FC loss guidelines

[DCM5150]

I've been searching through the website and the forums and can't seem to find any guidelines on how much FC loss should be expected. In one post you see people say that 4ppm is far too high and something is wrong. In another, they will post that 3-4 could be normal. Some of the more consistent posts have said that about 50% loss (I assume that is the total FC level, i.e. if my target is 10ppm, then I might lose 5ppm during the day).

So my question is: Are there guidelines on what are normal or reasonable FC loss over a 24 hours period? (I know the best way to see if something is wrong is for an OCLT of less than 1)

My FC loss seems to be about 3.5-4 now that I am out of the SLAM, but I haven't done an OCLT is about a week, so I will do that again.

Last nights numbers: FC - 8.5 (9.0 target), CC - 0, pH - 7.8, CYA - 80, TA - 240 (pool guy said this is mainly due to our water, but I am starting to try to bring this down)

 

[JohnT]

Chlorine loss depends on many things. Bather load, CYA, climate and time of year are probably the biggest things that affect it. It really comes down to eliminating things like algae or low CYA from the equation, and then you'll know what your pool's inherent chlorine use is.

One caveat is to make sure you have an accurate CYA level. The test is a little tricky and many people get fooled by it. The dot needs to be really, truly not visible.

 

[jblizzle]

As John said, there is no straight answer.

Assuming there is nothing in the water consuming the FC, the higher the CYA, the lower the % of FC lost to the sun. Also at a given CYA, the higher the FC level, the % loss may stay roughly the same, but the ppm lose would be higher.

A good balanced pool is usually around 2-4ppm lose per day, depending on CYA level, waste load, and sun exposure.

 

[ping]

Run the OCLT again and that will rule out or confirm organics as the culprit.

I would say you are on the high side of chlorine use at this time of year.

 

[woodyp]

Anywhere from 2 to 4 may be........YOUR.......normal. Mine is 4 in summer all day heat and sun in Texas with CYA at 60. Only heavy bather load on weekends.

 

[kris77]

I would typically lose 2-3ppm daily last year when the solar cover was off. When the cover was on, I could go 3 days and only lose 2-3ppm.

 

[freetime]

Isn't the elephant in the room CYA 80? (OP didn't mention a SWG)

I think the trick to minimizing FC loss is to try to match CYA level to the individual pool i.e. sun exposure, surface vs depth etc. Obviously cleaning/backwashing and removing organic debris promptly (leaves, dead insects etc.) all helps too.

My pool was using 3-4ppm FC per day at CYA=0, at CYA=30 the daily FC loss was down to about 1ppm - 1.5ppm per day, so I didn't take the CYA any higher

There is a detailed explanation and charts in this thread but I found it easy enough to just raise CYA in steps until I hit a nice low FC loss level

Chemgeek made a comment in the above linked thread: "Nevertheless, the absolute loss of chlorine is greater at higher CYA levels (keeping HOCl constant) and is the downside to a "high CYA & high Chlorine" approach."

So my guess is that the OP should be able to reduce FC loss by replacing some water to reduce CYA and therefore being able to use a lower target FC

 

[chem geek]

I have now edited the post you refer to since after I wrote that before 2007 (it was copied over from The PoolForum) we found that there is some sort of CYA shielding effect such that higher FC and CYA levels at the same FC/CYA ratio do not lose as much chlorine. Mark's experiments described in this post and this post (in the same thread) showed this effect which I roughly modeled for an SWG pool situation in this post. I speculated on some mechanisms in this post.

So thank you for catching that error. We start out basing our understanding on the known chemistry, but when there is a mismatch seen in real pools and especially when confirmed by experiment then we change our understanding and try to figure out the additional chemistry or other factors. In this case, we have an educated guess but have not found any definitive confirmation that CYA or chlorine bound to CYA absorbs UV in the 300-380 region of interest where enough UV gets through the atmosphere and could result in chlorine breakdown.

 

[umpalumpajay]

The dot needs to be really, truly not visible.

This is a big help to me - and im glad i really REALLY tried to see the dot and add more drops if i could. thanks!

 

[chem geek]

See this link on the Taylor website for what it should look like. And remember that you can pour the solution back into the mixing bottle to test as many times as you like. You can also get CYA Standard Solution to see what a true 50 ppm CYA looks like. Finally, you should view the tube with your back to the sun so that the tube is shaded (i.e. in strong indirect light) and you look down straight into it.

 

[iflyjetzzz]

I would typically lose 2-3ppm daily last year when the solar cover was off. When the cover was on, I could go 3 days and only lose 2-3ppm.

I found that my pool cover really lowered chemical usage. Both Chlorine and Muriatic Acid.

 

[jeffbrig]

Fwiw, my pool doesn't seem to lose more than 1 or 1.5ppm each day, even in August. But my CYA is a little high, and almost nothing ever falls in the pool.

 

[ewkearns]

My pool, currently under major repair, had another way of losing chlorine..... numerous and major leaks.....

 

[chem geek]

Fwiw, my pool doesn't seem to lose more than 1 or 1.5ppm each day, even in August. But my CYA is a little high, and almost nothing ever falls in the pool.

Is your pool exposed to direct sunlight and has no cover? That's an unusually low loss otherwise.

 

[jeffbrig]

Yes pool is in full sun, no trees, just a couple of palms. No issues with pollen, leaves, or anything else falling in.

 

[jeffbrig]

Just for my understanding, why would no cover and full sun cause lower FC loss? Just trying to understand the mechanism...

 

[jblizzle]

Just for my understanding, why would no cover and full sun cause lower FC loss? Just trying to understand the mechanism...

It wouldn't ... it should be the opposite.

 

[jeffbrig]

That's what I would have thought, since sunlight breaks down FC. I guess I misunderstood chem geek's statement.

 

[chem geek]

Yeah, I meant your FC loss is unusually low given no cover and direct sunlight. The high CYA helps and I assume you are keeping the FC at a level according to the Chlorine / CYA Chart

 

[jeffbrig]

Yep! Pool math is my friend...