15 ppm drop in FC on hot day when starting at 30 ppm.

[lalittle]

I had my FC level up to 30 ppm for a shock. At the end of a full day of not adding any chlorine -- which was a full day of sun where the air temperature got up to around 90 -- the FC was down to 15 ppm. I read that the chlorine level drops much more quickly when it's at high levels, but I was wondering if a 15 ppm drop is still normal in this situation. I don't have a SWG, there was no tablet feeder in the pool, my CYA level is around 60, and the pool water temp was in the high 70s to perhaps the very low 80s.

The 15 ppm level was around dusk, so I did an OCLT by measuring the level again the next morning about 30 minutes before sunrise (which was a little under 9 hours later.) To measure the higher Cl levels, I diluted the water with half bottled water to do the chlorine tests, and then doubled the results. It still showed 15 ppm, and no measurable CC (using a Taylor test kit.) As far as I've read, this should indicate that there was no algae in the pool, but I just want to confirm that the 15 ppm drop is normal in this situation.

To summarize, my evening measurements at about 8:45 pm where the same as my measurements at 5:30 am:

FC 15
CC 0
CYA 60
pH 7.4
TA 75

Thanks,

Larry

 

[JasonLion]

Yes, normal (or close enough). The amount of chlorine lost each day is a percentage of your starting level. If your starting level is twice as high, you lose twice as much.

 

[lalittle]

So does the percentage that is lost on any given day track equally with the total amount -- i.e. is the general rule of thumb that about half of the existing chlorine is used up on a hot day? In other words, if I had been at 6 ppm on the same day, would I have ended up with about 3 ppm?

Thanks,

Larry

 

[lalittle]

Just to update, I only lost 3 ppm today (starting from 15 ppm), compared to the 15 ppm I lost when starting at 30 ppm on the previous day. That means that my Cl loss rate was 50% per day when starting at 30 ppm, but only 20% per day when starting at 15 ppm. It wasn't as hot today (low 80s compared to around 90 the previous day), but it would seem that either the chlorine loss per day is not just a straight percentage of where you start, or that the loss is MUCH more sensitive to air temperature, i.e. that only 7 or 8 degree difference can result in Cl loss rate slowing by 60% (from 50% down to 20%.)

Does this still sound right? My testing seems to show that if the Cl level is twice as high, you lose much MORE than twice as much in a day.

Thanks again for the help here,

Larry

 

[JasonLion]

The rate of chlorine loss is very sensitive to water temperature, number of people swimming, and the exact amount of sunlight hitting the pool (even wispy clouds make a difference). If all of those things remain constant, then it is a fixed percentage (what particular percentage is based on your CYA level).

It is certainly possible that you lost extra chlorine the first time for some unrelated reason, like a low level algae problem. Presumably there was some reason you had the pool at shock level. Without a lot more information it is impossible to be sure.

 

[lalittle]

Yes, I had previously done a shock, but I just want to confirm that even with the change in daytime FC percentage level drop, the OCLT is still the only thing I need to be concerned with. Since the OCLT showed no drop once it was at 15 ppm (it was 15 ppm at dusk and again at dawn the next morning), it's safe to assume that the shock was successful, correct?

Thanks,

Larry

 

[JasonLion]

Correct.

 

[lalittle]

Thanks for the confirmation.

Larry

 

[imwarren]

To measure the higher Cl levels, I diluted the water with half bottled water to do the chlorine tests, and then doubled the results. It still showed 15 ppm, and no measurable CC (using a Taylor test kit.) As far as I've read, this should indicate that there was no algae in the pool, but I just want to confirm that the 15 ppm drop is normal in this

Thanks,

Larry

Not sure if diluting will give you good enough results to know if you have fully passed an oclt you should really get a fas/dpd test from tftestkits.com. You still could have something in your water

 

[lalittle]

Not sure if diluting will give you good enough results to know if you have fully passed an oclt you should really get a fas/dpd test from tftestkits.com. You still could have something in your water

I am using an fas/dpd test -- I'm using the Taylor K2006 kit. If I didn't dilute, I'd have to use a prohibitively high number of drops of R-0871 reagent.

Larry

 

[Richard320]

Not sure if diluting will give you good enough results to know if you have fully passed an oclt you should really get a fas/dpd test from tftestkits.com. You still could have something in your water

I am using an fas/dpd test -- I'm using the Taylor K2006 kit. If I didn't dilute, I'd have to use a prohibitively high number of drops of R-0871 reagent.

Larry

Use a 10 ml sample. Each drop counts as .5 that way.

 

[lalittle]

Use a 10 ml sample. Each drop counts as .5 that way.

I'm already using the 10 ml sample. At 30 ppm chlorine, however, it would still take 60 drops of R-0871, so diluting by half substantially cuts down on the reagent use.

Thanks again for all the feedback,

Larry

 

[chem geek]

When shocking, you lose chlorine even faster than at the same proportional rate, though as a rough starting point that's the minimum you'd lose. At shock levels you lose more because that much more of the chlorine is unbound from CYA and has a faster loss rate. Even so, since most of the chlorine is still bound to CYA, most of the overall loss is still based on the proportion of FC. What's your normal percentage daily loss when not shocking? Is it less than 50%? I would guess that you would lose somewhat less than that rate when at normal FC levels (i.e. not shocking) given your 60 ppm CYA.

 

[duraleigh]

so diluting by half substantially cuts down on the reagent use.

and subsequently cuts the precision in half as well. Granted, I sell refills, but saving a few drops of reagent to obtain results that are less than dependable seems penny wise and pound foolish.