Some numbers .. Hardness ?

[Phillbo]

On the other hand, if I had my own SWCG system, I'd probably try to run it at 5 ppm FC (if the CYA were 70-80).

Thats my plan.

[mbar]

Ok, I get that, but if you put enough chlorine in manually, then the cell wouldn't have to generate as much....or am I just not getting it? So if you started out with 5ppms, and your cell was just making free chlorine as needed, then wouldn't it make enough fc to keep the level up to 5? Or is that what you mean 30% - 45% is? As you can see I don't have a SWG.

[Phillbo]

The reference to the 30- 45% is the setting on the SWG. It means that when it's running, it only produces chlorine XX% of the time. The more it runs, the more chlorine it produces.. so, there should be that magic % that produces just enough without burning the cell up too fast.

[chem geek]

Phillbo is correct, but perhaps looking at this graph may help. At any given level of CYA and FC, there is an amount of time it takes for half of the total chlorine to breakdown from sunlight (UV). You can see that the curves get close together at higher CYA which means that the half-life of chlorine is independent of the amount of chlorine. The half-life is the amount of time it takes for half of the chlorine to breakdown so if you have more chlorine in the pool, then more gets broken down and has to be replaced (because "half" of a larger number is bigger than "half" of a smaller number).

So if you try to maintain your pool at 5 ppm of chlorine instead of 3 ppm, you will lose up to 5/3 = 1.67 or 67% more chlorine per day and will have to make up for that somehow. In an SWCG system, you would turn up the percentage of time that the system operates -- namely 67% higher.

Now in addition to chlorine usage from the breakdown from sunlight, chlorine also gets used up by combining with and oxidizing organics and disinfecting pathogens (bacteria, viruses) and killing algae. This usage is dependent on how much of this stuff is in your pool and is therefore independent on the amount of chlorine in your pool.

You can distinguish between these two types of chlorine usage by seeing the usage overnight when the sun doesn't shine and compare this to the usage during the day. If you subtract your overnight "per hour" usage from your daytime "per hour" usage, the difference will be the loss from sunlight.

Typically, the usage (independent of chlorine level) oxidizing and disinfecting is pretty small, usually less than 0.5/day while the usage (dependent on chlorine level) from breakdown from sunlight is quite high with about half of the chlorine lost during the day (this varies a lot depending on the amount of sun exposure, latitude, etc.). So with this example, maintaining a 3 ppm FC would require generation of 0.5 + 3/2 = 2.0 ppm per day while maintaining a 5 ppm FC would require generation of 0.5 + 5/2 = 3.0 ppm per day. So in this case the increase in the salt cell is 50% (so if currently running at 35%, you would need to increase this to 35% * 1.5 = 52.5% (so 50%).

Richard

[Phillbo]

now my head hurts

[Watermom]

Maybe if the spelling in your thread title is fixed, it will make your head feel better!

(I fixed it for you.)

[Phillbo]

Talk about twilight zone... I thought I did fix that after someone told me to go to the advanced section.... Thanks.


I upped the setting just a bit ( about 40% on the dial) since I don't want to run the pump longer due to the pool staying clean at 7 hours a night. I retested after a few days and my FC is now at 4. I think I will leave it like that for a while and see how it does. It's a little low on the "best guess" chart but I thinkk it's fine since my CYA may be a little lower then reported. I find the test to have a little variance.