Cya / Fc Levels.

[robquick]

Hi,

As the CYA increases in a hot tub I believe the Chlorine becomes less effective. Does that mean the Chlorine level drops off quicker or that for example 5ppm FC with a CYA of 100 is less effective than a FC of 5ppm with a CYA of 50 ?

Thanks.

[chem geek]

The chlorine actually drops more slowly because it outgasses more slowly and reacts with chemicals more slowly, but such slower reaction rates make the chlorine less effective. 5 ppm FC with 100 ppm CYA is roughly half the active chlorine level of 5 ppm FC with 50 ppm CYA. If the active chlorine level gets too low, then it may not oxidize bather waste fast enough, especially if you soak every day, so such chemicals will build up possibly making the water dull/cloudy. At some point, the active chlorine level can get so low as to allow for bacteria to grow, especially those that tend to form biofilms which need to be killed more quickly before they get firmly established since they then become much more resistant to chlorine (they develop an outer layer that prevents the chlorine from penetrating quickly to call the bacteria within).