Normal "burn" rate for chlorine in a day

[chem geek]

So, 70 ounces of 6% bleach in 27,038 gallons is 1.25 ppm FC per day which with a level of 4.2 ppm FC is a loss of about 30% of the FC level per day. That's pretty good, especially with a CYA of 25 and no cover. The cooler water temp of 70F might be helping there. Keep us posted as it (hopefully) gets warmer for you in the summer. Not having any loss at night indicates that your chlorine loss is probably almost all due to breakdown from sunlight (and maybe a small amount with outgassing of hypochlorous acid).

Richard

[fcfrey]

So, 70 ounces of 6% bleach in 27,038 gallons is 1.25 ppm FC per day which with a level of 4.2 ppm FC is a loss of about 30% of the FC level per day. That's pretty good, especially with a CYA of 25 and no cover. The cooler water temp of 70F might be helping there. Keep us posted as it (hopefully) gets warmer for you in the summer. Not having any loss at night indicates that your chlorine loss is probably almost all due to breakdown from sunlight (and maybe a small amount with outgassing of hypochlorous acid).

Richard

Richard,
You never fail to amaze us with your knowledge.

I was trying to work the numbers to come up with your result and have not had any luck. Are your calculations in one of your posts in the China Shop?? Or could you run the numbers so I can see how you got there?

[chem geek]

For bleach and chlorinating liquid, you want to use the "Trade %" since that is the volume % of available chlorine. One gallon in 10,000 gallons produces an FC level equal to the Trade % (one liter in 10,000 liters does the same thing). This is because "percent" is per 100 so putting this into one in 10,000 is a net of one in 100*10,000 = 1,000,000 or one in a million which is "parts per million". 6% bleach has a trade % of 6.17 so that means that one gallon in 10,000 gallons will produce 6.17 ppm FC. So (10000/27038)*6.17 * (70/128) = 1.25 ppm FC. You can see that you scale down due to your larger pool (i.e. the same amount of chlorine results in less of a rise in FC since it's in a larger volume of water) and you scale to units of gallons using 128 ounces per gallon.

Then, 1.25 / 4.2 = 29.8%

For practical purposes, you can just use the % Weight of sodium hypochlorite, or 6%, in the calculation and not be that far off. You would get 1.21 ppm FC if you did that.

Richard

[fcfrey]

GREAT --- As usual you come through. I had the right numbers but was not taking the trade %.

What would you consider to be an acceptable burn off rate. You mentioned mine came out to ~30%

Thanks again ---

[chem geek]

Well, this is controversial because we haven't yet sorted out whether CYA gives better protection than shown in this graph and this is getting discussed in this thread. If CYA only protects chlorine via a roughly 6-7 hour half-life in direct noontime sun, then I would expect worst-case burn-off in places like Arizona, southern Texas and south Florida in peak summer to be up to half the FC in a day (the sun isn't directly overhead the entire time and that roughly compensates for the 14 hours or so of sunlight). Technically, the average sun intensity at the spring and vernal equinox (i.e. 12 hour days) is around 32% of the peak intensity (integral from 0 to 2*pi of the sine of the angle all divided by 2*pi) but summer days are around 14 hours so (approximately) 14*0.32 = 4.5 hours of equivalent direct noontime sun so an expected loss if chlorine not maintained constant (at a CYA of around 30 ppm so a half-life of around 6 hours) of around 1-exp((ln(0.5)/6) * 4.5) = 41%. I just use 50% to be conservative.

That is not the same as the burn rate if the chlorine level is kept constant. In that case, the burn rate is 1 - ( (ln(0.5)/6) * 4.5 ) = 52%. That is, maintaining the constant chlorine level results in a greater loss which makes sense. "Half-Life" numbers refer to declining values of chlorine over time (i.e. the time it takes to get to "half" the original chlorine level). So your pool is burning chlorine at around 30% / 52% = 58% of maximum. That seems very good for this time of year and your being in Pennsylvania -- and your number could be lower than expected due to not having complete sun exposure from sunrise to sunset (plus not all of the pool water is exposed to the sun when it's lower, etc. etc.... many many factors).

This site gives the altitude of the sun which today (at peak near noon) in Philadelphia is 70 degrees instead of 90 directly overhead so you get sin(70) = 94% of peak which really isn't so bad. By comparison, today in Los Angeles it's 76 degrees, in Phoenix it's 77 degrees, in Houston it's 80 degrees, and in Miami it's 84 degrees.

Richard