Does FC drop faster with higher CYA levels?

mbeezie

Member
May 14, 2021
21
CT
Pool Size
36000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Hayward Turbo Cell (T-CELL-5)
hi all!

This is my 2nd season with this pool. Last year i bumbled my way through and found TFpool way too late in the season, and i have a specific question about why my FC disappeared in the last two days.

Opened May 8th, 36k gallon in-ground pool with SWG, heater, and new VS pump, taylor test kit.

Pool company initially shocked it at opening. FC levels were 5-10 based on my Taylor kit for over a week. I was not running the SWG at first because the water was cold. For the last week the water temp is in the 80s. I got my CYA level up to 70-80 which gives me a target FC of 3-6.

On 5/15 i dropped the SWG from 50% to 20% because FC levels were still 5-10 (pump running about 14 hours per day)
On 5/16 i dropped SWG to 10% because FC levels were still 5-10
On 5/18 FC level finally dropped to 3-6. everything was in balance. pH 7.4, TA 90, salt 3400 ppm, CC zero
On 5/20 FC level still 3-6, nothing changed
On 5/22 FC/TC/CC levels all undetectable on the Taylor kit

I guess i should have checked 5/21 as well, but i didn't that day. I did not use such a high CYA level last year (i didn't know it was recommended) and i figured that higher CYA accomplishes three things (but maybe this is wrong)

1) lets my chlorine last longer
2) requires me to run a higher FC than i did last year (when i had a much lower CYA level)
3) lets me run my SWG less

Point # 3 is why i was OK with the idea of running it at 10% (last year i ran a much lower CYA and had the SWG running 60% generally), but i don't know why it suddenly went from 3-6 to undetectable two days later. I felt the CYA would give me a more gradual drop off of the FC levels.

Right now i'm running my SWG at 100% to try and bring it back up quickly (supposed to go in it for the first time this eason today!)

So what i don't understand is that steep drop off of the FC level from 5/20 to 5/22. The pool is perfectly clear and CC is zero, so i don't think it got eaten up by anything.
 

laprjns

LifeTime Supporter
Aug 14, 2012
506
Ellington, CT
Pool Size
30000
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Chlorine
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Pureline Crystal Pure 60,000
So what i don't understand is that steep drop off of the FC level from 5/20 to 5/22. The pool is perfectly clear and CC is zero, so i don't think it got eaten up by anything.
I don't think its a steep drop off at all and looks to me as reasonable based on what you have your SWG settings. Normal FC consumption is around 2 to 4 ppm per day. Your per day decrease based on 5/20 level of 3 -6 ppm (why the range?) is 1.5 to 3 ppm, which is within the normal daily consumption. You don't say what SWG model you have, but lets assume that it a Pentair IC60, which produces 2.0 lbs of chlorine per day (one of largest commercial SWGs available). Running it at 10% for 14 hrs would only produce 0.4 FC in your 36k gallon pool. This would make your daily losses at between 1.9 and 3.4 ppm.
Identifying your SWG model would help in answer your question.
 
Last edited:

Jimrahbe

Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
22,164
Bedford, TX
I got my CYA level up to 70-80 which gives me a target FC of 3-6.

M,

Not exactly... With a CYA of 80 your "Target" is 6 and your "Minimum" is 4..

The idea is to maintain your Target FC all the time... and never drop below the minimum. Personally, I set my SWCG so that it keeps my FC at my target or higher.. It just makes life so much easier. I never worry about a cloudy day.. if the FC goes up, (within reason) I don't care, it will be sunny soon. Maintaining my target means I don't have to adjust my SWGS' output very often.

It appears to me that you are trying to micro-manage your SWCG output. You can do that if you wish, but it is just too much of a pain for me.. My SWCG is there to make pool care easy, not to add to my work load.

Keeping your FC at your Target or higher means that you can test less often and still be assured of a well sanitized pool.

There is just no down side to running your FC a little high, but there is a huge downside of allowing it to go to low. I never walk too close to the edge.. :mrgreen:

Thanks,

Jim R.
 
