Safe levels for VERY high CYA?

[Desterline]

Split by moderator from HERE. Please start your own thread to ask your own questions. Thanks, jblizzle

We say it is safe if the FC is greater than the minimum and less than the shock FC level as listed in the FC/CYA Chart. AND you can see the bottom of the pool.

Kind of diverging from the original question, but help me understand this. Does this hold true if I have insanely high CYA levels? I'm limping along and delaying the refill,but with a CYA of 500, PoolMath says I should shock at nearly 200PPM of FC. So "safe to swim at shock level"???

As to the original question, I believe those levels are fine for swimming and wait about 1/2 hour after adding chemicals -with the pump running- before swimming.

 

[jblizzle]

Re: wait time after adding chemicals -

I am not sure if our recommendations extrapolate that far out of the normal ranges.

 

[timerguy]

You realize to slam the first time it will take 62 bottle of 8.25% Bleach? Why are you still delaying getting rid the insanely high CYA? It's been over a month since you first found your cya to be high.

 

[Desterline]

Split by moderator from HERE. Please start your own thread to ask your own questions. Thanks, jblizzle

Fair enough.

I am not sure if our recommendations extrapolate that far out of the normal ranges.

Yep, the "not sure" part is why we ask questions.

You realize to slam the first time it will take 62 bottle of 8.25% Bleach? Why are you still delaying getting rid the insanely high CYA? It's been over a month since you first found your cya to be high.

Well, mostly because at the moment I'm suffering a pretty severe case of sparklypoolitis and I'm busy with it's complications. You know, swimming, grilling out, enjoying a drink. It's tragic but I think I can suffer through.


I'm delaying as much as practical. Dry plaster and 100+ degrees doesn't seem to go so well together. Plus when I drain it, there's a whole snowball of other work I need to get into.

I've been holding FC at about 10-15. Through splashout, backflush and a couple heavy rains, my CYA has come down to about 350. (still way too darn high)

 

[Desterline]

So, any comment on my original question?
Often stated, "it's safe to swim with FC levels at shock level". Since shock level is a function of CYA level, where's the real limit?

 

[jblizzle]

I am going to move this to Chem 201, where Richard might see it and respond.

 

[chem geek]

If you are now at 350 ppm CYA, then 7.5% of that for the minimum FC to prevent algae growth would be 26 ppm. I'm not sure why you are asking about shock levels since it seems that your pool is doing OK without shocking. At high FC with high CYA the chlorine bound to CYA starts to become more of a factor in disinfection and oxidation, but we don't have good data as to algae inhibition. Your 15 ppm FC bound to CYA may be OK as a minimum, but having 26 ppm wouldn't be too much. The shock level equivalent to a normal 40% FC/CYA ratio might be only 50 ppm FC because of this effect (based on 50/150 + 50/350 = 0.3 + 16/150), but we really don't know.

It's hard to get an accurate pH reading with a high FC which is one of several reasons why we recommend getting the CYA lower. You are in uncharted territory.

 

[Desterline]

Thanks for the response. As is often the case when I learn something new, I come away with more questions than answers...
My pool seems to be sparkly at the moment. It's a puzzle as to why it's not a swamp, I like puzzles. I find myself surprised most days when I walk out and it's not a swamp. I know that day is coming. When it does, the solution is going to be the same as it is now - drain and refill. It's a cost-benefit balance. As far as I can see at the moment, the downside is higher chlorine usage (currently about 1/2 gallon of 10% per day). Feel free to point it out if I'm missing a more significant downside.

The maximum safe chlorine level question still stands. Since I got my head around the FC/CYA relationship, I've found the standard "it's safe to swim at shock level" to be intellectually unsatisfying. Even here on TFP, there's at least two different versions of the chart - one includes mustard shock and extends to CYA levels of 120, the other stops at 100 and doesn't include mustard. So even "per the FC/CYA chart" doesn't really lead to a definitive answer.

For practical purposes, it would seem the answer is about 40ppm (regular shock at CYA of 100). I think pursuing an answer beyond that is only academic.

 

[tim5055]

The maximum safe chlorine level question still stands. Since I got my head around the FC/CYA relationship, I've found the standard "it's safe to swim at shock level" to be intellectually unsatisfying. Even here on TFP, there's at least two different versions of the chart - one includes mustard shock and extends to CYA levels of 120, the other stops at 100 and doesn't include mustard. So even "per the FC/CYA chart" doesn't really lead to a definitive answer.

Both charts were created by Richard (chem geek) who answered above. As he said, "You are in uncharted territory". The differences in the two are really more about making the one in the Pool School a little easier to read by rounding the numbers.

Even Ben Powell's "Ben's Best Guess* Guide to Swimming Pool Chlorine" on which a lot of this is based only goes up to CYA 200.

