Overnight chlorine drop test - FC rises

Jaywalker

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Dec 4, 2009
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San Antonio, Texas
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I'm four days into a chlorine shock for mustard algae and for the last two days the morning Overnight Chlorine Drop Test reading is higher than the previous night. That's never happened to me before - am I missing something?

Assuming it isn't normal, I can think of three explanations: experimental error, tri-clor tab feeder leak, and maybe something like dead algae releasing chlorine to be measured.

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FAS test, CYA 50, target 29.
Yesterday morning the reading was 30.5 compared to the previous evening 30.0, using older (but still newer than a year-old) powder and reagent. I didn't intentionally add any chlorine yesterday. This morning the reading was 23.5 compared to last evening's 20.5.
 

Texas Splash

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Let's make sure of something first. If you are treating for MA, you first completed your normal SLAM right? A CYA of 50 would've been an FC of 20. If you passed all 3 SLAM criteria, then you do the elevated FC level for the MA treatment but only for 24 hrs. That's it. No OCLT there because you already did one for the regular SLAM.
 

Jaywalker

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Dec 4, 2009
96
San Antonio, Texas
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Let's make sure of something first. If you are treating for MA, you first completed your normal SLAM right? A CYA of 50 would've been an FC of 20. If you passed all 3 SLAM criteria, then you do the elevated FC level for the MA treatment but only for 24 hrs. That's it. No OCLT there because you already did one for the regular SLAM.
I did two days of FC 20 before starting the four days I mentioned in the original post. I didn't do the OVDT on the first two days, just the MA part.
 

Texas Splash

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I did two days of FC 20 before starting the four days I mentioned in the original post. I didn't do the OVDT on the first two days, just the MA part.
If you do indeed have MA, that was not the correct process. Maintaining a MA FC level for that long is not recommended, nor is an OCLT required as part of the MA process. Only 24 hrs for the MA level and only after completing the normal SLAM Process and passing those 3 criteria first.
 

Jaywalker

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Dec 4, 2009
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San Antonio, Texas
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I wonder if this recommended process has changed since the last time I had mustard algae... Since I go seven years or so between bouts, I'll check the current recommendation the next time.

I note, however, that I still don't know why my overnight chlorine drop test showed increases the following morning, and that was my question. any ideas?
 

proavia

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Is your tab feeder empty? Turning its dial off doesn't guarantee it actually doesn't have some water flowing thru it and putting chlorine into the pool.
 

Jaywalker

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Dec 4, 2009
96
San Antonio, Texas
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Is your tab feeder empty? Turning its dial off doesn't guarantee it actually doesn't have some water flowing thru it and putting chlorine into the pool.
No, it's just turned off; I expect it's leaking, since the pump's on 24-hours.

I was just reading about phosphates and how algae take them up to grow and release them when they die - I wondered if was a recognized phenomenon. Wasn't likely - I searched - but there was always the possibility I was using the wrong search terms. Thanks.