Estimating FC demand due to extra sunlight hours

AndyTN

Bronze Supporter
Mar 27, 2019
402
Memphis
Pool Size
26000
Surface
Vinyl
I just had a large tree over my pool get some major trimming which is yielding approx 3 hours additional sunlight per day in the morning. These 3 hours are from 9-12 so it isn't going to be solar noon but still much more direct sunlight. If I used to have FC loss of 1-1.5 each day based on 5-6 hours of sunlight, should I expect a 50% increase in FC demand? I know the correct answer for me is just going to be to do the testing to find out the difference but I am wondering if anyone has done any prior study/analysis.

Not all sunlight hours are equal and with clear skies right now at about 10am, our UV index is only 4. Memphis gets sun angle at approx 76 degrees during July so I know the intensity would be higher than for someone up North. The temp of the pool immediately went up about 5 degrees the days right after the tree limbs were removed.

Anyway, I am likely just trying to over-analyze but would like some thoughts on the impact of FC demand based on different degrees of sunlight during the day and total hours during the day.
 

Wobblerlorri

Bronze Supporter
Apr 16, 2019
1,758
Bremen, GA, 65 m west of Atlanta
Pool Size
4024
Surface
Vinyl
Chlorine
Liquid Chlorine
I've discovered that my pool needs about 2 cups of 6% chlorine (2 ppm) on overcast and cloudy days, and 3 cups (3 ppm) on bright sunny days. The amount of sunlight really does affect chlorine usage. What I would suggest is test every day for a week or two and make note of how much sun the pool got. That should give you a good baseline for dosing the pool according to the weather.
 

AUSpool

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TFP Guide
Sep 23, 2015
966
Brisbane, Australia.
Pool Size
5000
Surface
Plaster
Chlorine
Salt Water Generator
I would test and adjust. But it’s important to keep on top of major change, particularly an upward trend. I would increase the daily dose as per test results but also add an addition dose at each test to maintain your target FC.
 

mgtfp

Bronze Supporter
Mar 5, 2020
994
Melbourne, Australia
Pool Size
66000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Astral VX 7T
Just stumbled over this older thread. I was recently looking into the correlation between UV-load and FC-usage, after I found a great webpage from the Australian Government that tracks the UV-index across the country (I don't know if something similar is available for the US):

The data (one measurement per minute) can be directly saved as CSV or XLS file. In Excel, I calculated the sum over a whole day, giving me a measure for the total UV-load over a whole day as "UV-Index Minutes" (similar to Kilowatt Hours...).

On a mostly clear day, like November 30th 2020 in Melbourne, this gives a UV-load of about 4000-4500 UV-Index Minutes. On days like that, my FC-usage is about 20% of the FC-level.

On mostly cloudy days, like November 28th 2020 in Melbourne, the total UV-load is about 1500-2000 UV-Index Minutes. FC-usage is around 10% of the FC-level on these days.

There is of course some scattering when comparing FC-usage with the UV-index, depending on FC testing accuracy (sometimes you are lower, sometimes higher within the 0.5ppm test resolution), varying bather load, and of course different cloud coverage at the sensor location and our backyard. But the general correlation is pretty good. Dialing my SWG in to generate about 15% of my FC-level seems to keep FC constant over the course of a week with varying weather conditions (we're in Melbourne after all - four seasons in one day). When there are a couple of cloudy days in a row, I can turn the SWG down a bit, once we get more stable sunshine (usually after Christmas), I can turn it up. There will of course also be some systematic effects over time, like generally less shade on the pool once the sun stands higher in the sky around December 21st.

From the general shape of the calculated curves (without cloud coverage), you can see that you reach max UV-load as expected at about 1pm (daylight saving time), UV-index about 11 at this time of the year in Melbourne. Around 10am and 4pm you have about 50% of the max. UV-load.

If I now add up the UV-index values between 9-12 (again, daylight saving time), I see that the UV-load in that time window contributes to about 30% of the total daily UV-load. In other words: Before your tree was trimmed, your FC-loss due to UV should have been about 30% lower compared to now (assuming that the tree shaded 100% of the pool, and that the general shape of the UV-index curve will be the same for your location).

Hope that helps. And yes, we are over-analyzing here. But we also spend too much time at home these days...

EDIT: I should add that my CYA is at about 80ppm.
 
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