Can't get to 0 CC

[tjwill]

Hi all,

I've had my pool for 2 years and by following everything I read here, my water has been crystal clear the whole time. This is my first time being stumped with the chemistry (and frankly, I might be making a big deal out of nothing). I use the TF-100 test kit and my CC levels have always been 0. A few days ago, I started measuring small levels of CC, usually just 0.5. I'm going on vacation in a week and didn't want to leave it until the last minute so I went straight to shock treatment. The water looks very clear and I don't see any algae blooms, but the CC level won't drop to 0. Last night, I passed the overnight test and technically the 0.5CC meets the requirement to stop shocking, but I'm still not comfortable with my vacation coming up. Am I making too big of a deal out of the low level CC or should I stick with the shocking and knock it all the way to 0?

FC 24
CC 0.5
TA 50
CH 250
CYA 40

 

[JasonLion]

Did you do anything new, or did anything unusual happen when you first started measuring CC? For example, adding sequestrant for the first time can create CC for a couple of days, as can a major wind storm blowing debris into the pool, as can (in a different sense) getting new test reagents.

 

[tjwill]

Thanks for such a quick reply......and interesting too. Nothing new had happened when I first started seeing some CC, other than a weekend of use, but the first day I started shocking, we got a major thunderstorm with wind, lost power, trash cans blown down the street, pool filled with debris. Is it possible that what would have been a short shocking process was masked by all this debris in my pool? I ran the robot the entire next day and cleared it all out, but it was in there for about 24 hours.

 

[JasonLion]

That is plausible, but not likely. If the CC started earlier, it is unlikely to be persisting because of the storm. The events are close together, but the timing doesn't really match up, and one cause instead of two is much more likely.

Another possibility: have you been using a solar cover? Solar covers tend to encourage CC.

By the by, you may never know. With the normal test kit, CC at 0.5 really means CC above 0 and not higher than 0.5. Levels down around 0.1 are very difficult to trace, and fairly common (though hardly universal).

 

[tjwill]

No, I don't have a solar cover. I agree that it's a pretty low level. Should I keep with the shock levels until I get it down to 0?

 

[chem geek]

If you don't have significant overnight chlorine loss and the pool is crystal clear, then I wouldn't worry about the CC of 0.5 ppm. As Jason noted, it's probably a lot less. To see how low it actually is, you can use a 25 ml water sample so that each FAS-DPD drop represents 0.2 ppm, but I'd wait to do this after your FC drops from the shock level to normal levels so you don't waste so much FAS reagent in the FC part of the test.

My own pool will often vacillate between 0 ppm CC and 0.2 ppm CC. I have a mostly opaque pool cover, but the pool is open every day for 1-2 hours in sunlight. Even the 0.2 ppm CC is likely to be <= 0.1 ppm since it is a very faint pink and even a partial drop clears it. If I were to use a 10 ml sample size, however, I might see the very faint pink and count the drop to clear it as <= 0.5 ppm CC. There is absolutely nothing wrong with that.

CC is not a terrible thing, especially when it is very low. There are many different types of CC and the most common which is monochloramine (chlorine combined with ammonia) is not a problem at all at low levels and is found in chloraminated tap water at around 1 ppm or so. At low levels it won't be noticeable. The other common CC is chlorourea (chlorine combined with urea) and it is even less volatile and less noticeable. There are other CC that are volatile and irritating such as nitrogen trichloride, but these are a problem at levels below what the CC test will detect so the CC test is really just being used as a proxy that their might be a problem, not that there is one. If you don't notice any obnoxious smells (i.e. "bad pool smell" often associated with poorly maintained or higher bather-load indoor commercial/public pools), then don't worry about the <= 0.5 ppm CC at all.

 

[tjwill]

Thanks chem geek.

It was odd to me because my pool has always read totally clear on the CC and it was driving me nuts because no matter how long I shock it, it still doesn't seem to clear. I took one reading this afternoon that came up clear and I thought I had licked it, but then I took another an hour later and it was still there. I'm going to bring it down from shock and just keep an eye on it for a few days and make sure it doesn't rise.

Appreciate the insight. I've been much more of an observer than a poster here, but have learned so much reading about other people's experiences. This site is fantastic and I really appreciate the service that the experts on here do for the rest of us. I'm so proud of my water, but it has more to do with you all than me.

 

[chem geek]

Part of the reason you get a clear reading is that the last drop in the FC portion of the test can overshoot to provide excess FAS reagent that applies to the CC part of the test when you add the R-0003. That's why the test accuracy can be no better than +/- 1 drop for each test (FC and CC). If your FC level is anywhere from 4.05 to 4.54 (assuming you can only see faint pink with at least 0.05 ppm), then that last drop will tell you the FC is 4.5 (basically it rounds up, depending on how well you see the faint pink), but if you only have a CC of 0.1, then any FC from 4.05 to 4.50 will be clear. However, if your FC is between 4.50 and 4.54, then the 0.1 ppm CC will put you over the 0.05 visual threshold and you'll see faint pink and think you've got <= 0.5 ppm CC.

So the bottom line is that most of the time when you've got a low CC but are using a 10 ml sample size, you will measure clear unless your FC happens to be close to a half ppm edge in which case you'll "see" the CC, but it's really small. That's why I say if you really want to see what is going on, use a 25 ml sample size and I'll bet your CC is still only one drop, but now you'll know that means <= 0.2 ppm.