Overnight Free Chlorine Test - Margin of Error?

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alanrmeadows

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May 5, 2010
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The overnight free chlorine test at high levels of chlorine (16ppm-19ppm) is starting to baffle me. For the last 3-4 days, in my unprofessional estimation, my pool has looked great. However, I still can't get the overnight free chlorine test to show absolutely no loss in chlorine. Last night at 9:00 pm I got two back-to-back readings of 18 & 19. I used the EXACT amount of pool water on both tests, so I'm pretty sure that's not the problem. This morning at 6:00 am I got back-to-back readings of 17 & 17.5. Is it possible that there is some margin of error for these tests...like possibly the size of the drop that comes out of the solution...or something else? I kept my pool covered all winter and have been shocking for a week. And again, the pool likes great and is sparkling clear. My CC's are consistently at .5. What do you think? Should I continue to shock? Is my overnight FC loss such that I'm probably OK?

Thanks,
Alan
17,500 gallons
In Ground
Fiberglass
Sand Filter
South Arkansas
 
With CC's consistently .5 and a loss of 1ppm or greater, you need to keep shocking.

What times are you performing the tests?
Are you holding the bottle vertical and dropping the drops about 1 per second or longer?
Are you dropping till the water is clear and adding one more drop just to make sure?

You can do a 25ml test if you want to. That makes each drop count .2 instead of .5 ppm.

We had a person a while back that couldn't pass the OCLT and come to find out they were doing the evening test late enough but not performing the morning test till about 8am and the sun was well up on the pool.
 
Would you please post your other numbers as well (ph, CH, TA, CYA, water temp.). Also, don't forget to brush that pool and get all the surfaces fully exposed to the chlorine.

And just in case...did you circulate the pool for at least one hour prior to taking your readings? Sounds like you are doing everything correctly, just wanted to make sure on the circulation. Mahalo.
 
PH 7.8
TA 100
CYA 50
CH 260
73 Degrees Water Temp.

I test at 9 pm and 6 am.
I hold the bottle vertical and drop consistently.
I am dropping until the water is completely clear.

Any other ideas?
 
OP posted...
My CC's are consistently at .5
Click to expand...
that actually passes the three criteria folks on this forum have developed as having a sanitized pool. It's the loss of GREATER than .5ppm of FC that (sort of) fails the test.

We normally suggest..
1. Your water is crystal clear.
2. You lose less than 1.0 FC during the FC overnight loss test
3. Your CC's test is .5 or less

So apparently all OP lacks is the FC loss and he/she is very, very close to that.

I would continue to hold shock value for FC at least one more night and probably two. If you cannot get the loss down to less than one by then (and it's still not greater than 1), I think it will be fine to let your FC come on down into operating range of 4-6ppm.

I have a feeling you will get that "less than 1ppm loss" soon, however. You are very close.

BBB is not neccessarily about following all these guides to the absolute T but rather knowing what to do with the information the testing provides. It seems to me that's exactly what you are doing....nice work! :lol:
 
I thought I would let you know some of the other minor details of my testing process...maybe there's something else I'm doing that will not allow me to get a consistent reading each time I test:

1) I go to the same place in the pool (about the mid-point of the pool between the shallow and deep end) and get a water sample about elbow deep.
2) I pour out a little bit of water from the top of the sample just in case I got some water closer to the surface.
3) I take the water from the sample and go ahead and "wash out" my chlorine cylinder and shake it as dry as possible.
4) I fill to the 10mL line using the bottom of the meniscus as my line.
5) I put in a heaping scoop of the powder and swirl for a few seconds.
6) I start dropping vertical and swirl for about 3 seconds between each drop and continue to drop until water is crystal clear.
7) I then wash out the chlorine cylinder with the same water sample and then repeat steps 4-7 with the same water that I got from step 1.

Do you see anything in my process that could be contributing to not getting the same reading when I do tests back to back?

Thanks,
Alan
 
Everything you describe sounds right. You should be able to get two sequential tests on the same sample to be +-1 drop.

Have you tried wiping the tip of the dropper bottle with a damp tissue? Sometimes static electricity accumulates on the dropper bottle tip and causes the drops to be the wrong size.
 

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OK. Are you ready for this one? Last night at 9:00pm (5 hours after putting additional bleach in the pool) I tested and got two consistent back to back readings of 17ppm. However, I ran out of my solution for testing chlorine that I had purchased last year after I completed these tests. So this morning at 6:00am I tested again with my NEW bottle of solution and got readings of 18.5 and 19. I got combined chlorine readings of .5 with my tests last night and this morning. What do you make of this? Do you think I should continue to shock?

Thanks,
Alan
 
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