I would say you seem to be handling te high CYA fine, just keep doing what you are doing. maybe bring the Min/Target up a tad and as keep having pool parties. Lots of cannonballs and other activity will increase that splashout and continue to get the number down

 

[RobbieH]

Ignore mustard shock level when you are determining whether it is safe to swim. The standard shock level is the one that counts.

 

[chem geek]

It's a puzzle as to why it's not a swamp, I like puzzles. I find myself surprised most days when I walk out and it's not a swamp. I know that day is coming. When it does, the solution is going to be the same as it is now - drain and refill. It's a cost-benefit balance. As far as I can see at the moment, the downside is higher chlorine usage (currently about 1/2 gallon of 10% per day). Feel free to point it out if I'm missing a more significant downside.

The maximum safe chlorine level question still stands. Since I got my head around the FC/CYA relationship, I've found the standard "it's safe to swim at shock level" to be intellectually unsatisfying.

It's not a mystery. Your pool is probably relatively poor in algae nutrients (phosphates or nitrates, most likely phosphates) and that slows down algae growth so that lower active chlorine levels (FC/CYA ratios) can still kill the algae. Your 1/2 gallon of 10% per day in 25,000 gallons is only 2 ppm FC per day which is about typical, but since your FC/CYA ratio is lower than normal perhaps this daily chlorine usage is higher than expected because there may be slow nascent algae growth occurring that isn't yet visible.

As for "safe to swim" it's not like there is a single magical cutoff above which there are serious problems and below which everything is fine. It's a continuum. The normal shock level of an FC/CYA ratio of 40% is equivalent in active chlorine level to around 0.6 ppm FC with no CYA so below that of many commercial/public pools especially indoors that have at least 1 ppm FC with no CYA. Even accounting for the higher FC of 32 ppm at 80 ppm CYA and accounting for both a higher water temperature of 88ºF and the chlorine bound to CYA then this is equivalent to no more than 1.6 ppm FC with no CYA.

 

[Desterline]

It's not a mystery. Your pool is probably relatively poor in algae nutrients (phosphates or nitrates, most likely phosphates) and that slows down algae growth so that lower active chlorine levels (FC/CYA ratios) can still kill the algae. Your 1/2 gallon of 10% per day in 25,000 gallons is only 2 ppm FC per day which is about typical, but since your FC/CYA ratio is lower than normal perhaps this daily chlorine usage is higher than expected because there may be slow nascent algae growth occurring that isn't yet visible.

I should update my sig line - Based on the last month or so of chlorine additions with before and after readings, I currently believe my pool is closer to 20k gallons.
As to the levels of phosphates/nitrates. I don't know. I don't have any numbers to back it up, but, subjectively, I don't think so. I have a bunch of landscaping too close to the water (picture earlier in this thread) and as a result I spend about 10 minutes a day netting leaves and sticks and flower blossoms out of the pool.

I've only had the house for about six weeks at this point. Initially I suspected the previous caretaker may have dumped it full of some algaecide and/or phosphate remover. Now I'm starting to wonder. I would think that six weeks would be enough time for any of that to be breaking down.

I've seen algae twice now. About three weeks ago I found a bit of green under the o-ring in my leaf catcher. A few days later I found a small patch (about 6") of light green slime in the corner of the stairs - definitely a low circulation area. A little brush work and adjusted a return eye for better circulation. Haven't seen any since.

As for "safe to swim" it's not like there is a single magical cutoff above which there are serious problems and below which everything is fine. It's a continuum. The normal shock level of an FC/CYA ratio of 40% is equivalent in active chlorine level to around 0.6 ppm FC with no CYA so below that of many commercial/public pools especially indoors that have at least 1 ppm FC with no CYA. Even accounting for the higher FC of 32 ppm at 80 ppm CYA and accounting for both a higher water temperature of 88ºF and the chlorine bound to CYA then this is equivalent to no more than 1.6 ppm FC with no CYA.

Now _that_ makes more sense (to me at least). I'm sure I'm missing some of the correct terminology here... And an equation... but the CYA is binding the chlorine and rendering it, well, irrelevant to the "safe" question. Hmmm, I'm struggling pretty hard with terms here, but I think that makes a bit more sense to me about sanitation and testing too. Can you link me to a more mathematical description of this so I can get my vocabulary up to speed?

Thanks

 

[chem geek]

If phosphates were removed or low, they do not increase again unless added to the pool water. You are right that regular algaecides such as linear quats and Polyquat break down from chlorine (though copper ions do not).

See the "Chlorine/CYA Relationship" section in the first post of the thread Certified Pool Operator (CPO) training -- What is not taught. You can see a diagram of the 10 chemical species of CYA and chlorine bound to CYA in the 1974 O'Brien paper.

Based on another paper measuring the rate of oxidiation of monochlorodimedone by chlorine (also linked to in that post) the chlorine bound to CYA is no more than 1/150th as reactive as hypochlorous acid so I used that in my estimates.