Basically, I need to know exactly how to use my FAS-DPD kit to test the Malibu Pool Shock (liquid chlorine 12.5%) itself. I'm suspicious that it is not 12.5% but much, much less.

In 6 straight days we've added enough each evening to bring our FC to shock levels of 28ppm. Starting FC was 9.0-12.0 and CC was 1.0 - 1.5.

Two of the seven applications yielded 16ppm FC an hour after adding it and the other five times it yielded 11.0 - 13.5 ppm. We lost 1 ppm FC or less by sunrise each time and the CC never changed.

Last night we added 3 gallons (128oz each of 12.5%) to an 18,000 gallon pool that had these readings before adding:

FC of 9.0
CC of 1.0
pH of 7.6 or 7.8
TA 120 or 150 before correction
CYA of 60 or 70
Pool water absolutely clear and still being aerated to keep pH from falling.

It yielded only 16 ppm one hour later so we added 2 more gallons at 10:00 pm.

This morning the FC was 12.5 and CC was 2.0, up for the first time in 6 days. Scratching my head - it has to be the pool shock.

How can I accurately determine what the strength of the pool shock we are buying really is?