TA - Need test advice.

[Crimson]

Got my TF100 test kit a few months ago. Been using it regularly and use the pool stores to 'sanity check' my results once a month when I go in for chlorine.

7/10/09 - Pool store tests TA at 110 (I'm about the same)
8/03/09 - Pool store tests TA at 80 (I'm about the same)
8/20/09 - Pool store tests TA at 150! (I'm at 70!)

WTF? My TA has been dropping consistently since I had my pool replastered. Pool store has been generally within 10-20 ppm of my TF100 tests. Now I am testing about 60-70 (It turns red at 6, more red at 7 drops).. so I am thinking I need to add baking soda.. so I go in to have the pool store verify my results and I get this! Sigh.

So I go to a different pool store, and they test it at 140.. I watched the lady do the test, and I counted the drops - It was 14. So now I am completely confused. I don't want to raise my TA if its really in the 140-150 range, but my testing is showing 60-70. Can anyone think of something I may be doing wrong with the test that could cause the results to be off? Its my understanding the 'static' effect on this test would result in my tests being higher than normal, not lower. And I now know how big the drops should be, so I don't think I am doing that part wrong.

Any ideas here? I am filling the container to the 25ml mark. Adding 2 drops of R7, swirling, 5 drops of R8, swirling.. then adding one drop of R9 at a time and swirling until it changes colors.. I get 6 drops until it changes, and 7 when it changes even more. So I guess the result would be 70.

 

[PaulR]

Same store? More to the point, same employee each time? People have varying degrees of training and skill. One thing you can be sure of, your training and skill haven't changed all that much...

Try this minor twist on your technique: After the R-0008, start swirling and keep swirling constantly, while you add drops of R-0009 one at a time, maybe 4-5 seconds between drops.

About the only reason for your TA to jump unexpectedly is if you added more water than usual lately, and your fill water has high TA. SO, I would be biased to trust your results even if you watch the pool store employee like a hawk and they do it exactly the same way you do.
--paulr

 

[Crimson]

Different store.. different chain.. different employee..

 

[chem geek]

If your pool was recently replastered, then the curing of that plaster would raise the pH (and CH) and TA so you have been likely adding acid over time. Your TA is probably dropping because part of the pH rise may be due to outgassing of carbon dioxide. Do you have any sources of aeration in your pool? Waterfalls, spillovers, etc.?

I would trust your own test numbers over that of the pool store. If you suspect your reagents (mostly the titrating drops), you could replace them.

 

[frustratedpoolmom]

Trust your own test results. From your description, I believe your TA to be 70, you should only add baking soda if your TA goes below 60.