total alkalinity vs ph -- questions from horticulture background

jseyfert3

Bronze Supporter
Oct 20, 2017
581
South-Central WI
It might be this post I'm thinking of. He's probably posted the equations in more than one post, but I think I'm done searching for the night. Good luck!

P.S. chem geek also was a large participator at a different forum, before something happened there and he switched to using only the TFP forum. But there is a bunch of chemistry-filled posts over on that forum as well. I'll let you look into that if you want more of his posts.
 

mrwoo

Well-known member
Nov 9, 2019
54
kalispell, mt
OMG that was a good read! Thanks for that link.

Here is a question and I've not found a "defacto" answer yet in searching.... just what does one want for CYA in a hot tub with an swcg? I've been told 20-30 but read in that very detailed thread more like 60-80 as it benefits swcg systems. So, I wonder just what IS the optimum CYA level for a hot tub (with a swcg)???

There is wide disagreement among SWG manufacturers about the ideal CYA level. The recommended ranges from manufacturers go from 30 all the way up to 140. If you do a complete survey you will find that we are about in the middle of the range of what manufacturers recommend, with some much lower and some much higher and some in the same general area we are in. We recommend CYA be 70-80 because of extensive experience collected from a large number of members and because of practical concerns with how the test kit works, not because of any particular manufacturers recommendations.
I now understand a TON more about what is going on, such as the effective kill times and rates of HOCI as a sanitizer as well as how CYA holds reserves and how to target a FC to CYA %. Wow, isn't it funny how a little knowledge makes you feel so good? I've read parts and pieces of that info from chem geek and others here and a couple other forums, but that particular thread had it all in one place. Simply outstanding!
 
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ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
12,807
Northern NJ
Here is a question and I've not found a "defacto" answer yet in searching.... just what does one want for CYA in a hot tub with an swcg? I've been told 20-30 but read in that very detailed thread more like 60-80 as it benefits swcg systems. So, I wonder just what IS the optimum CYA level for a hot tub (with a swcg)???
CYA of 20-30 gives you the benefit of buffering the harshness of chlorine. Using more CYA then that gives you the benefit of additional UV protection of the FC. If your hot tub is indoors or covered you don't gain anything from CYA over 30.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
12,807
Northern NJ
That's the level I've seen given the most and also what I plan on using when I refill next. But why are there so many 60-80 CYA for SWG recommendations on this site?
Chlorine / CYA Chart - Trouble Free Pool
Recommended Levels - Trouble Free Pool
Just wondering.
Thanks!
CYA 60-80 is for an outdoor pool with SWG. Is your hot tub outdoors and uncovered?

TFP guidelines are primarily aimed at outdoor pool owners. They can be adapted for indoor pools or spas with some adjustments.
 

mrwoo

Well-known member
Nov 9, 2019
54
kalispell, mt
Ahhhh that explains it. Mine is outdoors but covered and also has UV filter so in my case, if I understand all of this correctly, I am only concerned with 20-30 cya in order to have the chlorine be not as harsh and also to have a little buffer of chlorine for sanitizing purposes.
For reasons that I don't fully understand but also that don't concern me, a pool that uses swg needs much more cya than non swg. It was fascinating reading how the CYA and HOCI actually worked and I was trying to apply some of that to my hot tub but could not understand why higher CYA was preferable with swg but not really recommended in a spa.

Thanks again.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
12,807
Northern NJ
For reasons that I don't fully understand but also that don't concern me, a pool that uses swg needs much more cya than non swg. It was fascinating reading how the CYA and HOCI actually worked and I was trying to apply some of that to my hot tub but could not understand why higher CYA was preferable with swg but not really recommended in a spa.
A SWG generates chlorine slowly. Let's say a SWG generates the pools required 3ppm of chlorine over a 24 hour period. So the SWG adds 3/24 = 1/8 ppm of chlorine an hour.

At the same time the sun UV rays consume 3ppm of chlorine a day. But the sun will consume that 3ppm of chlorine over an 8 hour period or a 3/8 ppm loss of chlorine and hour during daylight.

With the pool losing 3X the amount of chlorine the SWG is producing during the day you either need to keep the FC high so that you don't have the FC drop below the minimum required FC, or provide sunscreen in the form of a higher CYA to reduce the FC loss. It is the higher CYA of 60-80 that lets you keep the FC at relatively low levels with a SWG compared to the non-SWG CYA/FC recommendations.
 
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AUSpool

Bronze Supporter
LifeTime Supporter
TFP Guide
Sep 23, 2015
733
Brisbane, Australia.
Ahhhh that explains it. Mine is outdoors but covered and also has UV filter so in my case, if I understand all of this correctly, I am only concerned with 20-30 cya in order to have the chlorine be not as harsh and also to have a little buffer of chlorine for sanitizing purposes.
For reasons that I don't fully understand but also that don't concern me, a pool that uses swg needs much more cya than non swg. It was fascinating reading how the CYA and HOCI actually worked and I was trying to apply some of that to my hot tub but could not understand why higher CYA was preferable with swg but not really recommended in a spa.

Thanks again.

