Drain & Fill Question

[waste]

NJ, have you considered the 'sheet method' which has been advocated here? (Place a sheet of plastic over the entire pool, pump the 'unwanted water out from under the sheet while simultainiously putting the fresh fill water on top - it allows you to keep the water level up without the dilution that Richard addressed). As Tenax wondered, how high is the cya and at what level do you want it?
Richard, thanks again for your knowledge and willingness to share it! (I actually didn't have to reach for my old chem textbook, nor the bottle of asprin , to understand this one !)

[Jakebear]

Here is the answer to your prayers.

Silage Tube ---- Yep, those big tubes that farmers blow cattle feed into for storage and feeding in the winter. You can buy them at most farm supply stores and they are available in various sizes -- 6, 8, 10, feet diameter and bigger they come in up to 8mil thick --- the way I figure it you could close one end --- lay it across the water of your pool and fill it with water. A 6' diameter 20' long tube should hold 565 cubic feet (3.1416 X Radius Squared X Length X 7.5 gal per cubic foot) which is equal to 4237 gallons ).

Water seeks its own level so the excess will go out the overflow or over the edge if you get a tanker to fill it with a big hose.

[chem geek]

Richard, thanks again for your knowledge and willingness to share it! (I actually didn't have to reach for my old chem textbook, nor the bottle of asprin , to understand this one !)

I must be softening a bit. The formula for how much pool water is needed to achieve a level of dilution using a continuous flow method is

% of Pool Volume = 100 * LN( 1 - (% Dilution / 100) )

Got a reputation to keep, you know.

[Jakebear]

% of Pool Volume = 100 * LN( 1 - (% Dilution / 100) )

Drawing a blank ---- What is "LN"

You do not need to worry about your rep EVER!!

[chem geek]

LN is probably from the French for "Logarithm Napier" otherwise known as natural logarithm which is a logarithm using a base of "e" (as opposed to 10 which is also a common base for logarithms). LN is also the abbreviation used in Excel formulas for the natural logarithm (base "e") so that's how I remembered it.

[CarlD]

Draining and refilling is simple:

If you have 20,000 gallons and your CYA is 100ppm, you need to dilute your water by half to get CYA to 50 ppm. By hook or by crook, you have to replace 10,000 gallons, whether slowly or quickly. If you drain 100 gallons at a time, you'll have to do that 100 times.

[chem geek]

Carl,

If you use a sheet to separate the fresh water that has no CYA from the pool water you are diluting that has CYA, then you are right, it doesn't matter how much drain or refill you do in what combination because there is no mixing of these two waters until you are finished adding and removing the total amount.

However, if you do not use a sheet and allow the water you are adding to mix with the existing pool water (say, by draining from one end of the pool while simultaneously filling from the other end, all while the pump is running), then it will take more water to achieve the same dilution compared to the sheet method.

A simple example will demonstrate this. Let's use your numbers of a pool with 20,000 gallons and 100 ppm CYA and you have fresh water you can add that has no CYA in it. If you want to get 50 ppm CYA then using a sheet to separate the fresh water from the pool water, you only need 10,000 gallons for dilution. However, let's do the dilution allowing the water to mix and let's do it in two steps with 5,000 gallons each. So, do the following:

1) Start with 20,000 gallons at 100 ppm CYA
2) Drain 5,000 gallons so you have 15,000 gallons at 100 ppm CYA (remember "ppm" is a concentration so draining doesn't change this, though evaporation would)
3) Add 5,000 gallons of fresh water. This gives you (5,000*0 + 15,000*100)/20,000 = 75 ppm CYA in 20,000 gallons
4) Drain 5,000 gallons again so you have 15,000 gallons at 75 ppm CYA
5) Add 5,000 gallons of fresh water. This gives you (5,000*0 + 15,000*75)/20,000 = 56.25 ppm CYA

Notice that replacing 10,000 gallons of water in two drain/refill steps only gets you to 56.25 ppm CYA and not 50 ppm CYA as you would get in one 10,000 gallon drain/refill step. If you replace less water each time and have more replacements, then you will get even less dilution. Notice that the formula is really

( (20,000 - 10,000/N) / 20,000 )^N * 100

where "N" is the number of water replacements. So doing only 100 gallons at a time would be 10,000/100 = 100 water replacements

( (20,000 - 10,000/100) / 20,000)^100 * 100 = 60.577 ppm CYA

"e" is defined as the limit of (1 + 1/N)^N as N goes to infinity so you can see the similarity of this with the above formula since the above formula in the limit as N goes to infinity (i.e. continuous dilution) is just e^(-0.5) * 100 = 60.653 ppm CYA which you can see is quite close to what happens with 100 water replacements.

The natural logarithm in the formula in my earlier post comes about because we are solving for how much fill water is needed as opposed to starting with a certain amount of fill water and solving for the dilution result.

Richard

[CarlD]

Richard:
I'm aware that if you drain off 200 gallons, then refill, your concentration of CYA changes. The next 200 gallons you drain will have a lower concentration so the refill will give you less "bang for the buck".

I deliberately decided to ignore that problem as it soon becomes an exercise in differential calculus--which I never use for fun or hobbies! I deliberately kept it SIMPLE for clarity.

I wanted to get to the REAL point: with excessively high CYA you cannot try to drain a little, then add a little to be effective.

Sometimes, trying to be very, very accurate causes you to obscure the REAL message and the real answer, so you have to watch for a "forest and the trees" situation: With excessively high CYA, if you want to lower it you must do a large drain and refill. Period.

The object is to give NJPool an effective option.