Chemical approach to determine pool size

MrAl

Member
May 15, 2020
5
Ozona, Fl
Hello all! Is there a way to determine pool volume by some kind of chemical addition at a known value and gauging the reaction in the pool itself? I should be somewhere between 13.5 and 15k I think, but I would really like to know for sure. Anyone have a proven method they could share?
 

sbcpool

Well-known member
Mar 10, 2015
265
Upland, CA
If you have a high-precision test and a precise source of a particular chemical it can be done. The hard part for me has been knowing the precise amount of chemical you have.

If it were me I would use something like CalHypo. I would first determine precisely how "strong" my CalHypo was by adding a precise amount to a precise amount of water. For example, adding 20 milligrams of CalHypo to 5 liters of distilled water (tap water already has chlorine). Then I would test that by filling the chlorine test kit to 25 mL so that each drop is equal to 0.1 ppm. If I measured 2.6 ppm I would know my CalHypo registers 65% chlorine on the test.

2.6 mg chlorine/L x 5 L = 13 mg chlorine
13 mg chlorine / 20 mg CalHypo = 65% chlorine


Then I would weigh out to a precise number of grams to add to the pool (maybe 200g or so). I would dissolve it in a pitcher first and then add it the moment the sun is no longer on the pool and let it mix for a couple of hours with the filter running. I would use the chlorine test filled to 25 mL again to measure the chlorine levels before and after the addition.

For example, you test your pool's chlorine to be 4.7 ppm. You add exactly 200 grams of CalHypo and after mixing you measure 5.9 ppm chlorine.

So we now know that 130 grams of chlorine (200 g x 65%) raises your pool by 1.2 mg/L (ppm).

The rest is math (and in school you asked why you needed this math):

130 g / 0.0012 g/L = 108,333 L / 3.7854 L/gallon = 28,619 gallons

You have a big pool :D

Don't forget each step has a margin of error, and they stack up as you move through the process. For example, if your scale is ± 0.001g your CalHypo could be anywhere from 62% to 68%. If the chlorine test is also ± 1 mg/L (ppm), that makes your CalHypo anywhere from 59% to 71%. Then you'll experience another ± 1 mg/L error when measuring the final chlorine level in the pool, making the possible range of actual pool size between 24,200 gallons and 34,100 gallons :oops:
 
Last edited:

sbcpool

Well-known member
Mar 10, 2015
265
Upland, CA
I suppose you could spend a few hundred on a ±0.0001g scale, and you could use 50 mL of water to decrease the margin of error on the chlorine test to ±0.05 mg/L (ppm), but even all those measures would still leave you with somewhere between 26,200 gallons and 30,600 gallons - a 7.8% margin of error.

Is all hope lost for this technique? If you're willing to drop a couple hundred on a scale, you might obtain more accuracy using that money for some ACS technical grade CalHypo and use the certificate of analysis to compute. The one I found lists 73.8% on the COA. If we assume that's accurate, we've tightened our margin of error to somewhere between 31,200 gallons and 33,900 gallons - ±4.1%.

For all that expense and trouble I'd rather just wait until I had to drain the pool and use one of these to measure how much water I put back in. They can be calibrated and are said to be ±1%, which in our case is far more accurate than the chemical method.

I'm not trying to waste everyone's time posting all of this, just trying to illustrate the depth of the rabbit hole that is seeking precision and how low the reward.
 

Bama Rambler

Mod Squad
TFP Expert
Bronze Supporter
LifeTime Supporter
Jun 22, 2009
23,864
SouthWest Alabama
It's really not as complicated as sbcpool indicates. You don't really need to be that precise. And you do it over time.
Just turn off your swcg and start dosing manually with bleach. Then test and pick a pool size and add as much bleach as PoolMath says it should take. Wait 30 minutes to an hour and test again to see if it hit the target. If it did then test and dose several more times to ensure repeatability and accuracy. If the resulting test didn't match the expected result, change the pool size the next time you test and add and see if the after addition test is closer to what you expect. You can keep changing the size in PoolMath each time you test and add until the results match the expected change for several times.

