61 FC after shock? Doesn't make sense

[fwb]

Bottom line: Added 8 gal 10% hypo to <1 FC pool, now testing shows I have 61 FC? Pool calculator said 8 gal would add 30 FC.

Tested after 24 hours running the pump. I added my pool water to the 10mL line, added bottled water to the 44 mL line for dilution, 2 powder scoops, and it took 28 reagent drops to turn the pink clear. 28*0.5*4.4 = 61.6 FC if I did my math right.


Background:
During July my SWG was having trouble keeping FC up and I noticed green algae started to form on the rock features (water was clear though). Had my SWG set to 100% and pump running 12 hours/day. My numbers at sunset were:

<0.5 FC
0 CC
75 CYA
7.4 PH
3500 salt
300 CH
90 TA
100 Phos
Air temp: 105* avg high

According to the SWG calculator on this site, my system should generate 7.2 FC per 24 hours, and since I ensured my SWG was clean, visibly producing chlorine gas, no error lights, water perfectly balanced, etc, my last resort was to shock the pool because something must be eating all my chlorine. Can't really do the OCLT because I have to run my SWG and pump and night due to peak electricity costs and don't want to go a night without adding chlorine since I'm falling behind. Hope this fixes it because I am out of ideas, but I don't understand the 61 FC reading after shocking. I used the pool size calculator to determine my pool size, and every chemical addition I've done in the past has resulted in accurate results for my calculated size.

 

[pabeader]

Why did you add the bottled water to the pool sample?

 

[fwb]

Why did you add the bottled water to the pool sample?

To dilute it so I didn't have to use an insane amount of reagent. I also tested the bottled water after to ensure it tested at 0.0 FC

 

[domct203]

Bottom line: Added 8 gal 10% hypo to <1 FC pool, now testing shows I have 61 FC? Pool calculator said 8 gal would add 30 FC.

Tested after 24 hours running the pump. I added my pool water to the 10mL line, added bottled water to the 44 mL line for dilution, 2 powder scoops, and it took 28 reagent drops to turn the pink clear. 28*0.5*4.4 = 61.6 FC if I did my math right.


Background:
During July my SWG was having trouble keeping FC up and I noticed green algae started to form on the rock features (water was clear though). Had my SWG set to 100% and pump running 12 hours/day. My numbers at sunset were:

<0.5 FC
0 CC
80 CYA
7.4 PH
3500 salt
300 CH
90 TA
100 Phos
Air temp: 105* avg high

According to the SWG calculator on this site, my system should generate 7.2 FC per 24 hours, and since I ensured my SWG was clean, visibly producing chlorine gas, no error lights, water perfectly balanced, etc, my last resort was to shock the pool because something must be eating all my chlorine. Can't really do the OCULT because I have to run my SWG and pump and night due to peak electricity costs. Hope this fixes it because I am out of ideas, but I don't understand the 61 FC reading after shocking. I used the pool size calculator to determine my pool size, and every chemical addition I've done in the past has resulted in accurate results for my calculated size.

128oz of 10% sodium hypochlorite will add 3.6ppm FC to your 28000 gallon pool, times 8 gallons is 28.8ppm of FC added. It is chemically impossible for the pool to be at a FC of 61ppm. Even if the bleach was 12.5%, that is still only 4.5ppm FC per gallon, or 36ppm for 8 gallons.

You have some serious testing error here.

Any time you dilute the sample you also decrease the accuracy.

Repeat the FC test without dilution.

 

[BestJoeyEver]

You can do an OCLT, just disconnect your SWG, but leave your pump running. Just make sure your FC is high enough that it's not going to drop too low before you start (and if it does, you need to SLAM anyway so it won't matter).

 

[needsajet]

Nice to meet you via TFP That's all good advice above.

If you want to save drops, and give up a bit of accuracy, you can use a 5 ml sample of pool water, one scoop powder, and then each drop = 1 ppm FC.

After your 24 hours, you had 14 ppm FC. It's the FC in the original 10 ml of pool water; each drop neutralized 0.5 ppm FC; unaffected by the dilution. A dilution would only work if you diluted the sample a known amount, and then withdrew 10 ml from the newly created dilution. Each drop would = 0.5 FC, which you would then multiply by whatever dilution factor you used. As mentioned, This is not recommended due to the likelihood of error.

