How long to wait before re-test

After adding liquid bleach, how long should I wait in order to get an accurate re-test?

I have a small algae bloom, so raised the cl to shock level. Following evening FAS-DPD said cl was 15 at 8 pm. I added 2 gal. of liquid bleach to my 40,000 gal pool, which should have raised cl to about 22.

Then tested the pool again at 9:45 pm to get a reading that I could compare to the next morning. At that point the cl reading was 18.5 and I wondered if bleach was weak. But the following morning at 7 am the cl level was 21. Pump was running 24/7.

The 7 am reading is about what I would have expected, but the 9:45 pm reading was not. What is the best practice for getting an accurate re-test?

Bill.

Did you add the bleach in front of a return? That can help it mix. Did you sample in a "dead" are with little water flow? Did you add the bleach in such an area? I would hink that with strong circulation, an hour and a half would be plenty of time to disperse the bleach - clearly it was not.

I spread the bleach around the whole perimeter of pool. Samples are taken on the back side of a return and between a skimmer, but there is pretty good circulation there. Always thought that was the best place, but I may try the front side of return.

Thanks.
Bill.

I'm guessing, based on having seen colored additives spread in a pool, but on a small pool (< 25,000 gallons) an hour should be OK, if the pool has both an operating skimmer AND main drain. With only one skimmer and one return, 2 hours would be safer.