Chlorine Levels!

[robquick]

Chem Geek,

I've recently had a problem raising the FC levels beyond 1ppm. We used the tub loads over the Christmas period, in hind sight based on your numbers I didn't add enough Dichlor after heavy bathing. I've been using Dichlor only with occasional shock since early November with no problem. Can you please clarify that your 3.5 tea spoon calculation per hour per person applies when using Dichlor only too as opposed to the Dichlor and bleach process? Six of us used the tub for nearly three hours on New Year's Eve, am I right in thinking we should have used 6x3x3.5 (divided by two because I have ozone) = 31.5 teaspoons of Dichlor ??? Any guidance appreciated !

[chem geek]

Yes, the 3-1/2 teaspoons per person-hour applies to Dichlor-only, BUT first of all this is just a rough guideline and secondly if you use only Dichlor than the CYA builds up making chlorine less effective. You can get to the point where the active chlorine level is too low so bather waste takes so long to oxidize that you can't keep up.

Anyway, your calculations seem correct, but the bottom line is that you add enough chlorine so that you measure a small residual after 24 hours (or just before your next soak if you intend for the chlorine to last that long). Personally, I'd switch to bleach at some point. Otherwise you'll need to change the water when the CYA gets so high as to make the active chlorine level too low.

[robquick]

Thanks for the reply.

My plan is to switch to bleach once I get my head around the basics which your advice is helping me to do ! If you use Dichlor only what life span would you get from the water roughly before the CYA level became a problem using a 1500l spa for 3 hours bathing per week? I'm guessing this is a 'how long is a piece of string' question however, just trying to work out how urgent it is for me to convert to the Dichlor/bleach process ?

Thanks again.

[chem geek]

It is a tough question because if the time between soaks is longer, then having the higher CYA isn't as much of a problem since there's more time to be able to oxidize the bather waste before the next soak. The higher CYA is more of a problem for heavily used spas that are used every day or two since then the CYA not only builds up quickly, but there's less time between soaks.

A rough rule of thumb is the standard Water Replacement Interval (WRI) = (1/3) x (Spa Size in Gallons) / (# of Bathers per Day) where I presume the soak time is around 20 minutes. So in person-hours this is (1/9) x (Spa Size in Gallons) / (Person-Hours per Day). So for your spa that's around (1/9) x (400) / (3/7) = 104 days or around 3-1/2 months. However, the water quality will get worse as time goes on until you change the water.

If you use Dichlor-then-bleach, then you can usually go at least twice as long as the above formula and the water quality will be in better shape so you'll notice less of a dramatic improvement in water quality when you do change the water.