HOCI Calculations

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7Iron

7Iron

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Jun 14, 2014
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Good morning,

If I have grasped the CYA/FC relationship notion:
1..that as CYA increases, there is less HOCl available/free/unbound for use as a sanitizer and/or algae controller,
2. and that HOCl is really what I am trying to maintain by adjusting FC,
3. and the HOCl level decreases (non linearly) as pH rises,
4. and that the available/free/unbound (not sure how it should be labeled) HOCl should be at least above .011 for sanitation and maybe ~.05 to prevent most algae.

When I became a TFP member/believer I began a spreadsheet containing all my test results and what I have added to my pool.

I found the following formula: .31*FC / (CYA-(1.8*FC)), but I think that it only approximates the actual HOCl. And it doesn't include any pH level impact. Is there a practical use formula that I can build into my spreadsheet??..I am hoping that it isn't an iterative calculatio.

Thanks
 
JasonLion said:
4) 0.07 to 0.11 is the level we recommend, or you could call that FC should be at least 7.5% of the CYA level.
Click to expand...

Stick with me JasonLion, I think I am getting close, but your 4th answer exploded my head.... If my pool is at 100ppm?? CYA that would be 7.5 FC using the CYA*7.5% rule of thumb recommendation and that foots with the 7 on the Chlorine/CYA chart...got that!!

But if you had 100ppm CYA and 7.5FC the HOCl would only be .031 using the formula [7.5/((2.7*100)-(4.9*7.5)+5)=.031] to the page where you sent me "see here for lots of detail" and then comparing it to the HOCI chart that is also provided the .031 foots between the 5FC and 10FC values... given the pH is 7.5....Mine is 7.2 and I think that would allow for .036 HOCl.....
HOCI.jpg
If I was at 60ppm CYA and achieved .11 HOCl my FC would be close to 13...if I was at 100ppm CYA to achieve .11 FC would have to be close to 21. .What dots have I connected incorrectly or not connected at all?
 
I think I made a mistake. It looks like I was low by a factor of 3. Try HOCl minimum 0.031 and shock level at 0.3.

I never use HOCl units. It is much easier to work with percentages of the CYA level of FC (assuming at least a little CYA in the water).

If you are trying to recreate the tables, there are other factors you need to keep track of. For example at very low CYA levels you tend to run out of chlorine too quickly. The chlorine is effective but then gets used up almost instantly so overall is ineffective. So you have to use certain minimum amounts of chlorine regardless of the CYA level.
 
There is already the Pool Equations spreadsheet, but you shouldn't need to make a spreadsheet for this. Just use the FC/CYA chart and ignore the effect of pH because with CYA it's not a huge effect. The FC/CYA rule is pretty simple as shown in more detail in the Chlorine/CYA Chart by Chemgeek where the values in the table are summarized with the following rough formulas (I've added FC equivalent with no CYA and HOCl levels):

NOTE: A reasonable approximation to the above table is the following:
.... "Min FC" is 7.5% of the CYA level (FC with no CYA equiv.: 0.06 ppm; HOCl: 0.03 ppm)
.... "Target FC" is 11.5% of the CYA level (FC with no CYA equiv.: 0.10 ppm; HOCl: 0.05 ppm)
.... "Yel/Mstrd Min" is 15% of the CYA level (FC with no CYA equiv.: 0.14 ppm; HOCl: 0.07 ppm)
.... "Shock FC" is 40% of the CYA level (FC with no CYA equiv.: 0.60 ppm; HOCl: 0.30 ppm)
.... "Yel/MstrdShock" is 60% of the CYA level. (FC with no CYA equiv.: 1.4 ppm; HOCl: 0.7 ppm)
Since chlorine is more effective at lower pH, it is normally recommended to lower the pH before shocking at a high FC level, especially for yellow/mustard algae where lowering the pH to 7.2 before shocking at the above levels is best (and remember that the pH test will not be valid during shocking due to high FC levels).
Click to expand...

and for saltwater chlorine generator pools, the rough rule is an FC that is 4.5% of the CYA level (FC with no CYA equiv.: 0.04 ppm; HOCl: 0.02 ppm)
 
Thanks guys,

And when I become confident with my CYA level, I will be right there with you...(no reason to make this hard/difficult). But there is thinking in my madness....I am trying to "hedge my bet" between FC levels at 60ppm or 100ppm CYA and a concern with high chlorine levels when my granddaughters (6 months and 2 years old) are using the pool.

If my calcs are accurate, for ~2days a week I am only going to maintain FC at 3 (2.9 [email protected])....that will give me sanitation (kill the bugs) at the 100ppm CYA and not concern me wrt high FC. And for the other 5 or 6 days, go to the other extreme and kill the algae and all known plant life in and around my pool:wink: at the recommend 12 FC

After only a few days of intense reading, I am not sure wrt to my math/calc, still not sure what is considered unsafe chlorine levels, but I do buy into that if there is such a level it is a function of CYA...

The only "good" article I have found on what is "too high" was the Canadian's National Collaborating Centre for Environmental Health article titled "POOL CHLORINATION AND CLOSURE GUIDELINES" Their conclusion,

"while there is no specific evidence to support the 10 ppm level used as an upper limit for pool closures based on health effects through ingestion, inhalation, and dermal exposure, animal studies on eye irritation do provide some evidence for the 10 ppm FAC limit"

And notice they did not give the answer...they just gave what isn't the answer


(I would link but I am unclear wrt to "hotlinking" in the forum rules)

Again thanks to both of you, for your insight, help and support!!!
 
Last edited:
FC at 10 with zero CYA is way way way too high and will result in hair loss, among other symptoms.