You can also, do this whenever you have to adjust pH or any other parameter just test before and after the addition to see if it meets the expected change over several additions. If it does, then the size you set in PoolMath is pretty close to the actual pool volume.
 
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sbcpool

Well-known member
Mar 10, 2015
265
Upland, CA
It's really not as complicated as sbcpool indicates. You don't really need to be that precise. And you do it over time.
Just turn off your swcg and start dosing manually with bleach.
What % of is the bleach you test with precisely (not what's on the label)? How do you insure that every bottle is the same? If you buy a month's worth or more, the last bottles will not be the same concentration as the first bottles. Sometimes I get a batch from the pool store that's really weak and sometimes really strong. I can't just blindly add x amount every time. I used to do it that way until I kept getting algae because of low FC.
 

Bama Rambler

Mod Squad
TFP Expert
Bronze Supporter
LifeTime Supporter
Jun 22, 2009
23,864
SouthWest Alabama
As long as you buy relatively fresh bleach and run the tests many times using different bottles of it you will get pretty close over time. Sure you could get a bad bottle of bleach, but over the long haul, and by using other parameters also, you can be pretty sure that it's close.
 

sbcpool

Well-known member
Mar 10, 2015
265
Upland, CA
That's how I've guestimated my pool size, too. I just thought OP wanted to know with some more precision. For dosing purposes a couple thousand gallons doesn't make a lot of difference in a 20,000+ gallon pool.
 

IceShadow

Gold Supporter
TFP Guide
Jun 8, 2019
1,563
Milwaukee, WI
If you have a high-precision test and a precise source of a particular chemical it can be done. The hard part for me has been knowing the precise amount of chemical you have.

If it were me I would use something like CalHypo. I would first determine precisely how "strong" my CalHypo was by adding a precise amount to a precise amount of water. For example, adding 20 milligrams of CalHypo to 5 liters of distilled water (tap water already has chlorine). Then I would test that by filling the chlorine test kit to 25 mL so that each drop is equal to 0.1 ppm. If I measured 2.6 ppm I would know my CalHypo registers 65% chlorine on the test.

2.6 mg chlorine/L x 5 L = 13 mg chlorine
13 mg chlorine / 20 mg CalHypo = 65% chlorine


Then I would weigh out to a precise number of grams to add to the pool (maybe 200g or so). I would dissolve it in a pitcher first and then add it the moment the sun is no longer on the pool and let it mix for a couple of hours with the filter running. I would use the chlorine test filled to 25 mL again to measure the chlorine levels before and after the addition.

For example, you test your pool's chlorine to be 4.7 ppm. You add exactly 200 grams of CalHypo and after mixing you measure 5.9 ppm chlorine.

So we now know that 130 grams of chlorine (200 g x 65%) raises your pool by 1.2 mg/L (ppm).

The rest is math (and in school you asked why you needed this math):

130 g / 0.0012 g/L = 108,333 L / 3.7854 L/gallon = 28,619 gallons

You have a big pool :D

Don't forget each step has a margin of error, and they stack up as you move through the process. For example, if your scale is ± 0.001g your CalHypo could be anywhere from 62% to 68%. If the chlorine test is also ± 1 mg/L (ppm), that makes your CalHypo anywhere from 59% to 71%. Then you'll experience another ± 1 mg/L error when measuring the final chlorine level in the pool, making the possible range of actual pool size between 24,200 gallons and 34,100 gallons :oops:
The link from ajw22 has all of the math needed using TA as the measuring stick. If you have a large enough sample, the TA test can be rather precise. :)
 

goody222

Silver Supporter
Feb 6, 2018
127
Chesapeake, VA
Ah, I too used the bleach method to determine my pool volume!! I have an curvy pool shape, an underwater bench, a shallow end that slopes down from 3.5 to 4', etc. The builder said it was 26,000 gallons according to their calculations. When I filled it the first time I measured the fill with my water meter and got around 21,500 gallons! So with the sun down, I measured the FC using the 25ml amount, let the pump run an hour, tested again, and played with the pool volume amount in Pool Math until the amount of bleach to add = 1 gallon. The result was 21,000 gallons pool volume!