Chlorine disappearing rapidly with CYA at 50

[TGosselin]

I just got my new TFT-100 test kit and went through some of the standard tests to see where I am. I had added enough bleach this morning (8.25%) to bring me to 4 ppm, but when I tested at 5 PM I am at .5 with the OTO test, so I tested with the FAS-DPD and also got .5 for FC. I also tested the CC and that is between 0 and .5 as well (very light pink one drop to clear). So TC is 1.0 by the FAS test.

Today was mostly sunny, and the pool is in sun for most of the day. Isn't CYA supposed to protect FC from sun?

Why is my chlorine going so fast?

FC = .5
CC = .5
TC = 1
CYA = 50
PH = 7.5
TA = 114

Any help appreciated.

 

[chem geek]

You may have algae growth and need to get ahead of it in order for the chlorine to hold. You also should not have gone to only 4 ppm FC in the morning because you want to end up there in the evening so should have targeted a higher level. At this point you should start a SLAM and add enough chlorine to get to 20 and see if it can hold overnight. Once it does, then you can let it drop, but be sure to add enough so that you don't go below 4 ppm FC for the next time you add chlorine.

If you are in a very sunny area (you can put your location in your profile), you might need a higher CYA level, but first get working with the 50 ppm since it's easier to SLAM at that level.

 

[TGosselin]

Thank you, chem geek, I'll have to give that a try starting tomorrow night, too late for today.

 

[TGosselin]

Started the SLAM and have been holding the FC at 20 with the additions so far:
Dropped to 15 on the first test (after a 5 hour interval); raised back to 20
Dropped to 18 on the overnight (9 hours); raised back to 20
Dropped to 17 after 3 hours with sun on the pool; raised back to 20

So far; so good....

 

[TGosselin]

Just tested again, not sure how long the interval was, but down to FC 10 with pool in full sun. Will bring it back to 20, but was wondering, is there a way to calculate the loss from sun?

 

[jmastron]

Just tested again, not sure how long the interval was, but down to FC 10 with pool in full sun. Will bring it back to 20, but was wondering, is there a way to calculate the loss from sun?

My understanding is that, much like the general chlorine levels (where the *minimum* you want is 4, so you have to go higher than that when you add), the SLAM is the same -- you want a minimum of 20ppm, so you should add enough bleach to get to, say, 25ppm, then test every few hours and keep adding so it never goes below 20.

Loss from the sun varies a lot from climate, water temperature, CYA level, and probably other factors -- for example, my pool at CYA 45 when uncovered in full sun at 87-89 degree water temp loses 3-4ppm during the day, so I have to be certain to dose it to 8 in the evening to keep it above 4 before the next evening.

The key to knowing you're done with the SLAM (and thus can let the FC drift down below 20) is *overnight* loss of less than 1ppm (in addition to CC < 0.5 and clear water)

 

[TGosselin]

Thanks, jmastron, I tested the FC before going to bed last night and it was at 15 - 15.5 (one extra drop seemed needed, but not sure it was), this morning I checked and FC is at 14 - 14.5 (again one extra drop seemed needed). Adding the extra drop in both cases may not have been needed as the test looked clear.

CC was 0 (added the reagent to the above test, and there was no change).

Water is clear, so I guess I'm ok now to allow the FC to drop to normal. I guess what I did worked although not perfectly executed.

Since I like to keep it 4 ppm, and based on what I'm hearing, I should shoot for a higher FC level in the evening than the desired reading, say 8 or 9, to allow a drop overnight and during the day to at a minimum of the desired reading of 4, then add chlorine to get back to the higher level overnight to repeat the cycle.

 

[pooldv]

Why do you like to keep it at 4? Is that from the pool store? That is too low and any hiccup will start algae growing and increase likelihood of person to person disease transmission and an unsanitized pool. You should keep it at 6-8 at all times.

It is safe to swim when FC is above minimum and below shock level for your CYA, Chlorine CYA Chart. So with CYA of 50 it is safe to swim when FC is between 4 and 20 ppm.

 

[TGosselin]

pooldv, guess I was reading and misinterpreting the CYA/Chlorine chart and just keeping it at minimum, rather than at target. OK that said, I just tested the FC in the pool and have gone from FC = 14.5 (at 8 AM EDT) to FC = 5.5 at 2:30 PM EDT which seems like a lot to me but the pool is in full sun from 9 am till past 4 PM in the summer.

This seems a bit quick, with 9 ppm being eaten by sun, so should I raise the CYA to combat this?

 

[Starman]

Given the size of your pool and a CYA of 50 your target range according to Pool Math is 4 - 8.

Is it possible that your CYA could be less than 50? say in the 35 range? Also wondering how you determined your pool's water (19200) volume.

Geographically, we're pretty close to one another. My 18x36 IG vinly pool is prettly much "dialed" in this year and I'm losing between 2 and 3 FC daily. My CYA is around 38-40. In general, I use the TF100 test kit for slamming and LaMotte's ColorQ test kit for daily monitoring.
Last year was my first one using the TroubleFree Pool method. My first year it took 13 days of Slamming before my pool passed the OLT. After SLAM day 5 I remember my pool looking clear, but the overnight tests indicated otherwise. Once I got over the SLAM hump it was smooth sailing. I closed my pool in Sept 2014 and didn't open it til Late May 2015. Took the cover off and the water looked great! My best opening ever! I'm a TPF believer.

I'd recommend you retest your CYA and use TPF's PoolMath calculator for daily recommendations.

 

[TGosselin]

Sorry for the delay in responding, Starman, been tied up with work. Looks like we are pretty close to one another! Been to Shelburne Falls several times. My pool is 16 by 32 and has a shallow end of 3 ft and a deep end of 8 (average of about 5 ft) so volume is (16*32)*5, I used the Pool Math calculator to get gallons.

I've been using TFP for a couple of years now, although not very accurately as I was not using a recommended test kit at the time, and relying on pool store testing. Hadn't had any trouble at all until this year, something just got a hold, and was eating the FC fast. SLAM took care of that, now I have a steady FC between 8 to 10 (adjusting daily) with a .1 drop overnight, and a daily drop with sun of about 3, not too bad. I am keeping at 10, since I have full sun all day, and guests often in the pool (only pool in the family, so we're popular!)