Alkalinty correction for CYA ???

What is floc, clarifier, stabilizer, cyanuric acid,
algaecide, brightener, dichlor, sodium hypo,
sodium bisulfate, ....??
hoodfigga
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Alkalinty correction for CYA ???

Postby hoodfigga » Tue 08 Apr, 2008 04:07

Once again I find myself confused about correcting or not correcting the alkanity reading on a taylor 2006 test kit for cya. As per the taylor website.

"When stabilized water is outside of the recommended range for pH or above the ideal range for CYA (7.4–7.6 ppm and 30–50 ppm, respectively) water balance calculations will be notably affected by the total alkalinity concentration unless a CYA correction factor is applied."

My current numbers are and my pool setup is noted in my signature.
Thank you.

fc 3
cc 0
ph 7.7
alk 70
cya 70
ch 325
temp 84F


15000GAL / Diamond Brite Classic / Aquarite SWG / 1.5hp Strarite / Hayward C17502 Cartridge Filter / AquaCal 155A Heat Pump /Sunny South Florida
chem geek
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Postby chem geek » Tue 08 Apr, 2008 13:04

You can use The Pool Calculator which will do the adjustment of TA for CYA automatically in its calculation of the Calcite Saturation Index.

The rough rule of thumb near a pH of 7.5 is to subtract one-third of the CYA value from the measured TA value for the adjusted TA that is used with the Taylor watergram.

Richard
Guest

Postby Guest » Wed 09 Apr, 2008 17:47

So in summary the correction for cya applied to the Total Alkalinity number is only used for the Calcite Saturation Index.

The total alkalinity number by itself is used as is with no correction.
chem geek
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Postby chem geek » Wed 09 Apr, 2008 18:47

Yes that's right, but when do you ever look at the TA number by itself? It is normally only looked at in conjunction with the saturation index. I suppose if one is looking at the normal ranges of 80-120 specified by traditional pool industry recommendations then you look at TA by itself, but technically it's better to look at the saturation index.

The only time TA by itself is of any reasonable meaning is making sure you have enough to prevent acidic conditions (such as Trichlor usage) from lowering the pH too quickly as TA is only a measure of preventing a drop in pH. A high TA, and technically it's a high adjusted TA, can lead to a pH rise due to carbon dioxide outgassing so even in this case it's really the adjusted TA that is more useful than the absolute TA number.

Richard
Guest

Postby Guest » Sun 13 Apr, 2008 13:07

Ok I was playing around with pool calculator values. Here is my thought. If chlorine is more effective as a disinfectant when the ph is low. Why not raise the TA (absolute) to 120 and drop the ph to 7.4? The saturation index checks out ok. For instance.

FC 3.0
CC 0
Ph 7.4
TA 120

CA 325
CYA 50
Salt 2800
TEmp 87F

csi .09
chem geek
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Postby chem geek » Sun 13 Apr, 2008 16:36

Three reasons why you wouldn't normally try a lower pH target.

1) You want a pH near 7.5 to be easiest on the eyes. Anywhere in the 7.2 to 7.8 range is OK, but tears are close to 7.5 on average.

2) A higher TA and a lower pH lead to faster outgassing of carbon dioxide and that causes the pH to rise unless you are using an acidic source of chlorine. The relative outgassing as a function of TA and pH is shown in this chart at a CYA of 30 (because it's really the adjusted TA that relates to the outgassing rate). Of course, the amount of aeration is key in determining the rate of outgassing and hence the rate of pH rise.

3) The traditional chart of chlorine effectiveness vs. pH is not correct when CYA is present. There isn't that much of a drop in chlorine effectiveness at higher pH when CYA is present. See this link for the traditional industry graph of chlorine vs. pH and the true graph when CYA is present. For example, with CYA, going from a pH of 7.5 to 8.0 has the hypochlorous acid concentration drop by 53% (roughly cut in half), but with 3 ppm FC and 30 ppm CYA going from a pH of 7.5 to 8.0 only has the hypochlorous acid concentration drop by 14%.

The bottom line is that you don't need to worry so much about chlorine's effectiveness vs. pH when CYA is present. Just keep the pH in a decent range not too far from 7.5. Then adjust other parameters accordingly. If your pool likes to hang out at a pH of 7.7 or 7.8, then that's not a problem so long as your overall water balance (saturation index) isn't extreme.

Richard

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