2ppm drop overnight

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G

Gig103

0
Jun 2, 2014
53
Phoenix, AZ
Hi there,

My water is still clear, but I noticed my FC dropped more than usual the other day, so after adding bleach and waiting an hour I tested last night at 10ppm, no free, PH of 7.8.

This morning I tested early to get an idea if an overnight loss and it came up at 8ppm. 2 overnight, but the water looks great!

Is this because my PH is high, reducing the chlorine effectiveness? Should I SLAM the pool? Or is there a different, less involved way to keep things in check when the water is clear?
 
A 2 ppm loss overnight measured with a FAS-DPD chlorine test means that something that shouldn't be there is consuming chlorine. It wouldn't have anything to do with PH. None of the other chlorine tests have enough precision to do an overnight loss test.

Could you post a complete set of water test results?
 
I will collect a fresh test set tonight after sundown. I was using FAS-DPD last night and today for my chlorine check. It was daylight by the time I tested today but mo direct light on the pool yet (about 8am from 1230 the night before).
 
Okay Jason (and everyone else), I ran a set of tests tonight, water sampled after the pump had been on 3 hours. Oh and FWIW the skimmer basket was jammed full of leaves - part of the problem?

FC 5.0 (EDIT2: was 8.0 this morning, 10ppm last night)
CC 0
pH 7.4 (I added muriatic acid at 8pm b/c I knew it was high from last night)
TA 150
CH 175
CYA 100 (Edit: This can't be lowered, my wife refuses to let me "waste" water. She hates the pool and doesn't care what the cost of the chlorine and effort is on my part since she never uses it)

So 2ppm overnight, and another 1ppm in the day. I re-read the SLAM and since my OCLT is > 1.0, I figure I need to keep at it. My question for the forum is if I should go to 39ppm per the chart or if a "mini" slam is all that's needed when water is clear?
 
Last edited:
How confident are you that the CYA is only 100 and not higher?

You need too SLAM to the full shock level. Although very odd (impossible?) that you lost less during the day than overnight.
 
jblizzle said:
How confident are you that the CYA is only 100 and not higher?

You need too SLAM to the full shock level. Although very odd (impossible?) that you lost less during the day than overnight.
Click to expand...

Thanks for confirming that the full level is required even when the water is clear.

Meanwhile I am confident in the 100. I used a half sample and it came up as 50. I haven't put anything but bleach in since Memorial Day when I started reading Pool School. I will make sure the chlorine is 39ppm tonight and check it in the morning. I know that isn't ideal monitoring frequency when starting a SLAM, but I will see what happens.

- - - Updated - - -

jblizzle said:
How confident are you that the CYA is only 100 and not higher?

You need too SLAM to the full shock level. Although very odd (impossible?) that you lost less during the day than overnight.
Click to expand...

Jason I checked my notes and you are right. It was 10ppm last night, 8ppm this morning (when I posted the thread), and 5ppm tonight.
 

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A more generous scoop of R0870 had a proper test result. Something is still chewing up chlorine and I'm not sure what. My wife commented how nice the pool looked today - could it be the cartridges in need of a rinse? If so, is it okay to stop the pump for 30 or so minutes to spray them off? Rules do say to run non-stop!
 
It will consume some chlorine if there are organics trapped in the filter. As I said, you need to clean it when the pressure rises which indicates that stuff is trapped in it.
 
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