reasons why the LSI is widely used and, sadly, it’s mostly due to technical laziness on the part of the pool industry. Pool water is not the same as boiler water or cooling tower water but, for the purposes of saturation calculations, the pool industry doesn’t care to recognize those “subtle” differences. However, both saturation indices should agree on the degree of scaling or etching. The Taylor Watergram is just the LSI with no ionic strength correction factor. Not sure what Orenda is using but it is very likely the LSI.
 
Thank you JoyfulNoise. I have now crunched the numbers and all is clear. The link was helpful!

My other question is on ORP. It is related to CSI, in that for a CSI near zero my ORP is low.

Here goes...to stop pH rise and to balance the CSI I maintain my pH at 7.8. However, the ORP at 7.8 is around 500 mV, well under the min 650 mV recommended. When the pH was at 7.3 the ORP would be around 625 mV but at this pH I had the zig-zag cycle of acid addition and bicarb (buffer) addition.

ORP does decrease with increasing pH, as noted on other posts in the forum and other literature. So this is not surprising.

Given that my pool is now rather stable wrt pH, ALK etc and the CSI is good, my thoughts are turning to why the ORP is low.

I have tested an independent ORP probe and my pool probe is accurate (or both are off-unlikely in my view).

So the other possibility is high organic content. My combined chlorine is essentially zero, but I am not sure if this means organic content is low.

Question: what is recommended to test organic content and more importantly to remove it? Shocking with chlorine apparently removes some but not all organics, even forming other unknown substances, while MPS also does not remove all organics.

I have read that enzymes can be added to remove organic content. Comments, advice, ideas?
 
In my opinion, so take it for what it’s worth, ORP is useless. As you can see from all of your testing, the ORP signal is sensitive to lots of different variables (even the presence of sunlight affects the signal). However, there is only one important parameter that matters for water sanitation and oxidation - free chlorine (FC) level (and more precisely, hypochlorous acid concentration). Chlorine is the primary sanitizer and oxidizer.

So the question that needs answering is this - in both the short term (24 hour period) and long term (months), how does the FC level correlate to the ORP reading? While the basic epidemiological literature talks about specific ORP levels being a indicator of sanitary water (pathogen kill rates) in highly controlled lab environments, most manufacturers of these dosing systems try to extend those concepts to something that is not controlled - pool water. With all the external factors that can affect the various chemical levels in pool water, it’s not at all unreasonable to see probe readings that drift around and correlate poorly to actual FC levels.

One specific issue that is an Achilles heel for ORP is the presence of the chlorine stabilizer cyanuric acid. As you have already stated, you must keep your CYA below 50ppm in order for the ORP probe to work correctly. In a lot of pools, that amount of CYA can be too low to protect the chlorine against high rates of UV photolysis. So, in order to make an ORP system work, one needs to use more chlorine than is necessary. CYA will also foul the probe membrane (as do other organic compounds) and so one must periodically clean the probe to keep it working correctly.

So why bother with ORP? It’s a proxy value that requires additional work to keep stable and tells you no more information that what one wouldn’t get from simply measuring FC each time and dosing appropriately. As an automation control point, it’s not very helpful because it’s too sensitive to drift and too influenced by other parameters unless one keeps the water in a condition that is harsh to bathers (high sanitizer levels and low stabilizer levels).

As for organic load, yes, there are dissolved substances in the water that can shift your ORP signal. The “standard” method of analysis would be something called a “total organic carbon” (TOC) Test. Unfortunately, that’s not something that can be easily measured pool side and typically requires a lab analysis. Combined chlorine is only an indicator of nitrogenous waste and the test itself is really only sensitive to a few types of combined chlorine. Chlorine can oxidize some simple organic compounds fully to inorganic carbon and nitrogen but, more often than not, chlorine creates organic chloramine, trihalomethanes (THMs), and disinfection by-products (DBPs). These are persistent pollutants until they outgas from the water or UV light from the sun slowly degrades them further.

I very much doubt the efficacy of any enzyme products made for pools as enzymes are substances that are easily inactivated by chlorine or UV light and they usually require very narrow ranges of pH in order to work. Enzymes usually require high levels of dissolved oxygen to work effectively and pool water is often oxygen deficient. So, in all likelihood, those products would do very little for you and add to the organic load in your pool.
 
