FC loss seems high

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Concho

LifeTime Supporter
May 23, 2012
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It seems like my FC loss seems high compared to other posts I have read. Here are some of my numbers:

PH 7.6
TA 190
CH 300
CYA 80-85

On Sunday night my FC was 11, on Monday night it was 5. I added a gallon of chlorine, got my FC up to 9.5 and on Tuesday night it was 5. I added a gallon to get it up to 10, and on Wednesday night it was 6. It seems like I'm losing about 1 ppm overnight and then usually about 3-4ppm during the day. My pool is clear and CC<.5. Do these numbers seem out of whack? Nobody has been swimming...except for a few turtles.
 
You almost certainly need to shock the pool. The only other possibility is that your CYA level is actually much lower than you think it is. How did you get that CYA test result, what kind of test kit?
 
That's what I was wondering....could my CYA be lower than what I'm reading. I just got done shocking last week...been trying to keep my FC between 6-11 range. I have the TF100 kit. My overnight loss has been 1ppm for the last 4-5 days.
 
Just as an aside...I called my pool store again playing dumb and asked what I should keep my FC level at...he said between 1-2...and didn't care about my CYA as long as below 200. My father also has a pool from a different company...I had him ask them the same question...they told him to keep his level between 2-3, and didn't ask him what his CYA level was. I tested his water and he was roughly 50 CYA. He is constantly shocking and wiping algae off the walls...I told him to keep his FC up more in the 4-8 range to see if that helps. So I don't get how these pool stores just throw out these low FC numbers regardless of CYA levels...wouldn't most of their customers be having problems? Is it safe to swim in a pool that has 1-2ppm FC if the CYA level was 150-200?
 
Concho said:
So I don't get how these pool stores just throw out these low FC numbers regardless of CYA levels...wouldn't most of their customers be having problems? Is it safe to swim in a pool that has 1-2ppm FC if the CYA level was 150-200?
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Some of their customers do have problems and go to the pool store for help where they happily sell them algicides, phosphate removers, clarifiers, flocculants, enzymes and other products to "fix" the problem. These products are more profitable (have higher margins) than chlorine and are incremental to the chlorine sales. It's not really the pool store's fault since they are not told the truth about the chlorine/CYA relationship from the manufacturers who make these expensive high-margin fix-it products in addition to selling chlorine. It's been the business model for so many decades that many people working for the manufacturers don't even realize it anymore and actually believe their own hype that "CYA doesn't matter".

Not every pool with a low FC/CYA ratio will get algae. The pool may be naturally low in algae nutrients (phosphates and nitrates) or may be shocked weekly which helps make up for too low a chlorine level during the week or an algicide may be used.

1-2 ppm FC with 150-200 ppm CYA will have a very low active chlorine level, but it's still fast enough to kill most bacteria faster than they can reproduce. So for residential pools, where person-to-person transmission of disease is not normally a concern, it would probably considered to be safe. The main problem with the low FC/CYA ratio is that algae can grow faster than chlorine can kill it (unless an algicide is used). When the FC is around 10% of the CYA level, then most fecal bacteria are killed at a rate of a 99.9% kill in 1 minute or less. With an FC that is 0.5% - 1% of the CYA level as in your example, the kill time extends to 10-20 minutes. Since bacteria take 15-60 minutes to double in population, one needs a 50% kill time faster than that to prevent uncontrolled bacterial growth. The 50% kill time in your example is around 1-2 minutes.

The other problem with such a low active chlorine level is that it will take proportionately longer to oxidize bather waste so the water may build up more monochloramine, dichloramine and other chemicals that might start to smell (and measure as CC) and the water might get dull-looking even if algae growth was inhibited in some other way (say, by an algicide).
 
Thanks for the info....would you happen to have any insight on why my FC is dropping so much during the daytime...other than me reading my CYA level wrong. Based on my testing I'm pretty sure its between 70-90. I get full sun from sunrise til about 4:30 pm.
 
Concho said:
My overnight loss has been 1ppm for the last 4-5 days.
Click to expand...

Do we have our answer right here?

Sounds like something's still consuming chlorine, albeit slowly. Last week's shocking of the pool probably killed most of whatever it is, but left just enough behind that it's fighting to hang on...

At least, that's how I'd interpret that.

