How fast does liquid chlorine mix if the pump is off?


Gold Supporter
May 5, 2019
Santa Clara Ca
The Experiment:
With the pump off, I poured 1 gallon of 10% chlorinating liquid into one corner of the deep end and began measuring the chlorine level at the opposite corner (shallow end) using Taylor 2006 FAS-DPD. I attached a spreadsheet which may be easier to read.
Time Elapsed Time Drops FC (PPM) LOCATION
7:15 PM -0:15 9 4.5 middle of pool
1 gallon 10% Chlorinating liquid poured into corner of deep end at 7:30 PM
7:30 PM 0:00 9 4.5 shallow end
8:00 PM 0:30 8 4 shallow end
9:00 PM 1:30 7 3.5 shallow end
9:15 AM 13:45 10 5 shallow end
9:15 AM 13:45 10 5 deep end
Pump run from 10:00 AM to 10:30 PM and from 1:00PM to 1:45 when sample was taken
1:45 PM 18:15 18 9 middle of pool
It was very clear that the chlorinating liquid does not mix rapidly with the pump off so I got a clear answer to my initial question.
My follow on questions are:
1) Where was the chlorine when the two corners had equilibrated? My guess is that it was sitting in the bottom of the deep end. Even though I had the gallon jug submerged up to the spout to minimize mixing when I slowly poured the chlorinating liquid, I could see the slightly yellow liquid flowing down towards the bottom. All samples were taken "elbow deep".
2) Why did the free chlorine drop after adding chlorine to the opposite end? If I had one only 1 reading I would ignore the but I had two measurements. Sunset here is around 7:30 now so sun was not a big factor. One factor may be the pool cover which sits on the water. We normally keep the pool cover closed and I kept it open for this experiment.

If I posted this in the regular forum, I'm sure I would be told "never add chemicals when the pump is off." I think that is good advice to protect the pool and people. I only run my pump for 1 hour per day. If I turn my pump on when adding chemicals, I don't know how to get it to automatically shut off after an hour without reprogramming controller each time. Maybe I will figure that out.

4 of our 6 grandkids live 8 blocks away. When we facetime them, it is hard to explain to the 2 year old and the 3 year old why they can't come visit. Stay safe.



TFP Expert
LifeTime Supporter
May 3, 2007
Pleasanton, CA
Pool Size
Salt Water Generator
The act of pouring the chlorine into water (i.e. momentum) combined with the slightly higher density would likely carry the CL towards the bottom of the pool.

As for the drop, that is hard to say. It could have been due to the slight current created by the entrance of the CL carrying less chlorinated water into the vicinity of the test area.