The only way you'll kill off the algae is with a SLAM Process. Follow the printed SLAM instructions to the letter. There is also a video that gives a visual idea of how it's done, but print out the instructions to keep them handy, and use those instructions. Before you start the SLAM, and while your FC is under 10 ppm, it's helpful to adjust pH to 7.2

It's quite normal to leave the pump running overnight for checking the OCLT during the SLAM. Your pump should be running 24/7 for killing off the algae, and this is part of the SLAM instructions. Just set the SWG to 0% when doing an OCLT.

Your SWG will keep up after the algae is killed off. The SJ40 in 28000 gallons will need approx. 10 hours per day at 100% to produce 3 ppm of chlorine.

 

[jeffchap]

Diluting your water also has the effect of diluting the reagent, so your test becomes invalid.

 

[fwb]

Thanks for the great answers everyone, I had a feeling it was a measurement error but I didn't understand why. My OCLT resulted in 1.5 FC loss over 8 hours. 7.5 to 6.0 FC. Not quite good enough... Should I dump in another 8 gal of chlorine(10%) to get back up to full shock level? Also, my CC is still <0.5 as it has been every time I've tested, why isn't higher if chlorine is being used up? Water still remains crystal clear.

Just cleaned my cartridge filter which should help, it was a mess and 5 psi over baseline. And my 55 lb shipment of boric acid just showed up, which I'm also hoping will help...Should I wait until post-SLAM to add that in or should I do it now?

 

[BestJoeyEver]

If you didn't pass the OCLT, then you're still in a SLAM and should follow that protocol which means regular additions of chlorine to get back to maintain the level.

Regarding your borates, normally adding borates will drive up your pH, necessitating testing. PH testing is not reliable at hight FC levels. Now the fact that you're using boric acid should in theory negate the pH rise, so you might be ok. Someone else might have a more definitive answer, but if it were me, I'd wait until my FC was in target to start the borate addition.

 

[needsajet]

Sounds like you're making good progress!

CC and algae are not related. When algae is oxidized by free chlorine, the chlorine becomes chloride. CC is created when FC combines with things like ammonia. It's the easiest criteria to meet at the end of a SLAM, and normally not a concern.

If PoolMath says 8 gals of 10% is the right number to get up to 31 ppm FC (for your CYA level of 80 ppm), then yep . It does sound about right to me. Have you been able to top up your FC 3 or 4 times per day?

I would hold off on borates for a while. Wait until after the SLAM and until after you get TA down to 50 or 60 ppm, and pH into a comfortable range.

 

[tim5055]

Should I dump in another 8 gal of chlorine(10%) to get back up to full shock level?

I think you are missing the key point of the SLAM Process process, the maintain part. A SLAM Process is not just dumping it in once, you do it multiple times a day keeping the pool at shock level as much of the time as you can. The minimum is 3 times testing and adjusting back to shock level each day, but more is better.

Also, my CC is still <0.5 as it has been every time I've tested, why isn't higher if chlorine is being used up?

Because you are in Las Vegas. CC is eliminated by the UV rays of the sun, lots of UV = little/no CC. In your case the CC is probably being eliminated as quickly as it is produced.

And my 55 lb shipment of boric acid just showed up, which I'm also hoping will help...Should I wait until post-SLAM to add that in or should I do it now?

Wait. Borates are the last ting you would want to do (totally optional process) and should only be added to your pool once it is balanced and you TA/pH are stable.

 

[COBrien]

Agree with tim5055. You have to maintain shock level as best you can.

When I SLAMed my pool, I was testing and adding morning, noon, and evening, as well as once or twice in the middle of the night. In my experience, nighttime is the best time to test and add, as your FC is able to fight algae without being eaten up by sunlight.

What follows is my personal experience, and not necessarily endorsed or supported by anyone else at TFP.
I see you have a plaster pool. If you're unable to test and add chlorine throughout the day, you can do like I did toward the end of my first SLAM. After testing my water and plugging my results into PoolMath, I would round up (i.e., if PoolMath called for 3.2 gallons, I'd add 4), then add one more gallon "for good measure." With plaster, it's easier to get away with this -- liners, not so much. This keeps the FC at or above shock level for longer, allowing you additional time between tests before the FC level falls too low.