A CYA of 30-40ppm would make testing and dosing a little easier. 20ppm often shows up as 0ppm. With a UV I would go with a CYA of 30-40ppm due to the possible degradation of FC by the UV. Although having said that I seem to recal that degradation of FC due to a UV steriliser is nowhere near that from natural UV from the sun. And I found in practical assessment that the loss of UV output over time is greater than the manufacturers suggested, ie., you need new bulbs more often to be comfortable that the pretty blue light is actually doing anything.

Higher CYA defiantly improves SWG efficiency in the presence of UV. We had a weekender with a little pool, over the cause of one week my FC jumped ~2ppm with no other change other than the addition of 20ppm of CYA.
 

mrwoo

Well-known member
Nov 9, 2019
54
kalispell, mt
A SWG generates chlorine slowly. Let's say a SWG generates the pools required 3ppm of chlorine over a 24 hour period. So the SWG adds 3/24 = 1/8 ppm of chlorine an hour.

At the same time the sun UV rays consume 3ppm of chlorine a day. But the sun will consume that 3ppm of chlorine over an 8 hour period or a 3/8 ppm loss of chlorine and hour during daylight.

With the pool losing 3X the amount of chlorine the SWG is producing during the day you either need to keep the FC high so that you don't have the FC drop below the minimum required FC, or provide sunscreen in the form of a higher CYA to reduce the FC loss. It is the higher CYA of 60-80 that lets you keep the FC at relatively low levels with a SWG compared to the non-SWG CYA/FC recommendations.
And that is my "Ah-hah" moment right there. Thank you so much. If I may respond with what I think I am hearing...
CYA, keeping chlorine it binds to from being degraded by UV (or much slower anyway) is what a pool/spa NEEDS assuming it's exposed to sunlight.
If one were using liquid chlorine, you would perhaps only have 40ppm of CYA because the FC would be replaced continually and in a large dose (end of day). One might want to increase the CYA in order to handle really large amounts of contaminants (always lots of people in pool) so the FC that is bound to the CYA provides enough "reinforcements" to get through a day - also meaning you add a lot more liquid chlorine because of consumption?

As for the SWG, I see now what you mean and why. It makes perfect sense at that, it's simply a numbers game, a war of attrition.

So here is a question then (sorry, I always have questions). If your SWG output 3ppm each day, and your CYA was let's say 60 ppm, wouldn't there at some point be a time when the FC that was bound to the CYA slowly was degraded by UV or freed from the CYA to sanitize, and that the 3ppm per day being generated by the SWG would not be enough to "fill up" the available sites on the CYA that are in excess? Do you test your pool to see how much is lost at a given CYA/FC rate and then adjust it so that the SWG's output finds equillibrium? Or do you periodically add some chlorine?

I so love it when the dots come together. I was beginning to think I might annoy you fine people with my same question over and over :oops:
 

mrwoo

Well-known member
Nov 9, 2019
54
kalispell, mt
A CYA of 30-40ppm would make testing and dosing a little easier. 20ppm often shows up as 0ppm. With a UV I would go with a CYA of 30-40ppm due to the possible degradation of FC by the UV. Although having said that I seem to recal that degradation of FC due to a UV steriliser is nowhere near that from natural UV from the sun. And I found in practical assessment that the loss of UV output over time is greater than the manufacturers suggested, ie., you need new bulbs more often to be comfortable that the pretty blue light is actually doing anything.

Higher CYA defiantly improves SWG efficiency in the presence of UV. We had a weekender with a little pool, over the cause of one week my FC jumped ~2ppm with no other change other than the addition of 20ppm of CYA.
Thanks for responding.
I think you are talking about a UV light built into the spa? As I don't have any sunlight UV exposure this time of year, and in the summer it have to pass through a dual layer polycarbonate roof panel which has UV filtering, I won't have to worry about that aspect. I don't think my spa has UV but I will check.

Oh, that's why the 30ppm is the minimum recommended because of testing for it's presence.
 

jseyfert3

Bronze Supporter
Oct 20, 2017
581
South-Central WI
Oh, that's why the 30ppm is the minimum recommended because of testing for it's presence.
Not sure that's actually true, back in 2007-2009 or so when chem geek the recommendation was 20-30 ppm (same as it still is), which isn't testable with the Taylor CYA test, since that is 30 ppm minimum (the TFT one goes to 20 at the low end). This is shown in the sticky How do I use Chlorine in my Spa (or pool)? from 2008 where they recommend adding 20-30 ppm by just tracking how much dichlor you add. It was then assumed you had that much CYA and that was that. Probably works okay if you change the water every 3 months, but I'm sort of under the impression that back then they figured the rate of CYA degredation in a hot tub was less than it actually is.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
12,807
Northern NJ
If one were using liquid chlorine, you would perhaps only have 40ppm of CYA because the FC would be replaced continually and in a large dose (end of day).
In a perfect world you would like to use higher CYA even with liqudi chlorine. But then it requires FC levels over 10 which cause problems with accurate pH testing. So CYA 30 - 50 is the sweet spot for liquid chlorine usage depending on how much daily FC loss a pool gets. In the intense sun of the Southwest a pool can lose 4 - 5 ppm of FC a day.

Do you test your pool to see how much is lost at a given CYA/FC rate and then adjust it so that the SWG's output finds equillibrium?
Correct. Over a period of time you dial in the SWG output so that it comes close to the daily FC loss during the day.