Pool Water Chemistry (Warning: Can Get Technical)

[stualden]

These are great - personally I find I need to stare at them quite a while to grasp them (I guess it's because you're dealing with 3 variables on two-dimensional graphs) but they convey more than words can.

Just a couple comments / question for clarification:

--What are the numerical values corresponding to "algae" and "bacteria" on your first graph? Do they correspond to the colors of the numbers in the table (i.e., red is below bact, green is above algae, black is in between)? Do these levels of HOCl correspond to particular levels of "oxidizing power" or "disinfecting power" that we commonly see quoted?

--Taking that algae line as a given, I read (roughly) the following minimum chlorine PPM for various CY PPM:

CYA..........Chlorine
5...........1
10..........2
20..........3
30..........4
50..........6

(Could you do another graph or chart which transforms the data into this format?) Unfortunately, these look even a bit higher than Ben's chart - discouraging!

--On the second graph, would you say the "take-away" is really that most of the chlorine-retention benefit of CYA is *already there* by 20 ppm, rather than "starts at" 20 ppm?

--The "no CYA" and "infinite CYA" entries are really points, right, not horizontal lines? (Hard to show clearly on this graph, though, I'll agree.)

--You talk about the "half-life" of chlorine, which gets at a concept you mentioned in that other post and I asked about there. I'm assuming that half-life here means the time in which the free chlorine ppm drops to 50% of what it was before. Perhaps you could also recast this graph to illustrate your point that "with twice as much chlorine, you lose it twice as fast." To me, this emphasizes the "double gotcha" with CYA - it forces you to bring your chlorine levels higher, which in turn means that your chlorine loss each day will be higher.

Thanks.

[chem geek]

These are great - personally I find I need to stare at them quite a while to grasp them (I guess it's because you're dealing with 3 variables on two-dimensional graphs) but they convey more than words can.

Just a couple comments / question for clarification:

--What are the numerical values corresponding to "algae" and "bacteria" on your first graph? Do they correspond to the colors of the numbers in the table (i.e., red is below bact, green is above algae, black is in between)? Do these levels of HOCl correspond to particular levels of "oxidizing power" or "disinfecting power" that we commonly see quoted?
Yes, below the "bacteria" line are red numbers in the chart and above the "algae" line are green numbers in the chart. The "bacteria" line corresponds to 0.011 ppm HOCl which appears to be about 650 mV ORP which is the minimum standard for disinfection in the U.S. and by WHO (though Germany has higher standards). I will add more data to my post to support this assertion, based on the Oregon Commercial Spas Study. However, the "algae" level is a guess since I cannot find good data to determine the level of chlorine needed to inhibit all algae, so I set an agressive level of 0.05 ppm HOCl. The real number could be 0.03 or less, or 0.1 or more, but given that Ben's table is based on reality, the real number is probably somewhere between 0.02 and 0.1 which correspond to the ranges in Ben's table (for the most part -- the shock section is the most inconsistent). I have been in communication with Ben on this and eventually he'll update his tables, but we want to get more real-world experiences from users first -- I plan to start another thread (non-technical) to get such data.

--Taking that algae line as a given, I read (roughly) the following minimum chlorine PPM for various CY PPM:

CYA..........Chlorine
5...........1
10..........2
20..........3
30..........4
50..........6

(Could you do another graph or chart which transforms the data into this format?) Unfortunately, these look even a bit higher than Ben's chart - discouraging!
See my response above. The 0.05 ppm HOCl level I set for "algae" is a guess so don't be distressed by it. Ben's table has low to high ranges that are roughly 0.02 ppm HOCl to 0.10 if you take combinations of "high chlorine & low CYA" to "low chlorine & high CYA". Please do not change any dosing behavior away from Ben's chart unless you are willing to take risks. Though I believe you will be fine from a disinfection point of view, we simply do not know enough about the algae prevention level to make a determination. You also don't want to "run out" of chlorine at any time, so Ben's approach of having a sufficient "buffer" of chlorine (stored in chlorinated cyanurates) is both conservative AND prudent.

