Adjusting TA down by 40 ppm when CYA > 100ppm; is this really necesary?

Each time I've gotten my water tested at our local pool store, they tell me to adjust TA down by 40 points, since my CYA measures 100 ppm. Is this really necessary? The pool store measures:

CYA = 100 ppm
TA = 170 ppm (prior to adjustment)

Not unless you have a concrete pool. If you do have a concrete pool or a SWCG (Salt Water Chlorine Generator), your TA (Alk) is too high.

Otherwise, it's not a problem, except that your pool will be 'twitchy' with respect to calcium and pH. If you add a shock dose of calcium hypochlorite, you might cloud your pool badly. Likewise, if there's much calcium in your pool, letting the pH go up a bit could result in a very cloudy pool.

You can lower the alkalinity pretty easily over time: http://www.poolsolutions.com/gd/lowe...p-by-step.html

Thank you so much! I do have a concrete pool. Indeed it was the cloudiness that led me to search for an independent source of information after I had shocked the pool with cal hypo. I'm so glad I found you guys. Pool is clear now after following all the recommendations here. Now I want to balance the chemistry and stay on this new, correct path!

Anyway, my original post above was too vague. Please allow me to clarify (no pun intended!).

My TA read 170ppm at the pool store (pH read 7.6), but they said since my CYA is 100ppm, the "true" reading is 130ppm. They said high CYA gives false readings of TA and therefore they subtracted 40ppm when recording my TA result in the computer. Is this bogus?

Since then I have brought my pH and TA down to 7.2 and 130 ppm respectively over about 3 days using muriatic acid. I'm on my way to ph 6.8 so I can begin aerating using my jets in the hot tub that shares water with the main pool. Once my Taylor K-2006c arrives I plan on a complete, independent, non-profit seeking chemical analysis and treatment plan. Thanks again to you wonderful folks for this forum that allows us to break free from pool store dependency!

The TA reading is a true reading of Total Alkalinity (TA). However, for calculating the Calcite (sometimes called Langelier) Saturation Index, one needs to adjust the TA by the a fraction of the CYA amount in order to get the carbonate alkalinity. All of this is done automatically for you if you use The Pool Calculator. You want your saturation index to be near 0 for a plaster pool. It just means that the water is saturated with calcium carbonate to protect plaster (which contains calcium carbonate) from dissolving or from forming scale. With a quality plaster job, you don't have to be that close to 0 in the index.

Of course, the bigger issue is your having a CYA that is too high. It will be harder for you to prevent algae growth using chlorine alone unless you use a very high FC level and if you run into trouble the shock level of chlorine will be impractical. It would be better for you to do a partial drain/refill to lower your CYA level and then follow the Best Guess Swimming Pool Chlorine Chart.