chlorine after SLAM

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tracyd

tracyd

Well-known member
Aug 11, 2013
61
North Kingstown, RI
Happy summer! After SLAM, should I expect my Free Chlorine to decrease rapidly in the hot sun (its 90 degrees/sunny here today)? Long story short, I brought my chlorine to 22 after the sun went down last night and only needed to add 1lb cal hypo throughout night to maintain 22 (at 10 pm). I was actually up all night checking the level every 1-2 hours. Right before the sun came up I checked it and the level held with no CC. I checked it at 9am and it dropped to 20. 12 pm 18. Should I be worried about this drop or is it to be expected?
these were my readings at 6am

CYA 50
TC 22
CC 0
PH 7.4
TA 80
I have another question as well. Im using cal hypo. It increases my PH. Is this temporary? I adjusted my ph w muriatic acid an hour after the initial SLAM because the PH was so high. Should I have done that? Is my PH going to bottom out after the chlorine burns off? My TA increased as well but I didnt add baking soda as it was 55 before the chlorine was added. I appreciate any input. Also, The reason I brought the chlorine level to 22 is because I never trust my eye with the CYA reading and would rather go alittle over than under and have to repeat the process.
 
It’s normal for chlorine demand to increase when there is a string of sunny days. Right now with the weather in the 90s and full sun and pool water in upper 80s, I lose probably 4 FC a day with the pool uncovered. Remember, the higher you keep you FC the quicker it falls because it burns at a %. I don’t recall details, but someone on here told me that when I had a similar situation.

Also remember that ph readings are not accurate when FC is 10+
 
You addressed a few items so let's be clear about everything, the most important right now - your suspected FC loss. You were testing the FC off & on over night, but it doesn't sound like you were actually performing an Overnight Chlorine Loss Test. I say this because you would've had to do the 2nd FC test early in the morning before the sun hit the water, but you didn't test the FC until 9:00 am. But yes the sun will steal your FC quickly when the FC is at higher SLAM levels during the day. But at night, you should still lose no more than 1 ppm. So before you add stabilizer to increase the CYA, be 100% sure you are algae-free and clearly passed the OCLT.
 
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Speaking of CYA, if you are at 50 right now, normally that's adequate enough for your region. But if you are sure about passing an OCLT and wish to increase the CYA a bit more you can, just be sure to keep the FC at the new appropriate level as noted on the FC/CYA Levels.
 
Yes I tested at 6am before the sun hit the water. My water was crystal clear, the only reason I did the SLAM was because my chlorine demand was more than it has been since owning the home 2009. I was adding 2+ lbs/ day just to maintain which is very unusual for my pool, heat and all.
 
tracyd said:
es I tested at 6am before the sun hit the water.
Click to expand...
Okay, very good. So after re-reading above, you are saying that the FC held (at 22) all night (with no chlorine added) and never dropped until daylight correct? If so, that's great.
 
I brought it up to 22 right after the sun went down the added an additional 1 lb at 10 pm which brought it back up to 22. 0600 it was 22 no CC, actually it didn’t have discernible CC yesterday prior to SLAM but my chlorine demand was higher than usual so I chose to SLAM.
 
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Okay, so my estimates show that 16 oz of that cal-hypo should have increased the FC by a little over 2.5 ppm. That'[s quite a bit. So right after the sun went down the FC was 22, then you added about 2.5 ppm more by 10:00 pm to maintain an FC of 22. This was ta a time when there was no sunlight out. It doesn't seem right. I understand initially your main question was about how much FC loss is considered reasonable during the day or after the SLAM. As stated, the FC will fall quicker post-SLAM because the ratio of FC to CYA is different and simply put there is more chlorine than CYA to bond to for protection. But at night that shouldn't be an issue which is why I'm a bit concerned about how much FC you had to add in the dark hours.

So I'll leave you with this....... Yes, you can expect the FC to fall quicker after the SLAM. But before you consider any other options like adding CYA, you might consider doing another OCLT to be sure. IMO, for the OCLT I would do my final dosing with liquid chlorine. I just find it more predictable and easier/quicker to dispense. That's just me. During the OCLT, you have the option to shut the pump off at some point, again (IMO) I would just leave it running. Low speed is fine. The important part is to keep the water circulating for an optimal/reliable FC reading. Be sure the final liquid chlorine you add has had at least 30 minutes to mix after dark or before your bedtime FC test. Then of course take the morning sample before the sun hits the water.

I hope this all helps. If you have questions or would like for me to address something else more specifically let me know.
 
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I just finished a SLAM and shared your concerns. My understanding is that CYA could not protect the elevated amount of chlorine in the water. Mine was dropping very fast (like 10-12 ppms in a day) until I got to my target levels. Now the number of lost ppms is much lower.
 
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