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  ForumSwimming Pool CareThe Deep End...Pool Water Chemistry  Welcome  to our new server and newforum software. If this is your first visit, be sure to check out the FAQ  by clicking the link above. You may have to register  beforeyou can post: click the register link above to proceed. To start viewing messages, select the forum that you wantto visit from the selection below. Thread: Pool Water Chemistry User Name   Password   Log in  Remember Me? Register HelpForgot your password?Home Pool School PoolMath Rules Store What's New? FAQCalendarForum ActionsQuick Links Advanced Search Results 1 to 20 of 73   Page 1 of  4 1 234Last Thread ToolsDisplay 05-25-2007, 06:56 PM Pool Water Chemistry This thread presents my findings so far on pool water chemistryincluding the following:1. More Accurate Calcite Saturation Index (CSI) toreplace Langelier Saturation Index (LSI) 2. Calculation of ppm HOCl (disinfecting chlorine) atvarious levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)3. Determination of pH and Alkalinity changes whenadding chemicals to the pool4. Various reaction rates including chlorine breakdownby UV Disinfecting Chlorine (HOCl) vs. Total Free Chlorine (FC)and Cyanuric Acid (CYA) The most important finding was how little disinfecting chlorine(HOCl) is left after chlorine combines with Cyanuric Acid (CYA)to get stored as chlorinated cyanurates (aka cloramides). Thechart at the following link shows this relationship. (I recentlydiscovered that all forms of chlorine are measured as ppmequivalents of chlorine gas, so all charts, graphs and thespreadsheet have now been updated to reflect this.)HOCl ChartNote that the red in the linked chart above represents a cutoff  #1 Join Date:Location:Posts:Mar 2007San Rafael, CA USA 8,657 chem geek   Senior Member   Forum  of 0.011 ppm HOCl which roughly corresponds to the 650 mVORP level that the U.S. and WHO set as the minimum required fordisinfection. The green color is a guess  at 0.05 ppm HOCl of theminimum level of chlorine needed to prevent algae. The actualnumber 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 onreal-world experience, I suspect the actual number will besomewhere in this range. So, red means bacterial growth whilegreen means possible algae growth. Blue is the safe area. The following shows this same data in graphical form with linesshowing the same two (probably correct) bacteria and (totallya guess) algae levels.The following is an approximate formula you can use so long asyour CYA ppm is at least 5 times your FC (the formula really fallsapart terribly below a ratio of CYA/FC of 3). (HOCl as ppm Cl2) = (FC as ppm Cl2) / ( 2.7*(ppm CYA) -4.9*(FC as ppm Cl2) + 5 ) and if you are interested in the FC for a given HOCl (toconstruct the equivalent of Ben's table, for example), you canuse the following which just solves for ppm FC from the above. (FC as ppm Cl2) = ( 2.7*(ppm CYA) + 5 ) / ( 4.9 + 1/(ppmHOCl) ) The constants in the above formulas are for a pH of 7.5 (whichis the only parameter that significantly affects these constants).With the spreadsheet I can easily calculate the constants forother pH, but remember that the above formulas areapproximate. For example, with FC of 3 and CYA of 15 theformula gives HOCl as 0.098 when the correct answer is 0.095.That's not terrible (about an 3% error). However, with FC of 5and CYA of 15 the formula gives HOCl as 0.239 while the correctanswer is 0.199 (about an 20% error) which isn't as good.A rough rule of thumb that applies at a pH of 7.5 is that theeffective chlorine level is reduced by a factor about equal to theppm of the CYA. So, a CYA of 30 ppm reduces the disinfectingchlorine (HOCl) level to about 1/30th of what it would be with noCYA.  The inverse of the above chart may be seen at this link:FC ChartThe chart columns from 0.02 to 0.1 ppm HOCl roughlycorrespond to Ben's Best Guess CYA Chart . Ben's chartconverted to show HOCl may be found here where you can seethat the rough Min FC corresponds to 0.03 ppm, the rough MaxFC corresponds to 0.07 ppm (implying an ideal target of 0.05ppm) and the shock table is not consistent, but probably impliesa minimum of 0.3 ppm, at least for green algae. User experienceindicates that hard-to-kill yellow or mustard algae (and maybeblack algae) may need 1.0 ppm HOCl for shock. User experiencewith black algae indicates that keeping active black algae fromgrowing requires around 0.07 ppm HOCl.A comparison of the traditional HOCl/OCl -  graph with the samegraph in the presence of CYA may be found at this post. Thisalso shows how CYA is a chlorine (specifically HOCl) buffer thatmakes HOCl concentration about half as sensitive to changes inpH.The srcinal source for the equilibrium constants was done in1973 (and published in 1974) where the recommended maximumCYA level was 25 ppm:J. O'Brien, J. Morris and J. Butler, “Equilibria in AqueousSolutions of Chlorinated Isocyanurateâ€, Chapter 14 in A. Rubin,ed., Chemistry of Water Supply, Treatment and Distribution,1973 Symposium, (published 1974), Ann Arbor Science, AnnArbor, MI, pp. 333-358.[EDIT] A searchable PDF of this paper may be found on a link onthis web page. [END-EDIT] A Little CYA Goes A Long WayNOTE:   The mechanism of protection of chlorine from sunlight by CYA is currently under review in this post . Higher CYA levelsmay protect even proportionately higher levels of chlorinemore, especially in deeper pools. The following is a graph showing that a large amount of thebenefit of CYA protection of chlorine from UV (sunlight) isalready there at around 20 ppm. This data is approximate, notonly because it is dependent on the amount of sun exposure,but because the rate constants themselves change with FC level(because there is a mix of two different rates of destruction --one from HOCl and the other from the chlorinated cyanurateswhich are more stable, but still breakdown from sunlight). Thelimiting half-life for HOCl/OCl- is 35 minutes which is consistentwith pool studies, but some experimental studies give 11.6minutes. The limiting half-life of the chlorinated cyanurates is 8.4hours though some other data shows it could be 6 hours.  The following graph combines the two concepts of needing morechlorine at higher CYA vs. the greater protection of chlorine byCYA. The graph shows the total chlorine (FC) loss rate inppm/hour vs. CYA at different HOCl levels. Remember that thisrate of loss will slow down as chlorine gets used up.Nevertheless, the absolute loss of chlorine is greater at higherCYA levels (keeping HOCl constant) and is the downside to a high CYA & high Chlorine approach. However, the primaryreason to have higher CYA and Chlorine is to have a sufficientbuffer of chlorine to prevent it from dropping to dangerouslevels. There is obviously a tradeoff here. Though using no CYAresults in the least amount of chlorine loss, the fact is that yousimply can't maintain a pool with only 0.05 ppm chlorineeverywhere in it -- hence a minimum level is needed as a buffer.Salt Water chlorine Generation (SWG) pools seem to require ahigher level of CYA, about 70-80 ppm, to operate efficiently. Thetheory is that the CYA is slow to store the chlorine as it isbeing generated so without enough CYA there is a build-up of chlorine that degrades the performance of the salt cell. I wouldprefer that the SWG manufacturers offer a larger lower-power(per length) cell that would work efficiently at lower CYAconcentrations. pH Rising If you find that your pH wants to keep rising, this may be due to

#5 - Control

Jul 23, 2017


Jul 23, 2017
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