PPM FC Drop Related to FC/CYA ?

Miniguy77

Well-known member
May 20, 2019
79
San Diego, CA
My CYA is 30. If I raise it to, say, 40-50, will that cause a smaller drop in FC in the course of a day due to the effect of sunlight? In other words, is it advisable to maintain a higher CYA within the normal range in order to minimize FC drop over a given period of time?
 
Last edited:

bmoreswim

Mod Squad
Gold Supporter
Platinum Supporter
LifeTime Supporter
Jul 16, 2012
5,721
Central MD
That is correct. CYA causes FC to last longer, primarily regarding it’s battle with the sun’s UV light. Regarding your overnight comment, there should generally be little to no decrease in FC overnight so long as the pool is not actively housing algae or other contaminants which would be using up FC to remove/kill them.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
16,219
Tucson, AZ
See this post by Richard. There are charts that show FC half-life versus CYA level.


In my own pool I have measured FC losses of less than 1ppm over a 24 hour period in the hot AZ sun because I use high CYA (90ppm is my target) and low FC levels (3ppm or less) so about a 3% FC/CYA ratio. I can get away with that because I carefully manage my water chemistry, use borates and treat for phosphates (PO4 < 150ppb). It is more time consuming on my part but I happen to have time to spare...
 

Miniguy77

Well-known member
May 20, 2019
79
San Diego, CA
See this post by Richard. There are charts that show FC half-life versus CYA level.


In my own pool I have measured FC losses of less than 1ppm over a 24 hour period in the hot AZ sun because I use high CYA (90ppm is my target) and low FC levels (3ppm or less) so about a 3% FC/CYA ratio. I can get away with that because I carefully manage my water chemistry, use borates and treat for phosphates (PO4 < 150ppb). It is more time consuming on my part but I happen to have time to spare...
The half life chart is very helpful. Thank you.
 

Miniguy77

Well-known member
May 20, 2019
79
San Diego, CA
See this post by Richard. There are charts that show FC half-life versus CYA level.


In my own pool I have measured FC losses of less than 1ppm over a 24 hour period in the hot AZ sun because I use high CYA (90ppm is my target) and low FC levels (3ppm or less) so about a 3% FC/CYA ratio. I can get away with that because I carefully manage my water chemistry, use borates and treat for phosphates (PO4 < 150ppb). It is more time consuming on my part but I happen to have time to spare...
I see you’re controlling phosphates. What is their effect in your unusual situation?
 

Etchase

Active member
May 22, 2019
33
Hawaii
See this post by Richard. There are charts that show FC half-life versus CYA level.


In my own pool I have measured FC losses of less than 1ppm over a 24 hour period in the hot AZ sun because I use high CYA (90ppm is my target) and low FC levels (3ppm or less) so about a 3% FC/CYA ratio. I can get away with that because I carefully manage my water chemistry, use borates and treat for phosphates (PO4 < 150ppb). It is more time consuming on my part but I happen to have time to spare...

Your losing less than 33% of your FC. Have you run it at a higher FC level to know more accurately what percentage you’re losing? I’m trying to understand if there is some unexplained shielding effect of high CYA values not explained by the half life chart.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
16,219
Tucson, AZ
I see you’re controlling phosphates. What is their effect in your unusual situation?
It’s just an “insurance policy”, like using an algaecide. My water has naturally low phosphates as my municipal supply doesn’t use any phosphates. So it takes very little phosphate remover (about 8oz) to drop my levels and I only have to treat once per year (usually in the springtime). So for my pool, it’s a cheap and easy thing to do to get a little extra insurance. Phosphates are simply nutrients needed for algae growth. When you keep them low, algae has s harder time growing. Phosphate removal is not a substitute for proper sanitation.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
16,219
Tucson, AZ
Your losing less than 33% of your FC. Have you run it at a higher FC level to know more accurately what percentage you’re losing? I’m trying to understand if there is some unexplained shielding effect of high CYA values not explained by the half life chart.
I haven’t measured that in a long time. The general consensus is that the UV shielding effect of CYA involves more than just the extinction of hypochlorous acid by UV. There could be some intrinsic absorption of UV by the cyanuric acid itself and/or a depth-effect where water at deeper levels sees a lot less UV than surface waters. Some careful experimentation with the right equipment could figure it out but most people prefer to use their pools for swimming rather than chemistry experiments. I get routinely yelled at by the family for messing around with the pool too much and not letting anyone swim :geek:
 
  • Like
Reactions: JoeSelf

Miniguy77

Well-known member
May 20, 2019
79
San Diego, CA
It’s just an “insurance policy”, like using an algaecide. My water has naturally low phosphates as my municipal supply doesn’t use any phosphates. So it takes very little phosphate remover (about 8oz) to drop my levels and I only have to treat once per year (usually in the springtime). So for my pool, it’s a cheap and easy thing to do to get a little extra insurance. Phosphates are simply nutrients needed for algae growth. When you keep them low, algae has s harder time growing. Phosphate removal is not a substitute for proper sanitation.
My phosphates are 1,000 ppb. Is there any downside to knocking them down if all other numbers are within the recommended ranges?
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
16,219
Tucson, AZ
Jus
My phosphates are 1,000 ppb. Is there any downside to knocking them down if all other numbers are within the recommended ranges?
Just time and money. You want to use a good commercial grade phosphate remover like Orenda PR-10000 or SeaKlear commercial grade. They cost more than the PhosFree stuff but they are more concentrated a don’t contain any clarifier or other junk.

I have a DE filter so my pool clears up in a matter of hours after treatment (it causes the water to become hazy/cloudy). With a cartridge filter, I’m not sure how long that would take but I do know a sand filter needs a filter aid to help clear the water. Sand filters can use either a polymeric clarifier or DE. Typically you want to treat the water, wait 24-36 hours for the pool to clear and then clean out the filter. The manufacturers typically recommend back washing but that’s not going to get rid of all the precipitated phosphate. So that’s why I usually do my treatment at the beginning of spring when I know I’ll be tearing down my filter for a deep cleaning. That way I’m sure all of the precipitated phosphate is out of the filter.

1000ppb isn’t the end of the world. Usually it takes more than that, around 3000 or more, to make water highly reactive to algae. You do need to check with your municipal supplier to see if they are using phosphates in delivery system. If they are, then treating for phosphates may not be worth it. I’d say if you have to treat more than once per year, then it costs too much. My original purchase of SeaKlear cost me $40. The amount I used was 8oz at a time. So I got 4 years out of that one bottle or $10/year in cost to me. That’s nothing. If I had to spend $40 or $50 per year doing it, then I wouldn’t bother.