chlorine level ?

pool info: 26,000 ingound vinyl, ph 7.3, cl 10.0 , cc 0.0, cya 75, alk 150, been using BBB method. plan to go swimming this weekend. decided to shock it 2 days ago with a goal of 30 ppm chlorine. i added bleach and tested and got 23ppm cl. I then added 5 more 3/4 gal bottles of 6% bleach as each bottle usually gives me about 2 ppm and retested and only got 26 ppm. i added 4 more bottles , waited 2 hours and retested and got 26ppm again. test water never really got clear when it changed from red but became kinda a pewter color. using ben's 233 kit with one year old dpd powder and drops. any idea why i couldn't get a higher cl reading ?

If you were adding this chlorine during the day, then it may have been getting broken down by sunlight. During direct noontime sun the chlorine half-life with a CYA of 75 ppm is around 7 hours. So at around 24 ppm FC, that's a loss rate of 12/7 = 1.7 ppm FC per hour (that's actually not the right calculation since the rate of loss declines as it approaches the "half" value of 12 ppm, but it makes the point -- the actual calculation shows that you lose almost 10%, or 2.4 ppm FC, in the first hour). That might explain the difficulty.

If you add it at night then any loss would have to be something else consuming the chlorine.

Richard

I was doing it during the middle of the day. pool does have a clear solar cover on it. interesting

That is interesting. So let me get this straight. Did you keep the solar cover on when you added the chlorine (presumably just moving it aside when you poured in the chlorine)? If so, then that would imply a loss from UV getting through the clear cover. I thought that the covers wouldn't let much UV through (visible light, yes, but not UV). If possible, it would be worth seeing if the same problem happens when the sun isn't on the pool.

I don't know anything about why the FAS-DPD test would go from red to pewter. The Taylor instructions say that high chlorine above 25 ppm (near where you are measuring) can result in a brown color and that to handle that you should either use more powder (they don't say how much, but I'd guess double the amount, or 4 scoops total -- at least for the Taylor kit since I'm not sure what Ben's uses) OR you can dilute the sample using filtered or bottled water and then multiply the result by your dilution ratio.

Richard

yes, the cover stayed on the entire time. it's a new 12 mil clear with diamond shaped bubbles. (works great at holding in heat) All tests were done with two scoops of dpd powder. I normally use one scoop and normal cl level for me is 8.

just retested, it's 4pm here we've had partly cloudy skies today. cl level was 20.5 with 0 cc and two scoops. with one scoop it was 23, 0 cc and it turned clear not the slightly grey color.

i have a very small area on the bottom of the pool that always has a very fine powder collect it is light brown to dark gold in color. also collects on the steps. if you pass your hand over it , it quickly dispereses. only collects where the circulation is the minium. when i first got the pool 3 years ago i let the cl go to zero for a day,. I was using the sticks. that was before i learned of this board. that is when it first appeared. i believe it is mustard algae. wanted to see if i could knock it out. wife now tells me that daughter wants some friends to come over saturday for swimming so i will have to let the cl level fall back down. i had read a thread on here about needing very high levels of cl to knock this stuff out. i might not get a chance to do it this summer.

i haven't visited the board since last summer, looks like a lot of new developments, Richard you've been doing a great job of helping alot of people , along with some other freq posters, thanks go to all of you for keeping the board strong during these tough times.

We don't usually suggest keeping a solar cover on while shocking. Pool needs to be able to vent and also, it might not be too good for the cover.

I agree completely with Watermom -- the cover prevents a lot of the chemistry of shocking from happening. You need your pool to be able to "breathe" and sunlight also helps break down combined chlorines that will form. The downside is that you will be losing chlorine during the day from exposure to sunlight. Not much to do about that except to keep adding chlorine. That's why we talk about seeing if the FC level holds overnight since that (plus virtually no CCs) means you're done with killing algae.

As for mustard/yellow algae, this "sticky" thread talks about it. You will need a lot of chlorine to kill it. With your high 75 ppm CYA, it would take at least 40 ppm FC and you'd have to start out with a pH of 7.2 before you added the chlorine. I think that in your case your better option is to do a partial drain/refill of the pool to get the CYA down to 30 ppm since that's where it should be anyway and then you'll only need about 20 ppm FC to kill this algae.

Richard

oh, didn't know about the solar cover and shock thing, thanks for the info.