--On the second graph, would you say the "take-away" is really that most of the chlorine-retention benefit of CYA is *already there* by 20 ppm, rather than "starts at" 20 ppm?
Yes, that is better phrasing and I will edit my post accordingly. Thank you.

--The "no CYA" and "infinite CYA" entries are really points, right, not horizontal lines? (Hard to show clearly on this graph, though, I'll agree.)
Yes, this is also true. Perhaps I can "truncate" the lines and/or dash them so that they look more like asymptotes which is what they represent.

--You talk about the "half-life" of chlorine, which gets at a concept you mentioned in that other post and I asked about there. I'm assuming that half-life here means the time in which the free chlorine ppm drops to 50% of what it was before. Perhaps you could also recast this graph to illustrate your point that "with twice as much chlorine, you lose it twice as fast." To me, this emphasizes the "double gotcha" with CYA - it forces you to bring your chlorine levels higher, which in turn means that your chlorine loss each day will be higher.
This is true, though I'm not sure how to graph this combination. Perhaps I can target a fixed ppm HOCl level and then the graph will show the relationship between FC and CYA that produce that HOCl level. On the same graph I can show the half-life of FC as a function of CYA (so CYA will be the X-axis and FC and half-life will be Y-axes). I'll try something and you (and others) can give me feedback/suggestions.

Thanks.

Thanks a million for you comments, questions, and suggestions. I have only just started this post and will add more and hopefully make things clearer. Right now I'm going to add a link to the spreadsheet that is sure to scare the bejesus out of most people! I'll also explain how the HOCl table and graph are only for a pH of 7.5 and will also post approximate formulas for calculating HOCl or FC given CYA levels.

[haze_1956]

If I understand the first chart correctly.

Red numbers denote - Level to low to kill bacteria or algae

Black numbers denotes - no bacteria but algae possible

Green numbers denotes - algae also killed.


If this is the case, perhaps you should switch the Black and Green colors.
A small point I know, but most people will associate the color green with algae growth. And it should help a layman to comprehend the chart.

[chem geek]

If I understand the first chart correctly.

Red numbers denote - Level to low to kill bacteria or algae

Black numbers denotes - no bacteria but algae possible

Green numbers denotes - algae also killed.


If this is the case, perhaps you should switch the Black and Green colors.
A small point I know, but most people will associate the color green with algae growth. And it should help a layman to comprehend the chart.

Yes, you understand the chart correctly. I was thinking of red being "bad" and green being "good", but I see your point that green, in the context of pools, probably means algae to most people. I'll change the colors in the table when I make some other changes I'm working on.

[haze_1956]

in the context of pools, probably means algae to most people

I personally made the "green means algae" association so I mentioned it.

But now you have me thinking completely in context of pools. How about these for full visual association?

Red = Germs
Green = Algae
Blue = Clean water

.

[aquarium]

If the pH is lowered to 7.2 do the charts change significantly?

[CToon]

THanks for taking the time to post this information. Having an understanding of the relationships is helpful

Following the Best Guess Chart has kept me out of trouble to this point. Through real world experience , I know it provides a margin of safety . I also know that things are better when I have FC towards the upper end of the scale. Now there's a little more insight as to how and why.

[chem geek]

If the pH is lowered to 7.2 do the charts change significantly?

I can best describe the change by an example. At a pH of 7.5, FC of 3.0 and CYA of 30.0 (and TDS of 550, Temp of 80ºF), the HOCl is 0.045.

At a pH of 7.2, with everything else the same, the HOCl is 0.052 which is not a huge change, but is still about a 15% change. The best thing to do is to download the spreadsheet and put in your actual numbers, but remember that except for disinfection, this is VERY preliminary so use with caution. Stick with Ben's best guess chart for now.

Richard