FC question

[nittany168]

I'd like to start off by saying that I've had such a great first pool season, and am so greatfull to everyone here for making it so. I have used the TF - 100 test kit with great sucess untill now. Here's my latest test#

FC 11.5
CC 0
TC 11.5
PH 7.8
TA 100
CH 400
CYA 80

10800 gal, SWG, pebble tec. I don't have the test stuff for the salt so I'm not sure where that is right now. We haven't been in the pool for the last week, and it has cooled considerably here in Phoenix in the last few days. I did turn my SWG down from 35% to 20% when I saw how high the FC was. Could this be from the cooler temps?? The FC has stayed right around 5 all season, so I'm at a loss as to why this happened.

Also I've noticed that my CH has creeped up quite a bit lately. This is my first pool and I still have to figure out how to replace some of the water.

 

[JasonLion]

Yes, it is the cooler temperatures. The rate at which chlorine gets used up depends on temperature, with higher temperatures using more chlorine.

 

[ping]

Yes, it is the cooler temperatures. The rate at which chlorine gets used up depends on temperature, with higher temperatures using more chlorine.

Jason, is it the actual temperature of the water or the sun intensity on the pool? I have solar heating and I can get my temperature to what ever level I want if I use a cover at night. If I control my water temperature at 90 degrees will that use more chlorine than if I keep the water at 85 degrees?

Thanks for all your help!!!

 

[chem geek]

It's both. When there is more direct sunlight, there is more chlorine loss independent of temperature. Even with no sun at all, there is chlorine loss that is higher at higher temperatures. In my pool, I have a mostly opaque cover which if I never open then the daily chlorine loss at around 88ºF is roughly 0.7-0.8 ppm FC per day but if the temperature drops to 80ºF then the loss goes down to around 0.5 ppm FC per day. At 50ºF the loss is less than 1 ppm FC per week -- maybe 0.1 ppm FC per day or something like that.

The losses from temperature and sunlight are independent and additive (i.e. you sum the two losses together for total loss). The loss from temperature is due to chemical reactions of chlorine with other substances in the pool water (including organics and nascent algae, if any), with the pool cover, with equipment, with the filter, etc. The loss from sunlight is directly from the UV in sunlight and independent of temperature. Note, however, that in the winter the angle of the sun is lower so the loss is lower as a result.