what time of day should i be worrying about minimum FC level?

crackers8199

Well-known member
Jun 5, 2014
132
Lake Elsinore, CA
i have been testing my water with both a waterguru sense and a TF-100 for about 2-3 weeks now, and i think i have everything pretty well dialed in. my ph has been reasonably stable around 7.6-7.7 (i'm only having to add acid once a week), and all the other numbers are pretty well in line with recommendations aside from CYA being a little low at 50. i plan to add CYA at some point soon, but based on recommendations from some members in the just getting started forum i'm feeling my pool out a bit first before adding any CYA (as well as making sure i'm actually doing the test correctly, the first week i measured it at 60 but it's hard to figure out when exactly that dot disappears).

my IC40 (way oversized for my 10k gallon pool, which is nice) is currently set at 20%, and the pump runs roughly 9 hours a day (5am-8am at a higher speed when electricity is cheaper for the vacuum to run, and 8am-noon and then 8pm-10pm at lower speeds). the SWG runs 8 hours each day (6am-noon, and 8pm-10pm), and i'm seeing the following:

* when the waterguru does its test every morning at 5:30am, it always measures between 3.5-4.5 (somewhere in that range). from what i've seen (both in my own tests and other peoples reports here), it seems that's probably measuring a bit low.

* when i measure FC/pH in mid-morning to around noon, FC is always at 5 on my K1000 kit.

* when i measure FC/pH later in the day (such as the test just done a few minutes ago around 6pm, when the sun is off the pool), FC measured 3 (which i know is a little south of the target level).

* in the two DPD tests i've done so far since getting my TF-100, both around noon, the FC levels were 6.5 the first week and then 6.0 the second.

this time of year, i have full sun on the pool pretty much from sunrise until around 4pm, and the sun is completely off the pool by around 5pm.

my question is: should i worry about upping the SWG percentage from 20 to 40 based on it being a little below target in early evening, or am i ok considering it's probably at or above the target for most of the rest of the day?
 
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ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
21,219
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Minimum FC for a CYA 50 is 3. You should never let the FC drop below that any time of the day. You are okay at 3.5-4. Just keep an eye on it. Increasing your CYA to 70 with a FC target of 5 will give you a bit of a safety buffer.

Your lowest FC level will be around sunset as the sun has burned off all the chlorine for the day.
 

crackers8199

Well-known member
Jun 5, 2014
132
Lake Elsinore, CA
Minimum FC for a CYA 50 is 3. You should never let the FC drop below that any time of the day. You are okay at 3.5-4. Just keep an eye on it. Increasing your CYA to 70 with a FC target of 5 will give you a bit of a safety buffer.

Your lowest FC level will be around sunset as the sun has burned off all the chlorine for the day.
ok, thanks. if i end up needing to up the output a little, would i be better off going from 20 to 40 or turn the SWG on at 5am with the pump for that extra hour first (since right now the pump runs 9 hours a day but the SWG only 8)?
 

borjis

LifeTime Supporter
Aug 19, 2014
3,333
Pacific NW
Another consideration is the number of swimmers.
Have say 4 people in the pool for 30 minutes. If nobody showered first it's a pretty well guarantee the FC will be lower when they get out.

I test & usually have to dose chlorine after guests are over. Your on swg but still good to be aware.
 

crackers8199

Well-known member
Jun 5, 2014
132
Lake Elsinore, CA
Another consideration is the number of swimmers.
Have say 4 people in the pool for 30 minutes. If nobody showered first it's a pretty well guarantee the FC will be lower when they get out.

I test & usually have to dose chlorine after guests are over. Your on swg but still good to be aware.
good point. normally it's just my wife and i (and probably will be for the foreseeable future with the pandemic), but i still think i might bump the percentage up to 40 just to give myself a buffer...or at least bump it up for a few days and see how the numbers react.
 
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JimMarshall

Gold Supporter
TFP Guide
Jun 5, 2017
1,003
Oil City PA
If i've done the math correctly, at 8 hours/20 percent, your SWG is generating 1.1 ppm of FC a day, which seems a bit low. Running the extra hour will bump to 1.3 PPM per day.

If bumping to 40% is your only available option, I think you're going to have to dial the run time back a bit

Maybe try the extra hour and bumping your CYA to 70
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
33,909
Laughlin, NV
Is 40% your lowest option increase from 20%? It seems like a bit too much of an increase. You're really close on your numbers. I would think you'd only want to bump up a bit.
Without a Pentair automation, that is the best you get with the Intellichlor.
 

crackers8199

Well-known member
Jun 5, 2014
132
Lake Elsinore, CA
If i've done the math correctly, at 8 hours/20 percent, your SWG is generating 1.1 ppm of FC a day, which seems a bit low. Running the extra hour will bump to 1.3 PPM per day.

If bumping to 40% is your only available option, I think you're going to have to dial the run time back a bit

Maybe try the extra hour and bumping your CYA to 70
how'd you do the math on that? i was trying to do it myself but i couldn't figure out the formula...

i am going to need to make a run to the pool store probably after tomorrow, because i'm almost out of acid (if my usual pattern holds i'll need to add some tomorrow). next week when my pH drifts up again i'll bump the CYA.

in the meanwhile, should i up the percentage to 40 and drop it back to running 6 hours a day instead of 8? would that work? would be easy enough to do, i can just remove the evening trips that turn the SWG on and off from the timer...

i'll go outside now and test the FC and see where i'm at in basically mid afternoon, and then again around 5:30 when the sun is off the pool for the day. as of now it'll have full sun for about another hour and 15 min. when i tested at 9:30 this morning, it was at 5 on the K1000.

edit: right now (2:45pm) i'm still at 5 w/ the K1000, or at least somewhere between 3-5. to me it looked like a 5, but either way it definitely was definitely a darker shade of yellow than 3. i'll test again at 5:30-ish and if the past is any indicator it'll probably be down at 3.
 
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JimMarshall

Gold Supporter
TFP Guide
Jun 5, 2017
1,003
Oil City PA
how'd you do the math on that? i was trying to do it myself but i couldn't figure out the formula...

i am going to need to make a run to the pool store probably after tomorrow, because i'm almost out of acid (if my usual pattern holds i'll need to add some tomorrow). next week when my pH drifts up again i'll bump the CYA.

in the meanwhile, should i up the percentage to 40 and drop it back to running 6 hours a day instead of 8? would that work? would be easy enough to do, i can just remove the evening trips that turn the SWG on and off from the timer...

i'll go outside now and test the FC and see where i'm at in basically mid afternoon, and then again around 5:30 when the sun is off the pool for the day. as of now it'll have full sun for about another hour and 15 min. when i tested at 9:30 this morning, it was at 5 on the K1000.

edit: right now (2:45pm) i'm still at 5 w/ the K1000, or at least somewhere between 3-5. to me it looked like a 5, but either way it definitely was definitely a darker shade of yellow than 3. i'll test again at 5:30-ish and if the past is any indicator it'll probably be down at 3.
You can do the math on SWG runtime using the PoolMath app.
 
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JimMarshall

Gold Supporter
TFP Guide
Jun 5, 2017
1,003
Oil City PA
Where you are at in CA I’m guessing you’ve got a fairly intense UV load. I’d guess you’d probably use 2.0 ppm per day? That would be 7 hours at 40%
@mknauss i think he’s fairly close to your neck of the woods any input?
 
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