If My Cya Is High, How Much Fc?

[tedinelkgrove]

My CYA is really high. According to my test kit it is around ~200ppm. From reading posts on the forum, I need to keep my FC high in order to combat the high CYA level. But being my test kit only shows a maximum color index equivalent to 5ppm for chlorine, how can I tell what my true FC level is? I use the Leslie's Complete Poolcare DPD Test Kit. I just did a Chlorine test and FC and TC had same color (a little darker than the 5ppm level), so CC is ~0.

After I figure this out, what level of FC do I want? I see a lot of recommendations for FC to be at 10% of the CYA level. But 20ppm of FC seems really high.

I was using Cl tabs, but I have stopped due to high CYA and CH. I want to start a liquid bleach protocol for maintaining proper FC levels. I will drain the pool when the heat subsides to bring CYA and CH back down. It is 100+ here in the valley and I was reading never drain a pool in hot weather.

16000 gallon pool

pH = 7.6

TA = 100

CH = 750...ouch!

However, pool looks great and water is sparkling.

[chem geek]

I'm glad that your pool water isn't cloudy from over-saturation with calcium carbonate, but you actually have a very low carbonate alkalinity since your CYA level is so high (about 60-70 ppm of the 100 ppm TA is due to CYA). You are actually fortunate that your CH is as high as 750 since it is protecting your pool plaster in spite of the low carbonate alkalinity (due to the high CYA). If your CH is truly 750, then that's going to be hard to manage long-term. If the CH is high due to some use of Cal-Hypo, then if your fill water is lower in CH, dilution will help. However, evaporation and refill simply adds to your pool water whatever is in the fill water so use of a pool cover can help considerably (in your case in a hot climate, a thin white or light colored opaque pool cover to limit evaporation, but not heat the pool, would work). Remember the following chemical rules (facts) that are independent of concentration of product and of pool size. This will help you prevent a buildup of CYA or CH.

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.

For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.

For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

Since you have essentially a re-branded Taylor K-2005 test kit, you can just get the FAS-DPD chlorine test that won't bleach out, will measure to within 0.2 ppm FC and CC, and can measure up to 50 ppm at tftestkits.net here.

Since your pool doesn't have algae, you may be fortunate to have low algae nutrient levels, either very low phosphates or nitrates or both. Though a phosphate remover could be used in a maintenance dose to continue this no-algae situation, it won't do anything for helping chlorine be effective at the high CYA level so at higher bather loads or organics getting into the pool, it could cloud more easily and require shocking to clear (unless the FC was kept up higher). Since you have a technical background, you can read at least the introductory sections of the 1974 paper in this link that definitively determined the chlorine/CYA relationship in terms of the equilibrium constants for all related species.

As crazy as 20 ppm FC with 200 ppm CYA sounds, it is technically equivalent to around 0.1 ppm FC with no CYA -- that is, the active chlorine (hypochlorous acid) level in these two situations is the same. Obviously, you wouldn't want to drink quantities of water at 20 ppm FC -- the EPA limit is 4 ppm FC based on drinking water standards of 6-8 quarts per day every day, but that's not the same as swimming in the water. Dermal absorption of CYA is minimal according to this paper and I'd expect the same for the chlorinated cyanurate species (which is where most of the FC is contained). Also, the minimum FC of 7.5% of the CYA level is conservative for worst-case pools rich in algal nutrients (I have such a pool due to 300-500 ppb phosphates in the fill water and blown-in fertilized soil) and accounts for manual dosing variations as well. An FC of 10 ppm with 200 ppm CYA would probably prevent algae growth if consistently maintained even in a "difficult" pool. This is especially true if there is even a minor algaecidal effect with the chlorine bound to CYA though that has not been seen in general.

Anyway, the best thing to do is to dilute the water to lower the CYA level to a more manageable range, no higher than 80 ppm (which should be fine to protect the chlorine from sunlight even in your area). Also note that the CYA will be a part of the TA level so the actual carbonate alkalinity that is relevant for protecting plaster uses an adjusted TA. You can use The Pool Calculator for that where right now it looks like your pool is fairly balanced. As for methods to dilute the water safely, you can do partial drain refills or you can do continuous drain/refill (though that uses more water) or you can use the "sheet" or "silage bag" method where a large sheet is put over the pool and water drained from underneath while new water added on top.

You can learn a lot more at the Pool School.

