Chlorine lost to sunlight no matter what!

[SL1]

My mother's pool maintenance company has been dumping enormous amounts of chlorine into her pool this summer, but always gets 0 ppm at their next weekly visit. I began doing some testing and found that adding 2.5 lbs of granuals to the 23.500 gallon pool give about 3 ppm chlorine. If I add it about noon, it is down to 0 ppm by 4 pm. But, if I add it about 6 pm, it stays at 1-3 ppm until after noon the next day, when the sun hits the pool again. So, I would think I need more cyanuric acid to stabilize the chlorine in the presence of UV light. But, the pool maintenance company test says it already has 60 ppm cyanurinc acid and that adding more won't help. They found something above 50 ppm nitrates and are blaming the chlorine loss on that. They say draining the pool and replacing the water is the answer. Before I go to that effort, I want to be confident that it will help. The water is clear, but we have been having problems with recurring pink algea on the walls.

Any thoughts?

SL1

[chem geek]

Shock the pool as described here and then maintain an FC level appropriate to your CYA level as shown here which means a minimum FC level of 5 ppm (4.5, actually) to prevent algae from coming back.

[Guest]

OK, I'm new to this, so please check out my understanding of all that I just read in your links and the links to those links:

I need to get my FC level up to 24 ppm to effectively shock the pool, and I need to keep it there until ALL the algea is dead. After that, the FC level will not keep dropping real fast at normal FC levels (?).

This is NOT what my mother's pool maintenance company is telling me (or doing). They "shock" it once a week in the morning and don't check it again for a week. They put about 10 lbs of dichlor into a 23,500 gallon pool, which the Pool Calculator tells me should put the FC level up to about 30 ppm. But, when I check it by the end of the afternoon, it is ALL gone. Also, the FC level seems to go only about HALF as high as the Pool Calculator says it should when I add about 2.5 lbs of dichlor. (This seems to tell me that I have a LOT of something in the water, but I'm not sure what. It is very clear, but there is a tendency to get on slime on the surfaces.

BUT, if I add the dichlor when the sun is not on the pool, the FC level DOES stay up until the sun hits it again. In one of the links to your links, that was supposed to indicate that the algea is dead. And, at 60 ppm, it does not seem that the water is low on UV stabilizer (CYA).

So, I am not really getting this. The pool company says I need to replace water to get nitrates down, but the links here don't seem to worry about nitrates.

What do I believe? I guess I should devote a weekend to trying to properly shock the pool with bleach and see what happens before I do the much greater task of partially draining and refilling.

Comments or suggestions. please.

SL1

[pool kid]

with a level of nitrates shock the pool ten times the amount you normally would with liquid chlorine. say if you have a 20,000 gallon pool. you would need twenty bags of shock to get rid of nitrates. liquid chlorine is a lot cheaper and more effective so you would use 3 five gallon drums of liquid shock. I came up with three because 1 five gallon liquid equals 7 bags. so I divide seven into 20 and get 2.8 so i round up to 3. try it. it works!

[chem geek]

For every 10 ppm Free Chlorine (FC) added by Dichlor, it also increases Cyanuric Acid (CYA) by 9 ppm. So if the pool service is shocking with Dichlor every week, then the true CYA level is off the charts making the chlorine essentially ineffective. In other words, the 60 ppm CYA can't be right. Are you testing with your own good test kit? You can get the Taylor K-2006 kit at a good online price here or the TF100 kit from tftestkits(dot)com here with the latter kit having 36% more volume of reagents so is comparably priced per test.

Your pool service should never be using Dichlor as a shock. It only makes the problem worse by increasing the CYA level making chlorine less effective.

If the CYA is extraordinarily high, dilution (partial drain/refill) is the normal way to reduce it. Pool Kid's super-shocking approach *might* reduce the CYA level and he/she believes it reduces the nitrate level, but I'm not recommending that.

For a pool service to add chlorine only once a week, they should be using an unstabilized source of chlorine such as chlorinating liquid and they do need to have the CYA level be fairly high, but would raise the FC fairly high as well. For example, there is a pool service in Arizona (and other very hot direct sunlight climates) that keeps their pools at a CYA level of 100 ppm and raise the FC to 14 ppm with it dropping to around 4 ppm the next week when they raise it again. They service thousands of pools and none ever get algae (unless they are late in dosing the pool).

Richard

[pool kid]

with having FC at 14ppm doesn't that start to cause burns for children and some adults?

[chem geek]

Not when the CYA level is at 100 ppm. The FC number by itself is meaningless in terms of the "active" chlorine concentration, that is of hypochlorous acid. Roughly speaking, the ratio of FC to CYA is proportional to the concentration of active chlorine. 14 ppm FC with 100 ppm CYA at a pH of 7.5 has the same amount of active chlorine as 4.3 ppm FC with 30 ppm CYA or 0.14 ppm FC with no CYA. Technical details about this are here and you can see a comparison graph of the amount of active chlorine vs. pH in the traditional industry graph that is only true when there is no CYA compared to the graph that is true when CYA is present here. I talked to the pool service in Arizona and not a single customer has any problems with stinging eyes or dry skin or anything normally associated with high chlorine levels.

My wife experiences this difference that CYA makes every year when the elasticity of her swimsuits gets shot after just one winter swim season in an indoor community center pool and her skin is flaky and hair frizzy as well. In our own outdoor pool, there have been none of these symptoms for 4+ years. The difference is that our outdoor pool has 3-4 ppm FC with 30 ppm CYA while the indoor pool has about 2 ppm FC with no CYA. That ends up being a factor of 20 difference in "active" chlorine concentration with the indoor pool essentially being over-chlorinated.

This issue and misconception comes about because the stabilized chlorine industry has been saying that "CYA doesn't matter; only FC matters" for many years and this also leads to "CYA is only needed in outdoor pools to protect chlorine from sunlight" because they never talk about how CYA reduces the amount of active chlorine by orders of magnitude even though the science behind this has been known since at least 1974. If they admitted the significant reduction in chlorine effectiveness, this would lead to questions like "won't higher CYA levels then allow algae to grow faster than chlorine can kill it" and people might realize that all they had to do was increase their FC level rather than buy a profitable algaecide. Or they might realize that the stabilized chlorine products themselves increase CYA levels rapidly in pools without lots of dilution (such as pools with cartridge filters where there is no backwashing).

Richard