Trying to lower Ph but getting odd results

[bownut]

Last night my numbers were the following:
FC 17.5
CC .5
Ph 8.2
TA 70

I added 1lb 6oz lb of dry acid, half the recommended amount by the PoolPal app to get to 7.6. The container recommended nearly that amount per 10k gallons. My pool is 20x40 with 3 foot shallow and 8 foot deep ends, so about 33,000 gallons. I distributed in the deep end and ran the pump overnight. I did half the amount this because I am getting errant results when I use the full amount suggested. So today I get:

FC 16.5
CC .5
TA 80
Ph 7.5

I don't understand why my results are not quite coming out like they should. I have never had this problem before. My Ph is about where it should be but using half of what was recommended to get there by PoolPal and a third by the Ph Down bottle. Any idea what is wrong?

 

[mknauss]

pH testing is inaccurate with FC above 10 ppm. Never add chemicals based on a pH test done with FC above 10 ppm.

You should also not use dry acid. It contains sulfates that will destroy metals, especially your SWCG. Use muriatic acid.

 

[bownut]

pH testing is inaccurate with FC above 10 ppm. Never add chemicals based on a pH test done with FC above 10 ppm.

You should also not use dry acid. It contains sulfates that will destroy metals, especially your SWCG. Use muriatic acid.

Thank you for the information. I had no idea about the dry acid. I'll wait to retest when my FC drifts down.