Minimizing CYA to Maybe ~30

[sparcel]

I live in the San Jose CA area and do not have a SWG. Over the winter rains and pumping water out my CYA dropped below 30 and I can now bring it up to whatever I want. I see the commonly recommended range of 30-50, and some on TFP suggesting higher - even without an SWG. For maximum life of my solar cover I'd like to minimize my FC level, and therefore lean toward a CYA of maybe 30. But I don't want to double my chlorine demand. Last year my cya was ~50 most of the season and I tended to consume ~2ppm / day. I do have a Stenner pump, so maintaining a level FC is easy. Obviously I can just slowly raise the CYA and see how it goes - but I thought I'd ask.

Somewhat related to this I have a question about chlorine loss as relates to higher levels of FC. Anecdotally I've noticed that at higher levels of FC, perhaps over 8-12, such as during a SLAM, that the FC drop is much higher than at lower levels. Is this a well known phenomenon? Is there a formula for it? Obviously during the early part of a SLAM there can be algae that is consuming the FC, but I think I have seen high FC drops even at the end of a SLAM when the overnight test passes.

 

[MSchutzer]

I'm not that far away from you in Redwood City and I also have a bubble cover on the pool except when swimming. I run my CYA level at 30ppm and typically consume about 1 to 1.5ppm chlorine per day in mid summer. I typically dose to 7ppm when adding LC so that I drift down through the 4 - 6 ppm range between the two days I go between testing. The bubble cover makes a big difference in the Chlorine consumption.

But regardless of the CYA and CL level you will only get 1 to 2 years at most out of a cover before the bubbles start to disintegrate all over.

 

[sparcel]

Thanks. I guess I've been lucky. I got 3 summers out of my last Sun2Solar 12mil, and 5 years for the one before that. But I was maintaining FC closer to 3 at the time - except for the last 6 months before the last cover started falling apart when I was at an FC of 5 to 6. I don't know if it was just at the end of its life anyway, or if ramping up the FC impacted it.

 

[mgtfp]

Somewhat related to this I have a question about chlorine loss as relates to higher levels of FC. Anecdotally I've noticed that at higher levels of FC, perhaps over 8-12, such as during a SLAM, that the FC drop is much higher than at lower levels. Is this a well known phenomenon? Is there a formula for it?

Chlorine loss is a percentage of the FC-level. If you double the FC and keep all other relevant parameters like CYA, UV intensity, temperature, pH, constant, your chlorine loss will double.

If you raise CYA proportionally to FC, keeping the FC/CYA ratio and therefore the amount of active chlorine (HOCl) constant, you will loose a little less chlorine at higher FC (at the same FC/CYA ratio), because there are other effects beyond the sheer amount of chlorine bound to CYA that improve the protection of FC from UV at higher CYA levels. Like shielding of FC in lower water layers by CYA in higher layers. This is one of the reasons why TFP recommends higher CYA levels with SWGs (where the risk of falling below min FC and having to SLAM is low) to increase the lifetime of an SWG by having to run it less. For manually chlorinated pools the risk of having to SLAM is higher, and TFP therefore recommends maintaining a CYA level that allows SLAMming if required.

For maximum life of my solar cover I'd like to minimize my FC level, and therefore lean toward a CYA of maybe 30.

I don't think that cover lifetime decreases due to higher FC at higher CYA, as long as the same FC/CYA ratio is maintained. At same FC/CYA ratios, you end up with same amounts of HOCl and OCl^-, which are the chlorine species that can potentially oxidise a cover and have an impact on lifetime. The rest of the FC is bound to CYA and quite unreactive.

What can have an impact, is the freshly injected chlorine before it is mixed into the water and bound to CYA, if the cover is on while the Stenner is injecting chlorine. You therefore want to minimise chlorine loss, where a higher CYA can be beneficial as explained above. I doubt that the difference between 30 and 50 is that significant, those benefits kick in more in the 70-80 range.

But you have a larger working window at CYA 50. You can let it drop down to 30 before having to add more CYA. When trying to maintain CYA 30, you can't really let it drop. Below 30, CYA's UV protection efficiency drops significantly, so you'll end up with higher FC demand. You'll also be blind for where your real CYA actually is. Once you're off the scale below 30, you don't really know if CYA is actually 20 or 10 or even 0, making it more likely that you overchlorinate, because you maintain an FC for CYA 30 when it actually is 10, i.e. your HOCl concentration will be 3 times higher than intended. If on the other hand your CYA drops from 50 to 30, you'll only overchlorinate by a factor of 1.66.

That's how I interpret TFP's recommended CYA range, you maintain CYA within this range. You can't really maintain any parameter at the lower acceptable boundary. By the time you realise that CYA has dropped, you can be well below the recommended range.

Long story short, if there are any advantages of different CYA levels in regards to the lifetime of your cover, I'd see them at the higher end of the recommended CYA range.

 

[sparcel]

Thanks @mgtfp, nice explanation. Makes sense. And thanks @MSchutzer for the local experience.