CYA levels and alkalinity

[sunnyhoosier]

My CYA levels have been running high from the Dichlor and Trichlor I had been adding to my pool the last 4 seasons. I finally wised up and have switched to the BBB method. I had my CYA level checked 4 days ago and it had dropped from 150 to 115 after 2 drains and refills. My problem is that I have been using the Taylor test kit -2005 and my alkalinity levels are 150 but when I have my water checked at the pool store the alkalinity has been around 80. Does the pool store make any adjustments to the alkalinity that I should be doing? According to Taylor you should factor in your ph and CYA into a formula to get the correct alkalinity reading. When I did this I got a reading of 115. Any suggestions would be appreciated.

Sunnyhoosier

 

[Poseidon]

sunnyhoosier,
Pool store testing is generally not reliable. The quick and dirty way to do this calculation is to subtract 1/4 of the CYA reading for a pH of 7.0-7.2 and 1/3 of the CYA for a pH of 7.4-8.0. Your outcome of TA 115 is pdc (pretty darn close).

 

[duraleigh]

Poseidon,

I am not aware of that formula. Is that applicable to all CYA testing? If so, we all need to get on board with it and start using it. Where did you find it?

 

[JasonLion]

Duraleigh - you might not have run into the correction factor because the default around here is to post "raw" numbers, ie without the correction. Unless your CYA is very high or your ALK is very low there is usually enough "play" in the values to not have to worry about calculating the adjustment. The time you need to worry about it is when all, or nearly all, of the ALK reading is coming from CYA. A typical case would be PH 7.5 ALK 90 CYA 45, corrected ALK 75. The difference between 75 and 90 will never really mater, so why worry about it. Now if you have PH 7.5 ALK 30 CYA 90 corrected ALK 0, then you would need to worry about it.

 

[Poseidon]

I believe waterbear posted that a while back at Poolforum (I could be wrong on that). Taylor seems to support it.
Taylor Chemistry Topics

 

[duraleigh]

sunnyhoosier,
Pool store testing is generally not reliable. The quick and dirty way to do this calculation is to subtract 1/4 of the CYA reading for a pH of 7.0-7.2 and 1/3 of the CYA for a pH of 7.4-8.0. Your outcome of TA 115 is pdc (pretty darn close).

Well, I completely misinterpreted Poseidon's post. To me, it seemed he was saying that the CYA test needed the correction factor. i.e. if your CYA test is 60 with a pH of 7.5, then your CYA is REALLY only 40.

That's my fault for not reading the post above yours carefully, Poseidon. I'm sorry I misunderstood you.

 

[Poseidon]

duraleigh,
No biggie.

 

[Guest]

I believe waterbear posted that a while back at Poolforum (I could be wrong on that). Taylor seems to support it.
Taylor Chemistry Topics

More than once I'm afraid. IMHO, the correction is more important when using stabilized chlorine that benefits from a higher TA to maintain pH stability and usualy means high stabilizer levels in the water if the stabilized chlorine has been in use for a while.

 

[chem geek]

My understanding is that the main purpose for adjusting the Total Alkalinity number to account for the alkalinity from CYA is to calculate Carbonate Alkalinity since that is what is important for calculating the saturation index. And has been pointed out, at normal pH of 7.5, 30 ppm CYA only contributes 10 ppm to TA so isn't a big deal while 90 ppm CYA contributes 30 ppm so is a little more important. So unless you are calculating the saturation index or are doing calculations for rates of carbon dioxide outgassing (as in this table where I assumed a CYA level of 30 ppm), then adjusting the TA level doesn't matter much. As waterbear points out, it is far more important to have a higher TA level when using an acidic source of chlorine (e.g. a TA of 120 when using Trichlor). Otherwise, a lower TA with somewhat higher pH or CH works fine.

When we talk about lowering the TA level to reduce the tendency of the pH to rise, I have used a target of around 80 ppm as a lower limit for SWG pools where the CYA is kept closer to 80 ppm while I have used a target of around 60-70 ppm for non-SWG pools with lower CYA of 30 ppm. This is to have roughly the same carbonate alkalinity since that is what determines the rate of outgassing (along with pH and aeration).

Borates also contribute to TA and technically would need to adjust TA for the same reasons as above, but even 50 ppm Borates only contribute about 5 ppm towards TA at a pH of 7.5 (they contribute about 16 ppm TA at a pH of 8.0).

Richard