Chlorine Dissipation ~ Or Vacation Planning

[Holydoc]

I did a little experiment to see how long I could leave the pool and go on vacation if I brought it to SLAM level before I left. The goal here was to learn the rate the pool dissipated the chlorine so that I would know when I would need someone to come over and add chlorine. With CYA=70, my target FC=10 and the minimum would be FC=5 for my pool. My SLAM level is FC=28. SLAM level is advertised as the level to keep your pool FC if you want to rid your pool of a Algae problem without hurting your equipment by having your FC too high. So I knew this level for the FC would be safe. So I started the experiment...

Day 1 - Raise pool to 28ppm, SLAM level. 5 Gallons of Chlorine.
Day 2 - Loss of 7ppm. Basically 25% of the Chlorine. Current reading 21ppm
Day 3 - Loss of 5ppm. Basically 24% of the Chlorine. Current reading 16ppm
Day 4 - Loss of 3ppm. Basically 20% of the Chlorine. Current reading 13ppm
Day 5 - Loss of 3ppm. Basically 24% of the Chlorine. Current reading 10ppm, my target FC.
Day 6 - Loss of 3ppm. Basically 30% of the Chlorine. Current reading 7ppm. Need chlorine after this to keep pool from reaching 5ppm.

Of course there are a thousand variables (e.g., sun, temp, rain, time of year, etc.) that I am not accounting for, but it appears that the pool loses around 25% of its chlorine until it reaches its target FC which it then loses at a constant 3ppm. On Day 6, I could have someone come over and add half a gallon of Bleach to give me one more day or 5 gallons to give me another 6 days.

I hope this experiment helps others to test their pools to learn how your pool acts so that you can have a trouble free Vacation as well as a TFP.

 

[JoyfulNoise]

Very nice set of measurements. Water temp is a big factor as chlorine oxidation reactions tend to half/double every 13 degrees F. So cooler water would see lower losses. The use of a pool cover while on vacation would also significantly slow down FC loss as most of it is due to UV photolysis.

Another option, if water is cheap, is to raise the CYA level up to 100ppm (or even 120ppm) and then raise the FC to shock levels. The added CYA would protect the FC from UV loss and you'd see the loss rate on the low FC side slow down a lot. Of course, when coming back from vacation, one would have to drain half their pool water and then rebalance in order to restore the pool to proper levels.

And finally, an SWG or Stenner pump would greatly extend one's vacation time as the continuos chlorine additions would keep the FC above minimum levels for a longer time.

 

[jblizzle]

Wish you would have run it one more day. I bet you would have lost less than 3ppm.

 

[Holydoc]

Wish you would have run it one more day. I bet you would have lost less than 3ppm.

Jason,

I have my target FC=10ppm, so I religiously add chlorine once I get below that target. I was getting a bit anxious when the FC=7. My conclusion was that I lose 2.5ppm to 3ppm a day when I am at FC=10 from many previous chlorine additions. I just am worried about getting below the minimum readings.

At least now I know how to compute how many pucks I will need if I leave for over 6 days on a trip.

I now need to compute the cost of just running my pool up to shock level on the first of the week and then leaving it alone for the rest of the week, or continuing as I am and adding chlorine once a day. The first is a little more troublefree than the latter but is a bit more expensive. If shock levels are ok to swim in, then the extra cost may be worth the reduced work.

 

[jblizzle]

It would be more expensive to raise the FC up to shock level weekly due to the higher losses earlier in the week ... and your water would not be as balanced and would be harsher on skin/hair/suits than maintaining target levels.