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mbeezie

Member
May 14, 2021
21
CT
Pool Size
36000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Hayward Turbo Cell (T-CELL-5)
I don't think its a steep drop off at all and looks to me as reasonable based on what you have your SWG settings. Normal FC consumption is around 2 to 4 ppm per day. Your per day decrease based on 5/20 level of 3 -6 ppm (why the range?) is 1.5 to 3 ppm, which is within the normal daily consumption. You don't say what SWG model you have, but lets assume that it a Pentair IC60, which produces 2.0 lbs of chlorine per day (one of largest commercial SWGs available). Running it at 10% for 14 hrs would only produce 0.4 FC in your 36k gallon pool. This would make your daily losses at between 1.9 and 3.4 ppm.
Identifying your SWG model would help in answer your question.
thanks, that’s helpful. that’s the range the taylor kit gives me. how do you get a more precise measurement?
 

Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
4,969
NW Ohio
thanks, that’s helpful. that’s the range the taylor kit gives me. how do you get a more precise measurement?
He's just doing some math, actually testing that precision isn't necessary (or possible with consumer equipment).

We don't follow a set range for FC, it is a function of the CYA. You want to follow the FC/CYA Chart to make sure your FC and CYA levels are balanced to keep the chlorine level high enough to sanitize and oxidize waste, but low enough to be comfortable. There's a ton of wiggle room on the high end of the ranges but not so much at the low end, as Jim was saying, so don't be afraid to overshoot the target a little.
 

Saturn94

Bronze Supporter
Mar 11, 2015
1,195
SE Virginia
Pool Size
20000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
Which Taylor test kit are you using?
 

mbeezie

Member
May 14, 2021
21
CT
Pool Size
36000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Hayward Turbo Cell (T-CELL-5)
Ok here goes -- probably the dumbest question you've ever seen on this board...

When measuring Cl or Br with my Taylor kit (https://www.amazon.com/gp/product/B00HEAQO5O/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1), each colored box has two numbers, ie 1/2, 2/4, 3/6, 5/10 etc. Does this represent the range of either Cl or Br? Or are the numbers on the left for Cl and the right for Br?

For example, say i check it, and color match to the 3rd box up from the bottom. Does this mean that there is a range of Cl or Br of 3-6 ppm? or that the Cl is at 3 ppm, and if it were bromine, the reading would be 6 ppm?

I ask because as it rises with rising CYA, it seems harder to get an exact number. For example, this morning, my water tested at the 2nd box down from the top, or the '7.5-15' Does this mean my chlorine is anywhere in this range? or '7.5' for chlorine and '15' for bromine?
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
41,454
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
You do not have bromine in your pool, at least we hope not. Bromine is for spas that do not get impacted by sunlight.

That test block is rarely used. Use your FAS-DPD test, R0870 powder and R0871 reagent for FC testing.
The OTO yellow color matching is used if you get a weird result from the FAS-DPD - the OTO will tell you there is chlorine in the water. It is not accurate for what level it is.
 
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mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
41,454
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
The blocks of color are not ranges. The TC is the first number, ignore the second number, does not apply to you.

The FAS-DPD test is needed for FC/CC testing.
 
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mbeezie

Member
May 14, 2021
21
CT
Pool Size
36000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Hayward Turbo Cell (T-CELL-5)
Thanks for your help.

So those blocks of colors are not ranges, and the first number is for chlorine, while the second is for bromine (i have a bromine spa).

The instructions that come with the kit says that it does test free chlorine, it is actually the first test for the chlorine, followed by total chlorine, and then you subtract FC from TC to get CC.

I don't understand why you are recommending a different kit as this one does do FAS-DPD testing as well.

I appreciate you clearing up that those numbers are not ranges, that is very helpful!
 

Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
4,969
NW Ohio
I don't understand why you are recommending a different kit as this one does do FAS-DPD testing as well.
The DPD test is not the same as an FAS-DPD test. The FAS-DPD test does not involve color matching and renders results to 0.5 ppm precision (even to 0.2 ppm but it's rarely necessary). It measures FC and CC separately and precisely and can measure up to 50 ppm FC.
 

mbeezie

Member
May 14, 2021
21
CT
Pool Size
36000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Hayward Turbo Cell (T-CELL-5)
The DPD test is not the same as an FAS-DPD test. The FAS-DPD test does not involve color matching and renders results to 0.5 ppm precision (even to 0.2 ppm but it's rarely necessary). It measures FC and CC separately and precisely and can measure up to 50 ppm FC.

Thank you for finally explaining!