FC loss in SPA without use

[run53]

I've been curious about this......

I thought the only ways to lose FC was via sunlight or organics. However when I re-fill my spa, I add 2 tablespoons of DiChlor to get the FC to 13. Then 5 days later after no use or exposure to sunlight, FC =5.5.

What is consuming the free chlorine? Is it that the fill water has organics in it?

 

[JVTrain]

Some is lost to natural decay of hypochlorite. The higher temperature of spas contributes to this. Some is also lost to small amounts of organics being introduced even when not in use. 1-2 ppm FC loss per day in a clean spa is very normal.

EDIT:

Ozonators or UV lamps in spas can also contribute to some FC loss.

 

[run53]

Ok that makes sense then. Especially since I have an Ozonator

 

[JVTrain]

Some spa systems allow you to control ozonator run time, some do not. If you have a spa that lets you control run time, setting it to run only after soaks or timing your daily pump cycle times for after when you usually soak can maximize the benefits of an ozonator and limit the extra FC demand.

 

[chem geek]

Ozone and chlorine react with each other to produce oxygen gas and chloride and chlorate. If there is no bather load and no ozonator then chlorine loss will be a lot slower though is dependent on temperature. Chlorine slowly oxidizes CYA in the water and it will slowly outgas.