# Chlorine level "bounce?"

#### dntboles

##### Well-known member
I'm trying to understand chlorine consumption better.

Does FC drop more quickly if it's a higher level than target, or is it more of a constant drop, based on UV, organics, etc?

Hypothetical example (assuming constant level of UV, organics, etc):
Assumption: Add chlorine to bring FC to 6,0, and it theoretically drops 1.0 per day, reaching 2.0 after 4 days.
Scenario: Add chlorine to bring FC to 10.0, would it theoretically:
A. drop at 1.0 per day, taking 8 days to get to 2.0, or
B. does the higher starting FC level affect how quickly it drops off?

I've recently SLAMmed to clear up some haze from all the leaves falling into the pool and now the water's nice and clear and I'm still high on FC. I don't want to keep FC high if it's going to cause it to drop faster, meaning I'd use more chlorine. But if it's pretty much a straight line, I'd consider keeping it higher to knock out all the crud from the leaves that keep falling in the pool.... that is until someone tells me why I shouldn't. And I'm guessing there's a reason I shouldn't.

FC: 10
PH: 7.5 at last reading, before SLAM
TA: 60
CH: 350
CYA: 30
CSI: -0.49
TEMP: 51

#### JamesW

TFP Expert
The higher the FC/CYA ratio is, the faster you lose chlorine.

dntboles

#### dntboles

##### Well-known member
When we opened the pool initially the PB used tri-chlor tabs and that's what established our CYA of 30. We've just kept it there, knowing it could creep up (if we ever used tabs again temporarily) but not down.

At CYA = 30, Pool Math suggests target FC or 2.0 - 6.0
Pool Math also suggests CYA of 40, moving FC target up to 3.0 - 7.0

Would y'all recommend raising our CYA to 40 or just keeping it at 30?

TFP Expert

#### JJ_Tex

Gold Supporter
Bronze Supporter
If it were me, I would keep the CYA at 30 for this winter, and your FC at 5-8. That is a bit elevated from the published target (4-6) but that will give you some buffer for leaves and other stuff.

If you get a lot of sun, I would look to raise your CYA up to 40 or even 50 in the summer, and raise your FC accordingly.

#### CrystalRiver

##### Well-known member
CYA can and does creep down. It degrades over time (a couple PPM per month), and is lost through splash out and backwashing. None of that happens super fast, but it does occur and that's why CYA should be checked every month or two, even if nothing has been added.

#### dntboles

##### Well-known member
Thanks, y'all!

We're pretty shaded, which is nice in the summer when everyone else's water temp is getting too hot in the sun. But those same trees that provide shade in the summer have spent the last month or so dumping their leaves into the pool. I'm emptying skimmers and the Polaris once or twice a day.

#### mgtfp

Bronze Supporter
Does FC drop more quickly if it's a higher level than target, or is it more of a constant drop, based on UV, organics, etc?

Hypothetical example (assuming constant level of UV, organics, etc):
Assumption: Add chlorine to bring FC to 6,0, and it theoretically drops 1.0 per day, reaching 2.0 after 4 days.
Scenario: Add chlorine to bring FC to 10.0, would it theoretically:
A. drop at 1.0 per day, taking 8 days to get to 2.0, or
B. does the higher starting FC level affect how quickly it drops off?

B is correct. Assuming CYA remains constant, you loose pretty much a certain percentage of your FC due to UV. How much this loss actually is, depends on CYA and UV load.

In your example: 1ppm absolute loss at 6ppm means about 17% relative loss, resulting in 1.7ppm absolute loss at 10ppm.

It also means that the absolute loss per day decreases as FC goes down.

Last edited:

#### dntboles

##### Well-known member
@mgtfp the percentage part makes sense. X% of a small number is a smaller number than X% of the larger number.

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