Your proposed approach won't work for a couple of reasons. The most obvious being that if you set FC to 12 with CYA at 100 it won't come down to anything near as low as 3 in just a week unless something is seriously wrong. You will also have significant problems measuring the CYA level. And should you actually get algae you will very much regret having CYA that high. I really strongly recommend trying anything remotely related to what you are proposing.
 
Ok so my madness is more madness than solid thinking.......I do not ignore advice from experts..(except beginning the 7th of June, when I found TFP, I now ignore pool store experts)

I already have significant problems measuring my CYA (see several posts/threads from me)....

I am confident that my CYA is between 50 and 100ppm....From memory and using your pool calculator I can account for at least 30ppm being added during the past month. I just began keeping records of what I was adding 14 Jun.

I have a cartridge filter, heavy (3-6" ) frequent rain that I drain and I replaced about 1/3 pools water after my daughter's dog fell in and ripped the vinyl two months ago.

I just started using the FAS-DPD test method daily to determine FC when my K2006 arrived 20 Jun...since then I have targeted 6ppm (afternoon) and it has dropped as low as 3.8 (AM reading) after evening thunderstorms. I usually add 2.5 to 3 cups of 8.25% bleach in the morning and I keep a diamond clear cover on the pool to reduce heat loss. Don't know if that also buffers the UV caused chlorine loss...

I cant get my pH up (it is usually 7.1-7.2 at the AM test time with base demand testing to 7.6 ranging from 4 to 12 drops) the 1/2 box of 20 team that I have added since 20 Jun, is marginally keeping up with rain..

The pool store CYA readings ranged from 50 to 120. With most of the recent at the 100...I get 90-100 using the K2006 and 60 using the TF kit.

So what is your input on maintaining the 6ppm until I get so frustrated that I totally drain the pool or get confidence in the CYA test readings and what do I do (if anything) with the pH

Last weekend's full test panel results:

FC 7 (Total 7.2 or less-- it took one drop)(this AM 4.5 but I added 3 cups 8.25% bleach to take to 7 and then I expect 6ish this afternoon at use time)
CH 90
CYA 60 or 90-100
TA 150
pH 7.5 (this AM 7.4 with 5 drops of base demand to 7.6)
Temp 82F
Cu not tested (but last time tested 7 Jun it was at 1)
Borate not tested
TDS not tested

I haven't performed an overnight test in 10 days because of our afternoon/evening rain....We get more rain than Seattle year over year
 
What are you using to add chlorine? An acidic chlorine source is the only thing I can think of that would be driving the PH down.

Your chorine usage suggests CYA is at least slightly above 50 but definitely below 100. But that is assuming the cover is left off. With the cover on the CYA would presumably have to be noticeably lower to account for that FC loss.
 

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I am thinking that rain is the culprit bring my pH down... The bleach is the Walmart generic brand (Great Value) of 8.25% sodium hypochlorite.

So does the cover provide chlorine buffering from the sun?
 
7Iron said:
I am trying to "hedge my bet" between FC levels at 60ppm or 100ppm CYA and a concern with high chlorine levels when my granddaughters (6 months and 2 years old) are using the pool.
:
After only a few days of intense reading, I am not sure wrt to my math/calc, still not sure what is considered unsafe chlorine levels, but I do buy into that if there is such a level it is a function of CYA...

The only "good" article I have found on what is "too high" was the Canadian's National Collaborating Centre for Environmental Health article titled "POOL CHLORINATION AND CLOSURE GUIDELINES" Their conclusion,

"while there is no specific evidence to support the 10 ppm level used as an upper limit for pool closures based on health effects through ingestion, inhalation, and dermal exposure, animal studies on eye irritation do provide some evidence for the 10 ppm FAC limit"

And notice they did not give the answer...they just gave what isn't the answer


(I would link but I am unclear wrt to "hotlinking" in the forum rules)
Click to expand...
As Jason noted, you can't look at these FC guidelines that don't consider the HUGE moderating effect of CYA. You also need to maintain a minimum FC that is at least 7.5% of the CYA level if you want to prevent green and black algae growth regardless of algae nutrient (phosphate and nitrate) level in your pool.

So let's assume worst-case that the CYA level is 100 ppm (I doubt it's that high as I would trust even your varying CYA numbers much more than the pool store -- so it's much more likely to be in the 30-50 ppm range), and say your FC level is kept at 8 to 10 ppm FC. 10 ppm FC with 100 ppm CYA has an active chlorine level equivalent to 0.089 ppm FC with no CYA. At a pH of 7.5, 0.043 ppm is hypochlorous acid, 0.046 ppm is hypochlorite ion, and the rest 9.91 ppm is chlorine bound to CYA that is reactively equivalent to 1/150th that amount (see this paper) of 0.066 ppm hypochlorous acid, but only for oxidation, not for disinfection. So total reactivity is around 0.1 ppm which is very low and equivalent to a pool with only 0.2 ppm FC with no CYA. CYA has virtually no skin absorption (see this paper) so chlorine bound to CYA likely has none as well. At the other extreme of 10 ppm FC with 30 ppm CYA, the active chlorine level is equivalent worst-case to 0.44+0.06 = 0.5 ppm FC with no CYA.

If I were you, I'd target your FC level as if your CYA level were 50 ppm, not 100 ppm.

As for links to other sites, we encourage that so we can see original sources. The only rules against links are to those that are essentially advertising or selling which is not allowed on this forum and of course links to harmful (e.g. malware) or inappropriate (e.g. porn) sites are not allowed. Links to common products with low prices (e.g. bleach or chlorinating liquid) from members not affiliated with such companies are OK.
 
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