Dumb question

[stlmuelk]

Hi All, I'm new here and reading too much so this struck me...

Over at Pool Water Chemistry there are some nice charts.
What caught my eye was the "HOCI vs FC at different levels of CYA", and the "FC Half-Life vs CYA" charts.

My water is currently at CYA of 50. Looking at the first chart, the "safe" range for algae looks like it crosses 50ppm CYA at about 6ppm FC. Which to my mind means anything below 6 risks algae growth.
This can't be right! PoolMath says I need 4-8 ppm FC, and it always defaults to 6 for chlorine additions.

Then, if you combine that with the 2nd chart, "FC half-life", the scenario gets worse...
As an example, if I raise my water to 6ppm FC at 9AM just when the sun hits my pool, then by 3pm-ish my FC should be down to roughly 3ppm. So I'm starting the day at on the edge, and after only a few hours I'm way over that edge into algae-ville.

I'm sure I'm not reading this right because if I were it would mean I need to adding chlorine all day to stay above 6ppm FC when my CYA is 50.

What am I missing? Or should I just stay out of the "advanced sections" lol

 

[PoolStored]

I'm sure I'm not reading this right because if I were it would mean I need to adding chlorine all day to stay above 6ppm FC when my CYA is 50.

What am I missing? Or should I just stay out of the "advanced sections" lol

Nothing. Maybe "yes" on the second question. LOL.

The algae line is what the U.S. and WHO set as the minimum required for disinfection. Not the minimum in real life.

On the half life, that is correct. That is why we often tell people to dose chlorine to the higher end of the recommended range, or even above range, so that WHEN YOU TEST, you are still in range. Hence, never letting the pool FC get out of range (except on the high side )

 

[JoyfulNoise]

When you read the texts associated with those charts you will notice the follow -

The green color is a guess at 0.05 ppm HOCl of the minimum level of chlorine needed to prevent algae.

So, red means bacterial growth while green means possible algae growth.

The following shows this same data in graphical form with lines showing the same two (probably correct) "bacteria" and (totally a guess) "algae" levels.

When you dig deep into the scientific literature on algae (and there is very little science for controlling algae using chlorine), you find that most large scale algae blooms are controlled using about 1ppm FC with ZERO cyanuric acid present. That represents an HOCl level of about 0.5ppm which is roughly 10X the amount needed for disinfection of pathogenic bacteria. But that is an excess amount of FC needed to control an algae bloom which represent a HUGE amount of biological matter. What one needs to keep planktonic algae (free floating algae) from rapidly reproducing is very likely A LOT LESS than what is needed to bring an algae bloom under control.

So, when @chem geek created those charts, he tried to put a best possible guess line that would do reasonably well at killing algae but without overwhelming a pool with chlorine.

As for the UV photolysis. that chart ONLY takes into account UV interactions with hypochlorite (the main pathway for photolysis) and it doesn't account for other UV blocking mechanisms. You'll find that the half life plotted there is probably a bit overstated from reality as most pools see a 2-5ppm loss of FC in 24 hours.

So, as @PoolStored stated, you need to see what works in YOUR pool and then stick with it.

 

[stlmuelk]

Nothing. Maybe "yes" on the second question. LOL.

The algae line is what the U.S. and WHO set as the minimum required for disinfection. Not the minimum in real life.

On the half life, that is correct. That is why we often tell people to dose chlorine to the higher end of the recommended range, or even above range, so that WHEN YOU TEST, you are still in range. Hence, never letting the pool FC get out of range (except on the high side )

I was thinking I could get to a point where I add enough to get 8ppm when the sun goes down and let that ride for the next day. If the charts are accurate and the UV is high (it's "extreme" today) that should put me at roughly 2-3 ppm by the next evening. That's not including any environmental pressures.

 

[PoolStored]

I was thinking I could get to a point where I add enough to get 8ppm when the sun goes down and let that ride for the next day. If the charts are accurate and the UV is high (it's "extreme" today) that should put me at roughly 2-3 ppm by the next evening. That's not including any environmental pressures.

So, as @PoolStored stated, you need to see what works in YOUR pool and then stick with it.

It is typical, in a pool that is in the sun and uncovered, to lose 3-5ppm, NV/AZ/Texas being on the high end. With swimmers, it may be a bit higher. As @JoyfulNoise stated, the charts are a bit over stated.

Start at 9 or 10 and see where you end up. That would allow for a 5ppm loss and still stay above minimums. If you have a solar cover, put it on, that will help reduce FC demand. In MO, I'd be surprised, but not shocked, if you used 5ppm in JUNE. June and July are the worst for FC demand because the sun is so high in the sky. In the shoulder months, the demand is lower.

Solar covered, in Ohio, I lose 1.5-2.5 max.

 

[stlmuelk]

It is typical, in a pool that is in the sun and uncovered, to lose 3-5ppm Texas being on the high end. With swimmers, it may be a bit higher. As @JoyfulNoise stated, the charts are a bit over stated.

Start at 9 or 10 and see where you end up. That would allow for a 5ppm loss and still stay above minimums. If you have a solar cover, put it on, that will help reduce FC demand. In MO, I'd be surprised, but not shocked, if you used 5ppm in JUNE. June and July are the worst for FC demand because the sun is so high in the sky. In the shoulder months, the demand is lower.

Solar covered, in Ohio, I lose 1.5-2.5 max.

Yeah we're in a patch of super-sun/heat like a good part of the country. On top of that I'm in the midst of a slam that won't quit so I'm reading everything here (and watching Matt on youtube...hi Matt!)

 

[Newdude]

If the charts are accurate and the UV is high (it's "extreme" today) that should put me at roughly 2-3 ppm by the next evening. That's not including any environmental pressures.

There are too many daily scenarios to list. UV burnoff happens through the clouds, but usually less than blazing sun. You'd need the better part of a week of similar conditions to really get a feel for it and by then the conditions would change. Or the daily UV demand would increase (first half of the seaon) or decrease (second half of the season) shortly thereafter once you were all dialed in.

You really have remain fluid and use last week as your guide for this week, then fine tune on the fly if needed. Just a couple of weeks could be considerably off from each other. Or. Hear me out.

Run hot enough that you can sustain the max possible loss of that point in the season and still have plenty of wiggle room to spare.

 

[stlmuelk]

There are too many daily scenarios to list. UV burnoff happens through the clouds, but usually less than blazing sun. You'd need the better part of a week of similar conditions to really get a feel for it and by then the conditions would change. Or the daily UV demand would increase (first half of the seaon) or decrease (second half of the season) shortly thereafter once you were all dialed in.

You really have remain fluid and use last week as your guide for this week, then fine tune on the fly if needed. Just a couple of weeks could be considerably off from each other. Or. Hear me out.

Run hot enough that you can sustain the max possible loss of that point in the season and still have plenty of wiggle room to spare.

Yep between you and PoolStored I think that's the way. Gotta kill the hidden whatever first tho

 

[Newdude]

Overnight Chlorine Loss Test and SLAM Process if needed, or SLAM and pass the OCLT the first night if it wasn't needed.

 

[stlmuelk]

Overnight Chlorine Loss Test and SLAM Process if needed, or SLAM and pass the OCLT the first night if it wasn't needed.

Thanks, I'm on day 4. I have a cheap intex pool and today I found a new flap/seam going around the whole thing at the bottom I'd been missing with my brush. Hopefully that was it - we'll know after the next oclt.