CSI formula question

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pakrh8r

pakrh8r

0
May 15, 2013
51
Chicago, Illinois
Pool Size
5500
Surface
Vinyl
Chlorine
Liquid Chlorine
I will preface this by stating that I think the pool calculator is fantastic and have been using it since I found this site last year (thanks Jason Lion!!). I have been logging my test results in a spreadsheet and have been working on a log that will graphically show if my results are in the acceptable range. I wanted to make something so that I wouldn't have to jump back and forth between the calculator and my spreadsheet. I found JesseWV's graphical pool testing log (also very excellent) the other day but due to the way it is laid out (observations listed under the dashboard) it is very difficult to use on the laptop that I use for logging my results. I have been trying to figure out how to calcuate the CSI, and unfortunately ChemGeek's thread on the subject (http://www.troublefreepool.com/threads/558-Pool-Water-Chemistry) is way over my head. I've never taken any chemistry, and even if I did I don't think I's understand it :confused:). I found a simpler formula in many places online which is Calcium Saturation Index (CSI) = pH + TF + CF + AF - 12.1. I've also seen this listed as the Langelier Index. Are those one in the same? I guess my question is if that formula is close enough for my purposes. I notice my results are slightly different between that formula and the pool calculator's formula. The factors that follow were in tables on the websites that listed the CSI formula. Using a pH of 7.5, temp of 55 ( factor = 0.4), CH of 140 (factor = 1.8), alkalinity of 110 (factor = 2.1), TDS factor = 12.1 I get a result of -3.0. The pool calculator's result is -0.46. Am I ok to use the simpler formula, and if not can someone list and explain the way the pool calculator's formula works?
 
CSI and the Langelier Index (LSI) are closely related, but LSI has several shortcuts to make the calculation easier, which also introduce inaccuracies.

CSI is not something you can calculate by hand, you need a computer. In exchange it is much more precise than LSI. You might as well use PoolMath, and let it do the calculation for you.

Your example LSI calculation is way off. I think you have the TDS factor wrong. The TDS factor is nearly always going to be just a little below 10.
 
pakrh8r said:
I found a simpler formula in many places online which is Calcium Saturation Index (CSI) = pH + TF + CF + AF - 12.1. I've also seen this listed as the Langelier Index. Are those one in the same? I guess my question is if that formula is close enough for my purposes. I notice my results are slightly different between that formula and the pool calculator's formula. The factors that follow were in tables on the websites that listed the CSI formula. Using a pH of 7.5, temp of 55 ( factor = 0.4), CH of 140 (factor = 1.8), alkalinity of 110 (factor = 2.1), TDS factor = 12.1 I get a result of -3.0. The pool calculator's result is -0.46. Am I ok to use the simpler formula, and if not can someone list and explain the way the pool calculator's formula works?
Click to expand...

With the formula you gave, the LSI = 7.5 + 0.4 + 1.8 + 2.1 - 12.1 = -0.3, not the -3.0 you listed. The -0.3 is not that far off from The Pool Calculator or PoolMath result of -0.46 and as Jason noted our CSI calculation is more accurate and takes into account more factors including borates. Note that I don't get The Pool Calculator or PoolMath result you got unless I use a CYA of 60 ppm. The LSI formula you gave uses ADJUSTED TA so you need to adjust for CYA which at a pH of 7.5 is roughly subtracting 1/3rd of the CYA level from the TA result. In your case, this doesn't change the looked up TA factor since it still rounds to the same table entry.

The older formula you show has several flaws. Even the ANSI/APSP-11 2009 standard in Appendix A7.5 "Langelier Saturation Index (LSI)" uses different factors than you gave (they suggest using the closest factor looked up in the table). For your example it gives LSI = 7.5 + 0.3 + 2.0 + 1.8 - 12.1 = -0.5 which is basically the same as The Pool Calculator and PoolMath. Note that using the manual table lookups without interpolation can have an error of roughly 0.05 for each of four terms so an error range of +/- 0.2.

I cannot imagine how your calculating an adjusted TA, looking up each factor in tables, then doing the addition/subtraction could be any faster or easier than using PoolMath which is also more accurate.
 
Thanks for the replies guys. You were correct in that I had a typo in my post. The reading I got from my calculations was -0.3, not -3.0. The TDS factor I've seen everywhere, including when I just looked at the document chem geek linked to, lists the factor for TDS <1000 ppm as a constant 12.1 instead of 10 as Jason states. I'm not disputing you, just letting you know where that 12.1 came from. The factors that I was using are exactly the same as the linked document except that the versions I saw said to use the next higher factor if the number is not listed instead of the closest factor as the linked document states. I listed all of those factors in a table and used VLOOKUP() functions to pull the factors into a calculation, so I wasn't doing any manual labor in doing the calculation. I re-entered the same numbers into the pool calc app and still came up with -0.42, and that was using CYA of 35 so I'm not sure where we're differing there. I guess I'll continue to use the pool calc to get my CSI number instead of trying to be clever. While I've got you chem geek, you answered a CH question for me the other day regarding upping my CH of 140 for my above ground vinyl pool. I read in another of your posts somewhere that to bring the CSI closer to 0 one should increase CH. Would you recommend that I do that with my -0.42 number or should I wait until the weather and my water warm up to see how that affects the CSI? Thanks.
 
A vinyl liner pool doesn't need calcium so you don't need to worry about your CH being too low. I'm not sure why I said anything different -- perhaps I didn't realize you had a vinyl pool (can you link to the post where I wrote that -- I want to edit it to correct it). Your CSI can be very negative and not be a problem at all. It's low pH that is harmful to vinyl, not low calcium. There is not calcium carbonate to dissolve in vinyl (at least not in large amounts -- there is some filler in some vinyls). You can see the thread Did you know that Vinyl liners contain calcium? including my post at the end of that thread (caution: it's in The Deep End).
 
Chem geek - I just looked back at that post and it was actually jblizzle that answered my question regarding increasing CH for a vinyl pool. He did tell me that I do not need to worry about a low CH level with vinyl pools, so no need to worry about editing anything. I remember that was the answer but didn't accurately state that in my response above. Sorry for the confusion. He also said that a low CH level will not affect my equipment (heater, pump, etc.), but will a negative CSI affect any of that? My pool is only 3 or 4 years old and I want to keep everything in as good a shape as I can. I think I'll just take your word that CH won't affect the liner since I know better than to jump into the deep end :). Thanks.
 
There are several ways you can zoom out with Excel to help with smaller screen sizes. The easiest is holding down CTRL and using the mousewheel. However, I've found that setting a manual zoom level of 70% looks good on most smaller screens. Hope this helps and thanks for trying it out!

pakrh8r said:
... I found JesseWV's graphical pool testing log (also very excellent) the other day but due to the way it is laid out (observations listed under the dashboard) it is very difficult to use on the laptop that I use for logging my results.
Click to expand...
 
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