SLAM Confirmed - But FC falling the day afterward - Expected drop after SLAM? Ideas?

[bobandsherry]

Despite crystal clear water, pool being in a screen enclosure and CYA of 70, I was seeing a drop of 8FC per day. So I went through a 3 day SLAM.

Started SLAM the morning of 7/23, started with adding about 3 gallons of 10% chlorine (target 28FC). Checked water every hour or two and dumped in the required chlorine.

Nightly OCLT tests:

• First night (7/23): 7PM: 27.0 Next 9AM: 16.0 FAIL - SLAM continued
• Second night (7/24): 8PM: 29.0 Next 7AM: 24.0 FAIL (but closer) - SLAM continued
• Third night (7/25): 8PM: 26.5 Next 7AM: 26.5 PASS (or so I thought) - Added 24 oz Chlorine just to bring FC back to 28 and brought SWG back online, set to 60% (5.1PPM / day)

SLAM Success Checklist:

1. CC is non-existent (has been so since day one) - CHECK
2. OCLT - CHECK
3. Water is clear (it's sparkling) - CHECK

So all indications are everything is back to normal. The next day (yesterday, 7/26) I kept an eye on the FC level. Tests during the day:

• 7AM: 26.5 (added 24 oz chlorine to bring FC to 28, turned SWG back on at 60% which should generated 5.1PPM / day)
• Noon: 27.5
• 3PM: 22FC (added 90oz chlorine to bring FC back to 28)
• 5PM: 26.8
• 10PM: 27.0 (Looks like it's all settled for the evening)

I checked the water this evening (9PM, 7/27) and FC has fallen to 20.8. That's a drop of 11.3 (6.2 + 5.1) for the day. Soooooo..... does that seem like a reasonable drop? I'd hate to let it continue to fall and have to go back to SLAM. Ideas?

Thanks!

 

[dkimble]

I don't believe the slam only took 3 days, and that amount of FC loss fixed itself that fast... To me that seems unrealistic, but I am not an expert.

 

[BestJoeyEver]

The most likely culprit is unfortunately a testing error. Your post seems really organized, so I'm leaning towards you having things together, but I would suspect that either your "pass" on the OCLT was flawed, or your test where you saw the drop. My money would be on the 3rd OLCT. How confident are you in those test results? You had a pretty high drop to be fixed in 3 days (although it sounds like you were very diligent).

 

[PoolUmp]

I have discovered this year that FC loss when FC levels are at SLAM level is very high (during the day). OCLT is a good indicator. If you passed OCLT, try letting your levels drop down to normal level for CYA of 70, and see if it holds better. Mine sure did!

 

[BestJoeyEver]

That's a good point. Y a u could always let your levels drop and perform another OCLT.

 

[Akathisia]

I would also suggest that you could test with 10 mL water samples to save on reagent. 0.5 ppm per drop is fine enough detail--especially at high FC levels.

 

[domct203]

I have discovered this year that FC loss when FC levels are at SLAM level is very high (during the day). OCLT is a good indicator. If you passed OCLT, try letting your levels drop down to normal level for CYA of 70, and see if it holds better. Mine sure did!

Correct.

The higher you are from your FC daily target, the more FC you will lose to UV (sun).

Once your FC is below 20% of your CYA the daytime losses should stabilize.

 

[bobandsherry]

Thanks all for the comments/suggestions.

@BestJoeyEver - I am as confident on the 3rd OCLT test as any of the others. What I didn't mention, in addition to the 8PM on 7/25 test I also did a test at 11PM, it too showed 26.5. And did my test twice in the morning for the OCLT. So about as accurate of a test result as I could get.

@Akathisia - Yep, I was testing using the 10ml during the tests as I was SLAM'ing. I went back to the 25ml to be eliminate any +/- accuracy in the smaller sample size.

@PoolUmp and @domct203 - I'll let 'er drop and see if the FC starts to stabilize once it hits 14 or so.

I did another OCLT last night, I left the SWG running and results are in line with what I would expect. Results from last night - SWG @ 60% generates 5.1FC/day, so for 10 hours = 2.1FC

9PM: 20.2
7AM: 22.4

20.2 (9PM test) +2.1 (10hr SWG) = 22.3 which shows that my morning read at 7AM passes another OCLT (and that my SWG is generating as expected).

 

[denniswiseman]

While trying to do an OCLT I was under the impression that the SWCG shouldn't be on

 

[domct203]

While trying to do an OCLT I was under the impression that the SWCG shouldn't be on

That is correct. The entire point of the OCLT is to see if organics are consuming FC, that will be impossible to identify if you are supplementing FC with the SWG.

 

[bobandsherry]

All previous OCLT was done with SWG off, SWG remained off during SLAM until OCLT passed.

I wasn't planning a OCLT last night as I wasn't sure I'd get a morning test. Unless I'm totally off base, the SWG is only adding FC. My OCLT last night factored in the FC produced by SWG by adding to starting FC. Is something is wrong with that approach?

 

[denniswiseman]

I would be inclined to leave the SWCG off when slamming and OCLT and not try and factor in what you estimate it's producing

 

[domct203]

All previous OCLT was done with SWG off, SWG remained off during SLAM until OCLT passed.

I wasn't planning a OCLT last night as I wasn't sure I'd get a morning test. Unless I'm totally off base, the SWG is only adding FC. My OCLT last night factored in the FC produced by SWG by adding to starting FC. Is something is wrong with that approach?

I did another OCLT last night, I left the SWG running and results are in line with what I would expect.

You should not refer to it as an OCLT then as it is misleading, especially to a newbie reading through threads. A true OCLT is done without any additions of chlorine.

 

[bobandsherry]

I would be inclined to leave the SWCG off when slamming and OCLT and not try and factor in what you estimate it's producing

SWG was off during the SLAM. OCLT was past several nights earlier, so was not in SLAM.

You should not refer to it as an OCLT then as it is misleading, especially to a newbie reading through threads. A true OCLT is done without any additions of chlorine.

Yep, I should have just said I was validating the chlorine lost overnight, but that's really the meaning of OCLT (overnight chlorine loss test). But ignoring the semantics, is there anything fundamentally flawed with the validation I did last night?

 

[domct203]

.............is there anything fundamentally flawed with the validation I did last night?

Not at all, that is a good way to estimate SWG production if the previous nights OCLT was zero (I say estimate because there are so many variables in play).