FC levels versus Safety cover expected life question

[sunny beach]

Hi to all.
Brand new guy with a brand new pool here.
I've read tons of stuff on this amazing site, listened to other local pool owners, listened to my pool builder, read owners manuals from the newly installed equipment.
As you can imagine, tons of conflicting knowledge and advice.
My dumb newbie question is about the conflict I run into with recommendations for FC.

Pool school/ Pool calculator advises 2 to 6 ( shoot for 4 average?) for my pool size/test numbers
That number makes sense to me after my crash pool school reading, BUT, the maker of my safety cover (Cover Pools) recommends .6 to 1.75 FC for a better cover service life and to keep the cover open anytime the FC is to high to swim.

I have had no problems with any of the tests on my water so far. (LaMotte Color Q 7)
No cloudy water, never been shocked (June 10 opening).
I was running about a 3 FC average at first, then backed off to about 2.25 average in the last two weeks.
I seem to lose about .7 FC in a twenty four hour period, which is pretty good.(?)
I use 10% liquid pool chlorine.
Plaster, 13,500 gallons, hot (very) desert southwest climate, never tested more than a .25 CC so far, typically .15 or less.
Not a very heavy swimmer load on average.
(Grandkids on weekends sometimes)

Is the .6 to 1.75 FC recommendation really to low, considering my no troubles encountered so far?

I don't want to prematurely age and destroy the cover from too much FC.
Any thoughts on the matter?

Thanks in advance for your comments.

 

[linen]

Welcome to tfp, sunny beach

I can understand your trepidation due to the expense of your cover.

Does the cover manufacturer specify a cya (cyanuric acid, aka stabilizer, aka conditioner) level along with the FC level? If they do not, then their recommendation does not mean much...however it may still affect your warranty. What will damage the cover is the "active chlorine" level, also known as the "disinfecting chlorine" level.

For example at 0 ppm cya, and an FC level of 1 ppm, you have an "disinfecting chlorine" level of 0.484 ppm.

However, using tfp guidelines and a 30 ppm cya level (and it seems that is what you are using for your FC numbers above) at 6 ppm FC you would only have an "disinfecting chlorine" level of 0.106 ppm.

In the example above you should have less damage to the cover using tfp guidelines then the manufacturers guidelines (assuming they are not specifying adding cya in the pool). With that said, I would guess your cover manufacturer either has no idea of the Chlorine/cya relationship, or tends to ignore it like much of the pool industry does, so the warranty is still an issue.

 

[sunny beach]

Thanks for your quick response.
I don't think, off hand, that the CYA levels were mentioned in the Cover Pool recommendation.
My CYA runs about 35-39 according to the Color Q, which is admittedly known to have somewhat of a CYA measurement issue.
I do the CYA testing at the LaMotte recommended 70 to 80 degrees F by placing the test tube /sample under test in a 75 degree water bath, and then follow, to the letter, their other tips for best CYA results.

Perhaps this is not the right post to ask a new question, but how does Free Chlorine differ from "disinfecting" chlorine?
By simple definition, isn't that what Free Chlorine means?
e.g., The actual chlorine available to go to work breaking down stuff?
Please excuse my ignorance; There is a lot I still need to read and digest.
Thanks.

 

[jblizzle]

Here is a chart that shows what we are talking about:
http://richardfalk.home.comcast.net/~ri ... l/HOCl.htm

Think of the FC as all the chlorine that has not started to break anything down. But much of that is bound to the CYA which makes it pretty inert. The stuff not bound and ready to work can be thought of as the "disinfecting" chlorine.

 

[linen]

Perhaps this is not the right post to ask a new question

I think this is a great place for your questions.

how does Free Chlorine differ from "disinfecting" chlorine?

My very simple view of it is, Free chlorine is a measurement of the total available chlorine to sanitize the pool and is a sum of the "disinfecting chlorine" and the chlorine that is bound to cya (chlorinated cyanurates). The "disinfecting chlorine" also known as HOCL or Hypochlorus Acid is the stuff that is ready to do battle with chlorine, while the chlorinated cyanurates is chlorine being held in reserve. Chem geek used this analogy:

You can think of an analogy of people fighting a war where you have a group of "active" soldiers on the front lines fighting hand-to-hand combat with an enemy. You have many more soldiers in the rear that are not directly fighting and are in "reserve". When some soldiers in the front lines get killed or injured, you can replace them with some from the reserve, but the rate at which you will be able to wound or kill the enemy is only dependent on the number of soldiers you have on the front lines doing the hand-to-hand combat. It doesn't matter how many you have in reserve. The amount in reserve only tells you how long you can continue to fight -- not the RATE at which you can fight effectively.

So the key is to have enough soldiers on the front line to kill the enemy faster than the enemy is able to reproduce (double in size) which is what bacteria and algae do. So only the "active" front-line fighters determine whether you can prevent bacteria and algae from growing. The "reserve" is just there so you don't run out until the next time you add more chlorine to beef up both the reserve and the front lines.

