Overnight OCLT - 3.5ppm FC lost, yet chemistry and clarity look good?

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G

GregoryRG

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LifeTime Supporter
Jul 15, 2015
30
Scituate, RI
In a different post I was concerned that my SWCG wasn't putting out anything close to the maximum rate (like 50% of it). But the chemistry yesterday evening before I shutdown to specifically run the test was:


pH 7.5
FC 6.5
CC 0.5 (or less)
TA 80
CH 75
Borates 40 to 50
CYA 70 to 80 (between 70 and 80 on the black dot/graduated cylinder test)
Salt - 3300 ppm (I added 15 lbs - was at 3000 pm)

This AM:

FC 3.0
CC 0.5 or less

I am surprised (maybe I shouldn't be?!?) that I've lost 3.5 ppm FC overnight with great water clarity, and the above #'s. I guess I am to deduce from this that something organic is going on, but at the microscopic level??? I just let the pool sit, uncovered without the filter pump running. I checked my test twice this am and got the same results both times.

Also, any potential skewing of results based on:

1. It was windy/breezy last night and stuff got kicked up into the pool. Even fine sand to the point where I just cleaned the bottom again this AM after during so yesterday. Could any small leave debris, unfortunate insects, etc. impact FC usage overnight?

2. I added 15 lbs of NaCl @ 3pm to bring my salt level up some, ran just the filter pump, and agitated the water. Could this impact the results?

I'm heading out to SLAM it right now (it's covered right now, but I have a couple of floats keeping up the cover enough to run the FP).


Any thoughts or experiences with this?

Thanks,

Bob
 
Hello again Bob. Your numbers themselves (above) do look good. And I'm not sure if this will totally explain your OCLT, but a couple things I see:
- You started the OLCT at an FC of 6.5. That's actually too low. An OLCT should be at your SLAM FC, but never less than 10 ppm. So next time you perform an OCLT you may want to consider having it higher.
- Yes, organics can be present without you actually seeing the full effects. I've had that happen to me as well. Same with zero CCs. You can have algae and have no CC because the chlorine has not begun the breakdown or start the conversion process yet to transition to CC.

Starting a SLAM today is probably a good thing based on that amount of FC loss overnight. This evening/tomorrow, try the OCLT again but with your FC closer to the SLAM level and see if that gives you a better outcome.

FYI - Don't feel alone. I see/talk about this stuff everyday, and I'm having an OCLT issue myself with absolutely crystal clear water and no CCs. But in my case, I think I over-shot my FC too much (again), based on my CYA, which skewed my OCLT. :brickwall: It can be frustrating sometimes. I didn't have this much trouble last year. Hang in there, and good luck.
 
Thank! I started a SLAM cycle about an hour ago, added bleach to get me up to my SLAM level for my #'s (28 to 29 from pool math). I'll probably try to run the OCLT overnight as I just told my wife the pool was "closed" until further notice - neither she nor the kids took that news very well!


PS - I need to donate to TFP - it, and people like you, have been invaluable resources to me already (with only 20 or so posts to my credit).
 
I am another TFPer with crystal clear water, algae and chlorine consumption. A month or so ago I had to SLAM. My pool has been perfectly clear for 3 years, my FC dipped to 1.5 (tired and lazy!) and my SWG/FC couldn't keep up. Clear water, 0 CC. I got in and found little fuzzy patches of algae on some benches and rocks. Started my swim-n-slam for a week and a half. Also found algae behind the light and in the pool cleaner. It is a lot easier to brush every day in the pool! Kept the FC just under shock level and swam and brushed daily.
 
Update:

1. SLAM started @ 1:30 Brought FC to 31 (overshot by 3ppm), CC 0.5
2. Checked FC/CC @ 8:00 PM (couldn't get to it earlier) - FC 24.5, CC 0.5 (pulled cover and pool was in direct sun from 1:30 to 5:30 (75F ambient air temp).
3. Added 33oz Bleach @ 8:00 PM (pool math said FC should drive to about 28.5)
4. Checked @ 9:00 PM - FC 33.5, CC 0.5 (seems likes I should have added about half the amount of bleach I did, but at these high FC #'s, things aren't exact).
5. Total Bleach added for the day: 222 oz

OCLT ongoing, checking before work tomorrow.
 