If you get your CYA down to 80 ppm, then you'll maintain a minimum FC of 6 ppm which is quite manageable. If you wanted to have a lower FC for whatever reason, then to prevent algae growth you would need some sort of supplement at extra cost (phosphate remover, as I mentioned earlier, or PolyQuat 60 weekly). Another alternative is to use 50 ppm Borates in the pool as that is not only a pH buffer but a mild algaecide, but we don't have enough data to know exactly how good it is -- anecdotal evidence is very positive so far. Note that you can't get borate products in California at pool stores, but you can get 20 Mule Team Borax in grocery stores (along with Muriatic Acid in hardware stores) and you can order Boric Acid directly which is more pH balanced. This is optional, though I just added it to my pool this year.

Richard

[tedinelkgrove]

Thanks for the advice Richard. I am going to dilute the pool tonight and fill it to the top tile. My auto-fill seems to always keep it low (middle of bottom tile, 3 tiles total, even at high setting), so this gives me an excuse to fill it all the way up. I'll check FC/TC/CC in the morning, along with pH, CYA, and TA. Just to see if there is any change and I'll adjust according to the pool calculator. That is a great tool.

My supply water is relatively low in Ca/CaCO3, 18 and 78ppm respectively. This is according to a Sac County water report. I would imagine a refill when the temps cool will be warranted. Is it better to drain/refill during cooler weather?

Ted

[jkusmier]

Thanks for the advice Richard. I am going to dilute the pool tonight and fill it to the top tile. My auto-fill seems to always keep it low (middle of bottom tile, 3 tiles total, even at high setting), so this gives me an excuse to fill it all the way up. I'll check FC/TC/CC in the morning, along with pH, CYA, and TA. Just to see if there is any change and I'll adjust according to the pool calculator. That is a great tool.

My supply water is relatively low in Ca/CaCO3, 18 and 78ppm respectively. This is according to a Sac County water report. I would imagine a refill when the temps cool will be warranted. Is it better to drain/refill during cooler weather?

Ted

As a practical matter, given your CYA and CH levels, I wouldn't bother testing tomorrow AM - you'll see only nominal changes in your test results; you really need to drain 1/2 your pool volume and refill. That should not pose any problems w/ a gunite pool, and should reduce both your CYA and CH levels by half, leaving your CH within range for a gunite pool (250-400) upon refill although, as Richard noted your CYA will still be a bit high (100 is still too high but definitely much more manageable than your current level). While refill time will vary depending upon your local water pressure, my notes indicate I was able to re-fill roughly half of my 16,500Gal pool overnight w/ a single hose.

You'll need to add tremendous amounts of chlorine until you reduce your CYA level. You're right to adapt to bleach - not so much to maintain FC but instead to keep your CYA from getting out of control. Those jugs get heavy, so best to drain/refill ASAP.

[tedinelkgrove]

As a practical matter, given your CYA and CH levels, I wouldn't bother testing tomorrow AM - you'll see only nominal changes in your test results; you really need to drain 1/2 your pool volume and refill. That should not pose any problems w/ a gunite pool, and should reduce both your CYA and CH levels by half, leaving your CH within range for a gunite pool (250-400) upon refill although, as Richard noted your CYA will still be a bit high (100 is still too high but definitely much more manageable than your current level). While refill time will vary depending upon your local water pressure, my notes indicate I was able to re-fill roughly half of my 16,500Gal pool overnight w/ a single hose.

You'll need to add tremendous amounts of chlorine until you reduce your CYA level. You're right to adapt to bleach - not so much to maintain FC but instead to keep your CYA from getting out of control. Those jugs get heavy, so best to drain/refill ASAP.

How do I drain the pool? I assume I need a submersible pump and a hose long enough to go to the street/storm drain.

[chem geek]

Yes, a pool cover submersible pump or more powerful submersible pump can be used. You may need to drain to the sewer, not the street or storm drain, depending on the regulations in your area. Storm drains that directly flow to natural waters usually shouldn't have pool water dumped into them so as not to harm aquatic life. After all, the stuff that kills algae in your pool will kill organisms in natural waters as well, though it will obviously get diluted a lot. Going to a sewage treatment plant will mostly neutralize the algaecidal components of the water (including chlorine), though I don't think they do much about copper/silver, salt or borates.

Richard

[tedinelkgrove]

Richard, I order the FAS-DPD Chlorine test kit you recommended. That should really help out with determining how much chlorine (FC, etc.) I have in the pool. I also added a gallon of 10% bleach (from Lowe's). The Pool Calculator said I need to add 2 gallons based on my current chemical levels, but I'll hold off on adding more until I get he FAS-DPD test kit.

Thanks for everyone's help.

Ted