 

[sunny beach]

Okay, got it!
I must have missed this deeper detailed line of knowledge in my reading so far.
One more question if I may.
On the richardfalk chart/link, what do the three colors stand for?
I would assume poor, minimum, and maximum?

I need to do more reading tonight to determine what FC/CYA I need to test at for the best balance between effective sanitation/ cover life.
Has anyone else gone down this path in regards to FC and covers?

By the way, (if it matters), we desire to keep the cover closed whenever possible due to the almost constant wind blowing all manners of junk/sand into our pool. If at all, the wind usually abates during the night.

 

[jblizzle]

I would just ignore the colors, I am not sure they refer to anything useful.

There is no determining the FC/CYA range. You set you CYA and then keep the FC in the recommended range. If you are going to keep the cover on most of the time, then I would suggest setting the CYA around 30ppm so that you can keep the FC at lower levels.

 

[linen]

I think they mean this:

red means bacterial growth while green means possible algae growth. Blue is the safe area

also:

Note that the red in the linked chart above represents a cutoff of 0.011 ppm HOCl which roughly corresponds to the 650 mV ORP level that the U.S. and WHO set as the minimum required for disinfection. The green color is a guess at 0.05 ppm HOCl of the minimum level of chlorine needed to prevent algae. The actual number may be quite different, from 0.02 or less to 0.1 or more, but based on Ben's "Best Guess CYA Chart" which is based on real-world experience, I suspect the actual number will be somewhere in this range.

 

[chem geek]

Bubble-type covers only last 2 or 3 years typically, though they tend to last longer if they are covered up when rolled up to protect them from the sun. Safety covers (thin vinyl) last longer. I have an electric safety cover and it has a warranty that pays 100% for the first 3 years and then is pro-rated for the next 4 so after 7 years there is no warranty recovery. We have found that the cover lasts around 4 years before it starts to get too many holes to patch. Interestingly, the holes are always at the sides and tend to be near folds so I don't think it's just related to chlorine or sun but to physical wear depending on how it gets rolled up. The cover company says some people get 7 years out of there cover, but that it varies. I suspect it also depends on how often the cover is opened and closed. We use the pool pretty much every day though 7-9 months of the year.

 

[sunny beach]

Thanks to all for passing along your knowledge.

Could someone clarify, in layman terms, just how the CYA releases FC back into "sanitation" service?
Or, using the soldier analogy above, how do the reserves get placed back on the front line to do battle - if ever?
It seems like magic to me.

For the issue I'm trying to solve with the cover reacting to higher FC levels (according to the Mfg -and warranty holder), then CYA seems to be somewhat of a foe itself. I can get a higher HOCl with reduced CYA at the expense of high FC losses in direct sun with the cover opened. But then, because the cover is opened, the Cover Pools published FC limits really don't apply during that time.
Hmmm, perhaps will it make sense to lower my CYA to about 20, and keep the FC reading around 1.75 whenever the cover is closed? That leaves me with an okay-ish HOCl coupled with the lower FC as they suggest. Then, whenever the cover is opened, just accept the fact that I will have to feed chlorine to higer levels and more frequently due to the sun loss/ low CYA.

Does any of this sound like a good plan, or am I fretting about it too much?

 

[chem geek]

The primary chemical equilibrium with chlorine and CYA is the following:

HClCY^- + H₂O <<<---> HOCl + H₂CY^-
"Chlorine bound to CYA" + Water <<<---> Hypochlorous Acid + Cyanuric Acid

At equilibrium, there is a back and forth of the above at equal rates and because it is faster to move to the left than to the right this means there is a higher concentration of the species on the left than on the right since the net rate is the product of the concentrations with the rate constant. In other words, most of the chlorine is bound to CYA. As for how quickly it gets released, if the hypochlorous acid level on the right were to instantly go to zero, half the chlorine bound to CYA will be released every 0.25 seconds (technically it occurs through a triplet of other related reactions -- the one above releases half every 4 seconds). So when you measure chlorine in a chlorine test, the hypochlorous acid reacts with the dye and more is released from CYA in the time of the test. So FC measures mostly the reserve or reservoir of chlorine, not the active chlorine level. More technical detail about this is in this post

Since your pool will be covered most of the time, you don't need a high CYA level. I have 40 ppm in my pool and I used to have 30 ppm, but I only dose twice a week so given the swing from 3 to 6 ppm I'd rather have 40 ppm CYA. If the CYA is lower, then the swings in FC mean larger swings in active chlorine level so on the high end the chlorine will be more noticeable. As for your cover manufacturers recommendation, they haven't a clue about the chlorine/CYA relationship. All that matters for cover degradation is the active chlorine level, not the FC level, so it's the FC/CYA ratio that matters.

I think that 20 ppm CYA may be too low for you. It's harder to measure and won't protect as much from sunlight when the pool is open.