Lost 3.0 ppm last night and the night before. Still going @ 28 to 30 FC. I have a suspicion I'll find the source underneath the awful flap that Intex leaves at the bottom of the pool where they attach the pool bottom to the side wall/curtain. I've seen a post somewhere that someone actually trimmed it back. I haven't found anything in the 2 suction side inlets nor the single pressure-side out nor the drain plug at the bottom of the pool. I have to snorkel it tomorrow.
 
GregoryRG said:
Thank! I started a SLAM cycle about an hour ago, added bleach to get me up to my SLAM level for my #'s (28 to 29 from pool math). I'll probably try to run the OCLT overnight ...
Click to expand...
Just a note that the pump should be running for an OCLT. I noticed in your first post, when you described your overnight test procedure at normal chlorine levels, you had the pump off.

Keeping the pump running isn't in the OCLT instructions in Pool School, but it seems to be the universal opinion when you search on the topic. Maybe it's assumed that, since pump is running 24/7 during a SLAM, the pool owner will simply keep it running during the OCLT which (hopefully) marks the end of the SLAM.
 
Bob, I'm coming into this show late, but I just wanted to say don't let clear water or seemingly good test results fool you. I just went through this myself. My water was so clear that the sun sparkling off it hurt my eyes and the test results were right in line. But, my OCLT was a 2ppm loss. A quick 4 day SLAM and everything was back in order.

Just to give you the short history we went away for five days and I left the Stenner pump turned off. I came home to clear water but zero chlorine. I got chlorine in the water and I thought things were good. But over the next few weeks my chlorine consumption slowly went up, thus my clue to do the OCLT. So, something was in the water and growing slowly. It just never got to the point of bursting out and clouding the water.

This is what TFP is all about. Know your pool, and with regular accurate testing you can find little problems before they become big ones.
 

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GregoryRG said:
Thanks. Being a rookie, I never would have thought there was something lurking in what looked like perfect water!
Click to expand...
Therein lies the problem with pool store twice a month testing. We want you to know your pool and see little differences. You start using more chlorine, there is a reason. Sunnier days, big pool party or algae. When you know your pool you will know why!
 
I'm still going with the SLAM routine and am very surprised I'm still losing FC overnight (2.5 ppm). I can't find anything anywhere in the pool that could be source whatever is consuming the chlorine. Nothing visible - no algae, no organic debris, nothing! Here are my latest #'s as of this PM:

pH - 7.8 (may not be accurate due to the high FC)
FC - 28.5 (approximate SLAM level)
CC - less than 0.5
CH - 50
TA - 100 (seems to be getting driven up, as it was rock steady at 80 until starting the SLAM)
CYA - 70
Salt - 3200
Borates - 40ish

Any thoughts? Am I chasing a ghost? I thought I read somewhere that ammonia could be a culprit?
 
At this point I don't think it is ammonia. That usually only occurs at the beginning of a SLAM and it will consume all your chlorine in an hour or two, or less. Have you removed/checked/scrubbed lights, skimmers, ladders, pool toys, pool cleaners/vacs, etc? Those items can harbor or reintroduce algae to the pool. What about brushing daily? Brushing exposes the algae to the chlorine and kills it faster. Algae forms a biofilm over itself and is slower to kill without brushing.

Definitely ignore your PH test, it is not valid with FC above 10ppm.
 
Pooldv,

I have a simple Intex Ultra-Frame vinyl pool with no lights, etc. I've scrubbed the whole thing including all the two suction side inlets and one return, as well as my over-the-side skimmer. I've scrubbed behind the seams on the floor, at the floor-wall interface, etc. I didn't see any indications of algae (I actually snorkeled the whole time, getting up close to everything).

But, it's gone, whatever it was!:) Lost 0.5 ppm FC last night with 0.0 ppm CC.

Could not have done it without TFP and the Forum.
 
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