EDIT : I understand the OCLT suggests "1 ppm or less", but I'm thinking the CONSISTENT reading of 1 is indicative of a "low grade fever", if you will. I could, of course, be way off base here.
 
gboulton said:
Concho said:
My overnight loss has been 1ppm for the last 4-5 days.
Click to expand...

Do we have our answer right here?

Sounds like something's still consuming chlorine, albeit slowly. Last week's shocking of the pool probably killed most of whatever it is, but left just enough behind that it's fighting to hang on...

At least, that's how I'd interpret that.

EDIT : I understand the OCLT suggests "1 ppm or less", but I'm thinking the CONSISTENT reading of 1 is indicative of a "low grade fever", if you will. I could, of course, be way off base here.
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I'd agree from experience. The clues being dust/clouds when you brush and high FC loss. A little nook or cranny of un-circulated water, maybe an automatic fill valve chamber or an overflow pipe from the skimmer that's blocked or just clinging on to a bit of rough plaster in an area of poor circulation.
 

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Thanks for the tips. Another pool store tidbit for those interested....they told me not to add salt because you only get the benefits of salt if you have a SWG.... it's the chemical reaction that occurs that will give you the benefits of salt....and if I add salt without it I will throw off all my other chemistry...so don't do it!
 
adding salt

Does anyone know if adding salt can lower my FC numbers? I was at a 7.5 FC reading, added 160 lbs of salt, later added 1 gallon chlorine...just took a FC reading and got 3.5-4.0.....should this be happening?
 
Re: adding salt

Salt should not affect your FC.

I do not recall your other threads, were you previously going through the shock process?

EDIT: Now I see your other thread ... should have just left this question there to be consistent ... as I see this was already answered.

Topics merged. JasonLion
 
Concho said:
Thanks for the tips. Another pool store tidbit for those interested....they told me not to add salt because you only get the benefits of salt if you have a SWG.... it's the chemical reaction that occurs that will give you the benefits of salt....and if I add salt without it I will throw off all my other chemistry...so don't do it!
Click to expand...
This is absolutely, positively not true. The benefits that people find in SWG pools, other than the automated chlorine dosing, are related to the salt level, not to the SWG. There is less pressure on the eyes (burning/drying sensation) since the salt level in the pool at 3000 ppm is closer to that of human tears at 9000 ppm. The water is more buoyant and can feel softer/silkier to some. Skin wrinkles more slowly due to the higher salt level. None of these benefits have to do with the SWG -- they have to do with the increased salt level. You can get most of these benefits by going to around 2000 ppm which lessens the potential downsides of a higher salt level which are a usually small increased risk of corrosion or soft stone deterioration.

One other benefit is due to the SWG and that is being able to have a somewhat lower FC target relative to the CYA level. That is most likely due to the SWG though the salt might have some effect (we don't know). If you want that benefit, you can spend more money for an algicide (such as Polyquat 60 weekly) so you can target a lower FC level, but most people do not do that since the normally recommended FC/CYA levels are not a problem.

Having the salt will not throw off your other chemistry. It only has a small effect on the saturation index that you can easily compensate for with a slightly higher pH, TA or CH target.

Sounds like this pool store doesn't know what they are talking about.
 
I was also going to comment on that too, but did not want to post on the bus ... I was hoping that the OP, just left off the rolling eyes after that statement, but maybe there really is a misunderstanding.
 
Salt is not the problem ... unless maybe you used the wrong kind I suppose. What did you use?

I have not seen the full results for an OCLT with time and reading.

Your FC is certainly getting lower than you should allow it which certainly points to you needing to go through the shock process.

Posted with Tapatalk ... sorry if I sound short ... hate typing on phone :)
 
I used Morton pool salt...I had been shocking for a week...my last two OCLT went from FC 17 to FC 16....then the next day went from FC 9.5 to FC 8.5. Water was clear and CC < .5. I assumed I was done shocking...but was losing a fair amount of FC during the daytime, 3-4ppm with my CYA in the 80 range. Everything seemed fairly normal yesterday....my FC was 7.5, I decided to add the salt, a couple hours later added a gallon of chlorine to try and pump it up to the 11-12 range.., tested later that night and FC was down in the 4 range, then FC 0 at 6;30 in the morning.
 

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