Another FC/CYA question???

[Rangerman]

First - a set of numbers from last evening:

30,000 gallon inground vinyl, 7 hours full sun.

FC 2.4 (before adding evening's dose)
CC .4
Ph 7.4
TA 90
CYA 60 (has been creeping up due to addition of cya - started at 20)

I have been adjusting the level to 6ppm for FC each evening as the cya increases.
According to the calculator I've needed to add about 2.3 bottles (96'ers) of bleach each night to bring it back up.
I have read on these forums some folk only add half bottle or so daily (depends on pool size of course) to the water.
There is no visible algae, only the sun. Something is chewing up the chlorine really fast.
Should I be adjusting something else - because at this rate - it's about $60 a month in bleach alone? I'm not going to change back to the old ways because the water looks better than ever.
Thanks for the assistance.

 

[chem geek]

Going from 6 ppm FC to 2.4 is a lot, even during the day, especially since you've got 60 ppm CYA. It would be a good idea to test the overnight loss of chlorine as that will tell you how much is due to organics or non-visible algae vs. sunlight. I suspect that the sunlight is consuming about 2 ppm and some nascent algae is consuming 1.6 ppm. Initial algae growth is not visible. The water only starts to turn dull and then cloudy and finally green when there is enough algae (in clumps or filaments) to reflect/diffract the light.

If your overnight measurement shows chlorine consumption, then I suggest shocking at a 24 ppm FC level and holding that level until the overnight consumption is low (< 1 ppm FC). Then, at 60 ppm CYA, do not ever let the chlorine get below 4.4 ppm FC (see this chart). Though you are starting with 6 ppm FC, I think the fact that it is getting below the 4.5 ppm FC minimum during part of the day that is allowing for algae to grow and you are right now probably balanced at algae growth vs. killing -- hence seeing chlorine consumption with no visible sign of algae.

Since you have a vinyl pool, remember to very carefully and slowly add the chlorine to the pool. You want it dispersed and adding it too quickly or not in a return jet flow (or skimmer that some do) can have it settle to the bottom of the pool -- especially if it's an above-ground pool with no floor drain (so you can use a brush to mix up the water near the bottom). You can also pre-dilute the chlorine in a bucket of pool water if you want to play it extra safe. Since you have a vinyl pool, if you feel uncomfortable with the 24 ppm FC shock level, then use 15 ppm or 20 ppm instead -- it will just take a bit longer to clear the nascent algae (if that's what this is).

Richard

 

[Rangerman]

Going from 6 ppm FC to 2.4 is a lot, even during the day, especially since you've got 60 ppm CYA. It would be a good idea to test the overnight loss of chlorine as that will tell you how much is due to organics or non-visible algae vs. sunlight. I suspect that the sunlight is consuming about 2 ppm and some nascent algae is consuming 1.6 ppm. Initial algae growth is not visible. The water only starts to turn dull and then cloudy and finally green when there is enough algae (in clumps or filaments) to reflect/diffract the light.

If your overnight measurement shows chlorine consumption, then I suggest shocking at a 24 ppm FC level and holding that level until the overnight consumption is low (< 1 ppm FC). Then, at 60 ppm CYA, do not ever let the chlorine get below 4.4 ppm FC (see this chart). Though you are starting with 6 ppm FC, I think the fact that it is getting below the 4.5 ppm FC minimum during part of the day that is allowing for algae to grow and you are right now probably balanced at algae growth vs. killing -- hence seeing chlorine consumption with no visible sign of algae.

Since you have a vinyl pool, remember to very carefully and slowly add the chlorine to the pool. You want it dispersed and adding it too quickly or not in a return jet flow (or skimmer that some do) can have it settle to the bottom of the pool -- especially if it's an above-ground pool with no floor drain (so you can use a brush to mix up the water near the bottom). You can also pre-dilute the chlorine in a bucket of pool water if you want to play it extra safe. Since you have a vinyl pool, if you feel uncomfortable with the 24 ppm FC shock level, then use 15 ppm or 20 ppm instead -- it will just take a bit longer to clear the nascent algae (if that's what this is).