Thanks JoyfulNoise.

Would you know why the recommended level for CH for a pool with SWG is 350-450 ppm or so, which is about 100 ppm higher than a pool with no SWG? (Numbers from Pool School)

Also, CYA recommended levels are 70-80 approx. but I have an ORP meter which is not so helpful as a guide under TFP recommended levels. I keep my CYA under 50, as earlier posts, but I am thinking to go higher to 70-80? Is it that at these higher CYA levels the SWG operates more efficiently?
 
JoyfulNoise said:
In my opinion, so take it for what it’s worth, ORP is useless. As you can see from all of your testing, the ORP signal is sensitive to lots of different variables (even the presence of sunlight affects the signal). However, there is only one important parameter that matters for water sanitation and oxidation - free chlorine (FC) level (and more precisely, hypochlorous acid concentration). Chlorine is the primary sanitizer and oxidizer.

One specific issue that is an Achilles heel for ORP is the presence of the chlorine stabilizer cyanuric acid. As you have already stated, you must keep your CYA below 50ppm in order for the ORP probe to work correctly. In a lot of pools, that amount of CYA can be too low to protect the chlorine against high rates of UV photolysis. So, in order to make an ORP system work, one needs to use more chlorine than is necessary. CYA will also foul the probe membrane (as do other organic compounds) and so one must periodically clean the probe to keep it working correctly.
Click to expand...

Total Alkalinity can also affect ORP numbers, fortunately CYA levels are fairly stable over the short term, but TA isn't, high TA causes the pH to rise, and if you have a pH probe, which you really need for machines that calculate sanitizer levels from ORP, if it's automatically feeding acid to lower the pH, which also lowers TA, which affects the ORP number. Making ORP essentially a moving target...
 

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CFB said:
Thanks JoyfulNoise.

Would you know why the recommended level for CH for a pool with SWG is 350-450 ppm or so, which is about 100 ppm higher than a pool with no SWG? (Numbers from Pool School)

Also, CYA recommended levels are 70-80 approx. but I have an ORP meter which is not so helpful as a guide under TFP recommended levels. I keep my CYA under 50, as earlier posts, but I am thinking to go higher to 70-80? Is it that at these higher CYA levels the SWG operates more efficiently?
Click to expand...

There are two chemical balances that need to occur in pool water - sanitizer balance and saturation balance. The FC/CYA ratio takes care of the proper balance of sanitizer to stabilizer. It is based on the science of cyanuric acid buffering of chlorine and the sanitation levels needed to inactivate pathogens. Saturation balance deals with the concentration of scale-forming minerals dissolved in the pool water and the ability of pool water to absorb or emit those minerals. Calcium is the most-likely mineral to scale out of pool water.

The Recommended Chemical Levels in Pool School attempt to achieve the correct balance of the various components so that water is sanitary, clear and safe for pool surfaces and equipment. When setting the various target levels, TFP uses the calcite saturation index (CSI) to achieve the correct saturation balance. Since it’s easier to talk in ranges rather than exact targets, the ranges of CH are different for SWG pools because those pools need lower TA and higher CYA than manually chlorinated pools. If the CH levels were not adjusted higher, than the CSI could potentially fall out of acceptable limits.

There’s a great deal of variability from pool to pool in what works best. What works in my poolis not necessarily easy for someone else to achieve. So simply find the range of values that works best in your pool as long as you stay within sanitary limits and the saturation index is within bounds.

I would recommend NOT letting the ORP system control chlorine output. It will either fail to keep the sanitizer at appropriate levels OR it will cause your SWG to run constantly. They are really not well-suited for outdoor, residential pools. With an outdoor pool that gets a lot of sun and uses an SWG to add chlorine, you need the higher CYA levels to reduce the loss of chlorine from UV photolysis. Running at lower CYA levels means you’d have to run the SWG at higher output, which shortens its usable life, and you’ll have to run it longer which wastes energy from the pump being on.
 
Great, thank you-that is very clear.

Regarding ORP, my pool sits at around 500 mV with all else stable and a CSI of close to zero. Yes, I realize ORP is a moving target and depends on many variables, and trying to get it above 650 mV would require altering pool conditions (eg, lower pH, higher TA). For my pool a higher ORP means conditions that are not constant (eg, rising pH, use of lots of acid, must use low CYA). So, I use CSI as the overriding guide.

