FC Loss curve

[H2O_Keeper]

Perhaps this is somewhere in a chart at TFP...

I had typically tried to keep my FC towards the high end of the FC levels on the CYA/FC chart in Pool School. I noticed the pool calculator it goes a little higher for a CYA of 50 (4-6 vs 4-8).

I started to think, I know anywhere above 4FC as far as algae I should be fine. My question is centered more around extra losses of FC at a level of 8FC opposed to a level of 6FC. When I shock its not uncommon to drop several points of FC loss in a few hours. Thus there appears to be a steep curve of FC loss when you go above ideal FC/CYA levels.

Are there any curves or any data that could help support how much more FC usage (thus cost) one may go through trying to maintain higher FC levels on a regular basis (than necessary).

 

[JasonLion]

The amount of FC lost to sunlight is a percentage of the starting FC level, while the amount used for sanitizing in usually a fixed amount. So you might lose 45% to sunlight and have 0.5 ppm consumed in sanitizing. The loses due to sunlight usually are the clear majority.

So, roughly speaking the amount lost is proportional to the starting level. For example, doubling the starting level means almost twice as much goes away by the end of the day.

 

[H2O_Keeper]

Thanks I understand. I realize there are multiple factors involved here. To be more specific why I asked the question is that I recently installed a peristaltic I run nightly on a timer and am trying to dial in where I want it. I realize the 4-6 range is fine and I will shoot for 6 and that way if I am not putting enough in nightly I wont undershoot the 4 before realizing it. With just a few days of data I am using 1FC a day trying to hold a "7".

My question was that what if I bumped it up to say 8 or 10 to allow for unknowns as the water temp starts to rise in the next few weeks will I use more chemicals daily. I have no data on what summer consumption is because I was doing rough methods last year with test strips, have a TF-100 now. I realize there is more FC involved to reach say a 10, but once I am there will my nightly dose be more like 2FC to keep it there, or will I still consume 1FC a day (assuming temp constant).

I am assuming there is some relationship to extra FC burn-off once you exceed ideal FC/CYA ratios. Observation is from the beginning of this thread becase when we superchlorinate we quickly burn-off several points of FC (even with no contaminents). I observed this when I superchlorinated my pool a few times trying to clear dead algae and there was no CC any of the times I did it.

 

[JasonLion]

The higher your FC target, the more chlorine you will use, assuming everything else stays the same.

Your system can be calibrated exactly the same way a SWG is calibrated. Measure the FC level in the early evening once every several days, if it is too low increase chlorine additions, if it is too high reduce chlorine additions. The setting will change depending on the seasons and the bather load.

If you want one setting to last for a good while, then you need to set it higher than is currently required. For most people in most situations, it is best to only set it a little higher, and test FC and adjust the setting at least once every two or three weeks.

 

[H2O_Keeper]

Okay, assumption verified. Thank you Jason. Once I get things close to coming in everynight at "7" for my CYA of 50, I will tweak down to 6 and try and run it there. I will be checking it daily for a while(only have 3 days of data) to make sure I am not rising or loosing too much ground every day.

I like your logic of maybe overshooting just a little and let it burn off, this should reduce constant tweaking. Is a shift of 1FC reasonable to start to tweak? We all have constant variables with weather changes, bather load, daytime length, testkit limitations, etc. Or is a shift of 2FC more realistic real world so I am not constantly trying to dial it in?

 

[JasonLion]

The more you are familiar with your pool and how it behaves, the less margin you need. I usually only test the FC level after a sunny day, so I know the measurement will be a worst case measurement, and I add extra chlorine manually before and after pool parties to compensate for the extra bather load. If you don't already make compensations like that, then it is better to give yourself some extra margin on the FC target.