Is FC loss logarithmic?

[Richard320]

For the past few months - read: chilly - my pool has been losing about 1 ppm FC/day, not counting the losses when I dilute with rainwater. I was getting .5 CC off and on when testing FC, never any higher. The rain has stopped for the time being. Pool is all clean and balanced, but I noticed, down inside the main drain, what could be algae. It could also be real fine silt from the rain gutter fill water. All I know is that it looks a dark olive green. The water's too cold to swim down and get a closeup look, and there's no way to get in there with a brush, so I just don't know. Not wanting to risk it, I decided to shock. 16 should be high enough, but I had two full gallons of 12.5%, so I dumped it all in and started at 20.

FC loss was 3 ppm in 24 hours. I topped it back up with some fresh bleach. Again, 3 ppm loss. No sign of CC. The pool is in the sun pretty much all day. I haven't done the overnight test, yet.

What I'm wondering is: does one lose more FC daily as the level increases? You know, like how you lose air pressure faster the higher the starting pressure is? It would make sense to me, as I understand that bleach degrades faster the higher the concentration. Is it one of those logarithmic things?

 

[dmanb2b]

Yup...the more PPM of total chlorine is in the pool, the more ppm you'll lose per day

 

[woodyp]

Ok...so now I'm confused.
My pool is in full sun all day, and the whole reason I raised my CYA (which is supposed to protect the chlorine from too much burning off) to 60--------was to hopefully LESSEN the FC loss per day. Would I be just as well off at 40 CYA and maintaining minimums for that level--------or does this algorythm just apply to high shocking level scenarios as Richard is describing?

 

[dmanb2b]

I don't think you are that far off . You are correct in that CYA slows down the sunlight induced FC consumption rate, but that rate is still a certain % of all FC in the Pool. The higher that FC concentration is times the % burn rate, the more FC you will lose in ppm per day.

So where at let's say at an FC of 10ppm (CYA @70) you may be burning 2ppm/day driven by sunlight, you would likely lose 3ppm/day at an FC of 15ppm (CYA @ 70).

 

[woodyp]

Ok...............that helped a little bit!

 

[Beez]

For the past few months - read: chilly - my pool has been losing about 1 ppm FC/day, not counting the losses when I dilute with rainwater. I was getting .5 CC off and on when testing FC, never any higher. The rain has stopped for the time being. Pool is all clean and balanced, but I noticed, down inside the main drain, what could be algae. It could also be real fine silt from the rain gutter fill water. All I know is that it looks a dark olive green.

I could be wrong, but I have a feeling the water from your rain gutter is a ready source for algae. So ideally before(or soon after) a rain you would bump up the FC to compensate for an increased FC demand from the rain water.

 

[chem geek]

First off, the loss of chlorine due to sunlight is an exponential decay so yes it is logarithmic in that sense. The loss is a certain percentage of the FC level per time period for a given CYA level. However, it's not really a loss of FC itself, but a faster loss rate of the chlorine unbound to CYA (i.e. hypochlorous acid and hypochlorite ion) which is roughly proportional to the FC/CYA ratio and a much slower loss rate of the chlorine bound to CYA which is roughly proportional to the FC (or more accurately FC-(FC/CYA)). However, since there is so much more chlorine bound to CYA than unbound, much of the absolute loss comes from this chlorine bound to CYA.

Since a higher CYA level requires a higher FC level to prevent algae growth, pretty much keeping the FC/CYA ratio constant, one might think that the absolute loss from sunlight would be at least as high if not higher when the CYA level is higher assuming the FC is proportionately raised. In practice that doesn't happen because there is an additional non-linear CYA shielding effect that blocks UV from getting to lower depths. So even as you raise the FC and CYA levels together proportionately, one usually finds that the absolute FC loss from sunlight actually drops.

Technically, this comes from the fact that the intensity of UV falling off as a function of depth is a negative exponential function so going from a CYA of 40 ppm to 80 ppm can reduce the UV a foot or so below the surface by a factor of 3 which more than makes up for the factor of 2 higher FC level required to keep the FC/CYA ratio constant.

 

[Johnny B]

For the past few months - read: chilly - my pool has been losing about 1 ppm FC/day, not counting the losses when I dilute with rainwater. I was getting .5 CC off and on when testing FC, never any higher. The rain has stopped for the time being. Pool is all clean and balanced, but I noticed, down inside the main drain, what could be algae. It could also be real fine silt from the rain gutter fill water. All I know is that it looks a dark olive green.

I could be wrong, but I have a feeling the water from your rain gutter is a ready source for algae. So ideally before(or soon after) a rain you would bump up the FC to compensate for an increased FC demand from the rain water.

My thoughts too.
And I can't imagine non-organic debris being an issue too if your roof is shingled. So I'd filter it prior to letting it enter the pool, (w a nylon?)

 

[Richard320]

Time for an update

I kept it up at shock for a couple days. Last night I took a reading after the sun went down. 19.5 FC. This morning, before the sun hit the water, 19.0. I don't think it was algae, especially since it hasn't really changed appearance and I've been pulling all my water from the bottom drain.

I checked CYA when things got lighter. It's looking like about 30. That concerns me... with the dilution, things were all dropping about the same. I'd expect to see somewhere closer to 45. Does high FC affect the CYA test? On the other hand, it could very well be 30ish....it was blazing hot today - closest official weather station showed 98 at 4 PM :grrrr: - and if my test was correct, I lost 7 ppm FC today!!

This is going to take a lot more testing. And bleach. I hate to add any stabilizer right now; I was hoping to wait until May, when we leave town for ten days, and buy some pucks for the floater.

EDIT: It was testing error. I added enough bleach to kick it back up to 16, brushed a bit and let it circulate for an hour, and came up with 19.5. The problem, I believe, is that it was too dark and I was using the light in the speedstir. It seems to wash out the pink color. Light on, looks clear. Light off, even by the dim, distant patio light, still shows pink. So I actually only lost no more than 4 ppm.

And in answer to another question, the worst of the debris doesn't make it to the pool. I have a series of flexible downspout extensions that are corrugated. They act like a gold-panning sluice box. Only the finest of dirt makes it to the end. That is dumped into the spa, with the flow off, so it has a chance to settle before it spills into the pool. After a good storm, the spa looks awful, the pool looks good. Switching the valves to draw from spa and return to pool, I filter out almost all of it by brushing and keeping it stirred up. A little bit of back and forth with the valves to refill & dilute and some more filtering on spa only and it all looks great within half an hour.

 

[dmanb2b]

They act like a gold-panning sluice box. .

Richard...have you been watching Alaska Gold Rush again :lol: