Always amazed how much chlorine full sun sucks up

Ronald U.

Bronze Supporter
Every year I go through my early season dis-belief at how much my FC goes down every day. Right now it seems to go from 6 to 2 (or lower) every full sun day. The house is on the east side of the pool and large trees on west but for at least 8 hours the sun hits pool directly. I am now onto a new batch of chlorine, 10% from Home Depot, $9/ 3 gallons. Seems like its holding out a tiny bit better than slightly older 12.5% from Leslie's ($20/4 gallons) was. I store chlorine in hot garage, which may speed deterioration? Have fun out there in pool land! Much thanks to TFP for putting me on the right track.
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
43,001
Tucson, AZ
It is a little confusing, but while your FC target is higher, you should lose fewer ppm every day. Think of the CYA as protecting a % of the FC each day.
 

RebelBob

Well-known member
Jun 7, 2018
52
Virginia
@jblizzle
I have always wondered why the target level doesn’t increase as cya increases, i.e. it looks like the chart doesn’t give any credit for lower loss of chlorine until around a cya level of 70. Why is that?
 

RebelBob

Well-known member
Jun 7, 2018
52
Virginia
I Mispoke and I have got myself really confused. I meant the difference between target and minimum which seems to me should DECREASE some as cya goes from say 30 to 40 to recognize that burn off will be less.
 
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IceShadow

Gold Supporter
Jun 8, 2019
803
Milwaukee, WI
Minimum and target levels only have to do with what is required to maintain a sanitary pool (bacteria and algae killed), not with how much is lost per day.
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
43,001
Tucson, AZ
I Mispoke and I have got myself really confused. I meant the difference between target and minimum which seems to me should DECREASE some as cya goes from say 30 to 40 to recognize that burn off will be less.
It does not show up in the chart because it is masked by the rounding that is done. Going from 30 to 40ppm for CYA might result in you losing a fraction of a FC ppm less. But to be safe the charts round up.
 

RebelBob

Well-known member
Jun 7, 2018
52
Virginia
Thanks for explaining that. BTW, I have always interpreted “target” to mean what I should aim for when adding bleach each evening. Is that not correct?
 

thetekgeek

Bronze Supporter
Jun 8, 2018
117
Nebraska
Yes, target is what you should aim for, knowing that you will lose some to the sun and some to contaminants throughout the course of the day. If you can test at roughly the same time each day, you'll know exactly how much you lose each day and can adjust your target up or down within the target range so you never dip below minimum. Some days, I dose to the top of the range while others I shoot for middle depending on what I see in the forecast, or whether the solar cover is staying on longer.