Does this make sense?

Jun 14, 2012
11
PA
#1
Last night, after shocking for most of the afternoon, i took my final chlorine test for the night. At 830pm this is what i got FC: 18.5 and CC: 0.5. This morning at 530 i took a test and this is what i got, FC: 10.5 CC: 0.5. I had to work all day so nothing got added today. when i got home from work i took another test at 430 and got FC: 6 CC: 0.5.

what i don't understand is if organisms break down chlorine, and so does sunlight, why didn't i lost more FC during the day when the sun AND the organisms were breaking down the chlorine? it doesn't make any sense to me why i would only lose 4.5 FC during an 11 hour period where the pool probably recieved 8 hours of sun, but 8.5 during a 9 hour period at night. am i missing something? help please.

i tested everything else 2 days ago and my results were:
PH 7.5
TA 110
TH 200
CYA 40
 

danivdp

Well-known member
Mar 16, 2011
91
Sunny Central Florida
#2
I'm guessing you killed most of the organisms last night, which ate up more of your chlorine. Today during work, the majority were dead, and therefore, ate less chlorine :) You are in the home stretch!!
 

257WbyMag

TFP Expert
LifeTime Supporter
Feb 23, 2008
5,061
Denton, TX
#3
So you lost 8 ppm of your FC overnight. You aren't finished until you have gotten to where you lose no more than 1 ppm of FC overnight. Keep going! What test kit are you using?
 

TonyM

Well-known member
May 11, 2012
85
Middle Tennessee
#6
vanmic35 said:
I just don't understand why I lost more at night than during the day. I have a tf-100. Shouldn't I lose more during the day?
I thought the same thing when I was fighting algae, but learned a helpful tip - think of FC loss as a percentage instead of just the delta. when you look at it that way, you lost almost the same amount:

Night reading:
18.5 - 10.5 = 8 ; 8 / 18.5 = 43.24% loss

Day Reading
10.5 - 6 = 4.5 ; 4.5 / 10.5 = 42.86% loss

Tony
 

feetr2c

Well-known member
May 13, 2012
72
#8
TonyM said:
vanmic35 said:
I just don't understand why I lost more at night than during the day. I have a tf-100. Shouldn't I lose more during the day?
I thought the same thing when I was fighting algae, but learned a helpful tip - think of FC loss as a percentage instead of just the delta. when you look at it that way, you lost almost the same amount:

Night reading:
18.5 - 10.5 = 8 ; 8 / 18.5 = 43.24% loss

Day Reading
10.5 - 6 = 4.5 ; 4.5 / 10.5 = 42.86% loss

Tony

Hmmmm. Applying that same method,

10 - 2.0 = 8 ; 8 / 10 = 80% loss? Can't be. And in your above example, 6 is more than half of 10.5, so therefore your loss is over 50%.

It should be:
(10. 5 / 18.5) * 100 = 56.76% loss for his night reading and,

(6 / 10.5) * 100 = 57.14% loss for his day reading.

Your calculations would have worked if you carried it one step further and applied the inverse, i.e. subtracted your final answers from 100.

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TonyM

Well-known member
May 11, 2012
85
Middle Tennessee
#9
feetr2c said:
10 - 2.0 = 8 ; 8 / 10 = 80% loss? Can't be. And in your above example, 6 is more than half of 10.5, so therefore your loss is over 50%.
Yes, the above example would be an 80% loss.

If FC = 10 and then went to 2, that would be a loss of 8, or a 80% reduction. the ending number (2) is 20% of 10.

*The above example would also send me into a full-out algae panic, causing me to lose 80% of my chlorine inventory :shock:

feetr2c said:
It should be:
(10. 5 / 18.5) * 100 = 56.76% loss for his night reading and,

(6 / 10.5) * 100 = 57.14% loss for his day reading.
You are dividing the ending FC number by the original FC number, which is not the amount lost. You formula is determining the current level of chlorine (in percentage), assuming the high number (18.5) is the max and 0 is the low (which is why your number + my number = 100%).

Look at the first example; The original FC was 18.5. Just to make things simple, 50% loss (half) would mean the ending test would show 9.25 FC (18.5/2). Any number larger than half would be a percentage loss less than 50% - any number less than 9.25 would be a larger loss than 50%. He went from 18.5 to 10.5, which meant he lost less than half of the FC, or .4324324 (43.24%).

Enough with the math... lets go swimming :cool:
 

feetr2c

Well-known member
May 13, 2012
72
#10
My bad.... I only looked at your formula and read it that he lost 10.5 and ended up with 8, if which, then my calculations would've been correct. I understand now. And yes. A swim is in order :p

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