Notwithstanding the above, scientific studies clearly show that water with 650 mV or above kills pathogens/bacteria in seconds whereas water with around 500 mV the time extends to tens of minutes. So what’s the potential issue?

Is this increased time at lower ORP to kill “nasties” really a problem for swimmers in water? It is something that I have been thinking about for some time.
 
The CDC sets rules for pathogen kill times that are designed to yield a maximum protective effect. The CDC is concerned about public/commercial pools where disease transmission among bathers needs to be minimized.

As far as those studies go, yes, water with an ORP above 650mV is sanitizing BUT the water they use and the water in your pool are two very different things. Many of those studies never take into account the buffering effects of CYA on chlorine and often just look at free chlorine (FC) levels when doing their analyses. Hypochlorous acid (HOCl) is the active, sanitizing species in water and one that should be measured, not FC.

See Post #4 in this thread by Richard for lots of good information -

Pool Water Chemistry
 
Again, very helpful, thanks.

So, just a couple of things:

1. Why is CYA of 60 not recommended for a SWG pool?

2. In Pool School it also notes that there are advantages to running the SWG during the day when sun is shining on the pool. Do you know the advantages? I cannot seem to find them.
 
CFB said:
Again, very helpful, thanks.

So, just a couple of things:

1. Why is CYA of 60 not recommended for a SWG pool?
Click to expand...

It has generally been found by most SWG users that a higher CYA level allows more efficient use of their SWGs (shorter run times). One must still use the correct FC/CYA ratio (around 5% for SWG pools). SWGs add chlorine very slowly to the pool water (usually not much higher than a few hundred ppb per hour) and so higher CYA levels lower the FC loss rate (from UV photolysis) and allow the SWG to function properly. In my own pool, I typically see my FC drop faster and my SWG % output has to go higher as soon as my CYA drops below 70ppm. I typically keep my CYA up at 90ppm and my FC loss rate isn’t much more than 1.6ppm per day. That rate is low enough to allow my SWG to run at lower output and for less time this saving its useful life (SWGs are typically only rated at less than 8,000 to 10,000 hours of use before their coating fails).

CFB said:
2. In Pool School it also notes that there are advantages to running the SWG during the day when sun is shining on the pool. Do you know the advantages? I cannot seem to find them.
Click to expand...

I’m not entirely sure why it says that as I dont really see where the advantage is (I did not write that article so I have no idea what the author was thinking). I run my system during the day as I like to keep an eye on its operation. I know some people generate chlorine at night to avoid FC loss from UV and because utility rates are typically cheaper in the overnight hours. I think when you run the pool is up to you and your personal preference.
 
I have been looking at the FC=7.5% CYA rule. If we take 30 ppm and 50 ppm CYA we get 2.25 ppm FC and 3.75 ppm. I understand these are minimum levels.

But other posts suggest that HOCl = 0.05 ppm is what is required to prevent algae formation. This corresponds, from various posts and articles (eg, G Birch, B Lowry), roughly to FC 4 ppm (30 ppm CYA) and 6 ppm (50 ppm CYA).

Which should one adhere to? The 7.5% rule or 0.05 ppm HOCl? Comments, advice?
 
The 7.5% rule is a compromise based on the science and what has been observed in actual pools. 50ppb (0.05ppm) HOCl is what has been established as a minimum level at which chlorine can kill certain types of algae and marine plankton. However, there are other types of algae (yellow "mustard" algae, for example) which are much harder to kill. Also, as far as sanitation goes, eliminating pathogenic disease transmission (bacteria and virus') is much more critical. Pathogenic control is achieved at HOCl levels closer to 10ppb (0.01ppm) which also corresponds to an ORP level near the minimum allowable value by CDC studies (about 650mV). One then has to balance sanitation against the extinction rate of chlorine by UV photolysis which increases as HOCl concentration increases. If one were to simply use a higher percentage as a routine dosing amount, then there's a lot of chlorine that can go wasted to UV losses. So setting FC levels lower by using 7.5% of the CYA concentration pretty much does the trick for 99.9% of the pools out there and doesn't waste large amounts of chlorine.

As I have been known to say on occasion, this isn't rocket-fuel you're mixing up, it's pool water. It's best to not be sacrificing the good-enough to the elusive perfect....
 

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