Richard

I'll get a fresh set of numbers and post here to include the overnight FC consumption.

Thanks for the assistance Richard ...

 

[Rangerman]

chem geek and I have been trading information by PM over the last few days concerning excessive FC drop and CYA levels. Here's the up-date.

TUESDAY -- At 6pm the FC was at 2.6. I added bleach to push it to 6. Checked it at 1030pm and it was registering a 6.
WEDNESDAY MORNING -- At 6am it was still at 6. So - no consumption of FC overnight? Is that possible? As mentioned earlier, I get about 7-8 hours of sun every day. Doesn't seem likely that I would see a 3.4 ppm drop due to sunlight with the CYA at 55.

WEDNESDAY EVENING --
FC 2.8 (3.4ppm drop today in sun)
CC .0
Ph 7.4
TA 80
CYA 60

According to the calculator, it requires another 2 jugs of bleach tonight to bring it back up. This has been the pattern for the last several days. Rarely do I add less than 2 96 oz. bottles of 6% ...

What do you think?

 

[Rangerman]

Well, unless there are some weird errors in measurements, I'm stumped. If you had a drop overnight, then a nascent algae bloom would be a reasonable culprit, but with the chlorine holding at night and yet dropping so much during the day, I can't figure it out unless our ideas about CYA protection from sunlight are off somewhere (but others don't see it so strong -- usually none have more than a loss of half of the FC).

I'd hate for you to try adding more CYA to see if it helped protect the chlorine better because that would require you to have higher FC levels to maintain and if you did every get algae, then the shock levels would be out of this world.

 

[JasonLion]

My rule of thumb is 50% loss of chlorine each sunny day with CYA around 50. You aren't too far off of that. You loss is still a little more than I would expect, but not dramatically so. Your pool is fairly large, more than twice as large as the average here, so obviously you use more chlorine than most people. Very low levels of algae or some chemical being broken down by chlorine are possible, but the zero loss of FC overnight seems to rule that out.

 

[chem geek]

Jason,

The other weird thing in this particular situation is that he started originally with around 20 ppm CYA and he increased it to 60 because of the relatively large daily loss, but this increase in CYA didn't seem to help at all (with the corresponding increase in FC levels, of course). So with no loss overnight, I'm stumped as to the relatively large loss during the day. He also told me that most recently he hasn't been using the pool as I thought that bather load could account for more loss. So I'm stumped.

Richard

 

[Riles_J]

That chlorine usage doesn't sound that far out of whack to me. I can go from 6ppm to 3ppm in a day. As Jason said, the pool size is quite a bit larger than most. I typically add about 1 jug of bleach daily to my 19,000 gallon vinyl pool and I only get afternoon/evening sun.

Riles

 

[JasonLion]

The only other possibility I can think of is a CYA measurement problem. What brand of test are you using to measure CYA?

 

[Rangerman]

The only other possibility I can think of is a CYA measurement problem. What brand of test are you using to measure CYA?

Using the Taylor K-2006. Have checked this against a LaMotte kit ... same reading.

 

[JasonLion]

The Taylor test is very good, that shouldn't be a problem then. There is some variation, +-10, in the readings but that isn't really enough to completely explain things. I'm out of ideas. Assuming the max variation in the test readings in the correct direction gets you somewhat close to what I would expect, so not a huge mystery (from what I see here).

 

[Rangerman]

The other weird thing in this particular situation is that he started originally with around 20 ppm CYA and he increased it to 60 because of the relatively large daily loss, but this increase in CYA didn't seem to help at all (with the corresponding increase in FC levels, of course).

Richard

This is the part of the equation that stumps me as well. Practically no difference between 20 and 60ppm CYA in the amount
of loss.