Pool Size (Capacity) Estimation

[washburn1445]

I have a "free form"-shaped pool. Is there any way, short of performing some complicated calculus integration (or using some sci-fi "Prometheus" laser-scanning orb), to estimate the size of the pool?

I have a SWCG, and was thinking of adding enough salt to move the salinity level up by a certain percent, and determine the size by the change of salinity level?

Any suggestion would be welcome!

[PoolDoc]

Method #1: Measure the volume exactly, when you next empty and fill, by reading your meter before and after, and the deducting average water use during the period.

Method #2: 'Grid' the pool, by laying out a rectangle to enclose the pool. The sides should be divisible by 3. Use stakes or tent pegs to layout nylon lines, 3 long wise, and 3 cross wise, and count the 'squares. Count each section as 0, 1/2 or 1. Measure the depth in each 1/2 or 1 square. Leap frog the lines, to create the new squares. Calculate the volume of each square (9 sft x depth or 4.5 sft x depth). Sum the volumes.

It will be easier if you use graph paper to track your work. This should be within 20% of actual volume.

Method #3: Add 5 lbs exactly of calcium chloride dihydrate. Measure the change in hardness using the K2006. Calculate to the volume. Probably accurate to 25%.

Method #4. Layout a bounding rectangle or trapezoid that fully -- but barely -- encloses the pool. Layout the largest enclosed trapezoid that will fit IN the pool. Estimate the average depth. Calculate the volume of each solid (large trapezoid sft x depth; small trapezoid x depth) and estimate the pool's volume as in between the two. Probably accurate to 50%.

[PoolDoc]

PS. I'm sure there are other methods. Those are just the ones I've used, or that occurred to me just now.

[washburn1445]

Method #1: Measure the volume exactly, when you next empty and fill, by reading your meter before and after, and the deducting average water use during the period.

Method #2: 'Grid' the pool, by laying out a rectangle to enclose the pool. The sides should be divisible by 3. Use stakes or tent pegs to layout nylon lines, 3 long wise, and 3 cross wise, and count the 'squares. Count each section as 0, 1/2 or 1. Measure the depth in each 1/2 or 1 square. Leap frog the lines, to create the new squares. Calculate the volume of each square (9 sft x depth or 4.5 sft x depth). Sum the volumes.

It will be easier if you use graph paper to track your work. This should be within 20% of actual volume.

Wow. Thanks! Will probably try the second method. I'm guessing:

1. I can use a plumb line (something weighted) to measure the depth of the pool.
2. The accuracy of the measurement for this approach is based on how small I make the squares?

One side of the pool is higher than the other. Thus, it may be a slight challenge to secure the lines on that side of the pool. However, as that side is a wall with stacked slate, I can just stick something in horizontally with an eye-let.

As an aside based on your statement of "when you next empty and fill", is there any reason I would need to completely empty the pool on a regular basis? I presumed I would only ever need to empty to repaint or resurface the pool.

[PoolDoc]

1. I can use a plumb line (something weighted) to measure the depth of the pool.

Yes, or a folding ruler, or a pool pole with a yard stick taped on.

2. The accuracy of the measurement for this approach is based on how small I make the squares?

Exactly. But if you get too small, it will probably be hard to maintain equal squares throughout the measuring process.

As an aside based on your statement of "when you next empty and fill", is there any reason I would need to completely empty the pool on a regular basis? I presumed I would only ever need to empty to repaint or resurface the pool.

Also, correct. That's sort of and if-and-when note: If you do happen to drain, that would be when you want to get an exact volume.

[nefretrameses]

You can estimate surface area by measuring the number of gallons to raise your pool 1 inch. Requires a calm surface. Mark the current and desired level like on a skimmer I r piece of tape. Put the fill hose underwater. It shouldn't take a great deal of time, so maybe restrict other uses during the fill. Surface area = (Gallons to raise 1 inch x 12) /7.48 or Gallons x 1.604. Multiply surface area by average depth. Use a string with a knot tied every 1 or 2 feet. Pull the string across the length of the pool and use Doc's pole/yardstick to measure at each knot. Attach a small level with rubber bands to make the pole plumb. Add all the depths and divide by the number of measurements. There are other average depth estimating methods depending on the shape of your (pool's) bottom. Like shallow depth + deep depth /2. I was able to estimate my pool's gallons and confirm my result with expected response to chemical additions.

[washburn1445]

Exactly. But if you get too small, it will probably be hard to maintain equal squares throughout the measuring process.

I was just going to hang a lot of strings across the pool--I told the kids it will be like an archaeological dig. Haha.

You can estimate surface area by measuring the number of gallons to raise your pool 1 inch. Requires a calm surface. Mark the current and desired level like on a skimmer I r piece of tape. Put the fill hose underwater. It shouldn't take a great deal of time, so maybe restrict other uses during the fill. Surface area = (Gallons to raise 1 inch x 12) /7.48 or Gallons x 1.604. Multiply surface area by average depth. Use a string with a knot tied every 1 or 2 feet. Pull the string across the length of the pool and use Doc's pole/yardstick to measure at each knot. Attach a small level with rubber bands to make the pole plumb. Add all the depths and divide by the number of measurements. There are other average depth estimating methods depending on the shape of your (pool's) bottom. Like shallow depth + deep depth /2. I was able to estimate my pool's gallons and confirm my result with expected response to chemical additions.

Interesting. I found one the earlier planning sheets from the pool builder today (will be trying to contact them. I kind of doubt I will get anywhere, as I tried before and they were unresponsive), and it looks like although the pool is "free-form," they had planned the longer length to be 35' and the shorter one by 25' (the lengths being perpendicular to each other). I also know now the deepest part of the pool is 8' and the shallowest is 3'. The one complication is that there is a small "wading pool" section that is <1' deep (the previous owner of the house had a one-year-old and was planning to have another baby). As it will be easy (easier) to find the actual volume of that part of the pool, I presume I can just subtract that from the "surface area" approach.

[nefretrameses]

Exactly. I assume that the pool bottom has the same depth across the width of the pool at a particular point along the pools length. My pool has a 3 foot shallow and 6 foot deep end. Average depth (deep+shallow/2) = 4.5 ft. When I measure every foot , sum and divide by the number of measurements I got 4.1 feet. That's a 1200 gallon difference - about 10%. I also have curved corners where the bottom meets the sides. Its about a 2 foot radius, so I figured the surface area of the curved triangular area ((4x4)-(2x2x3.414))/4). Think of a circle inside a square of the same size. Multiply this area by the circumference of the pool to get cubic feet and multiply by 7.48 to get gallons. For my pool this adjustment was 51.5 cu. Ft. And 385 gallons (a subtraction). You can adjust for steps, swim out and other areas like yours in the same way. For my 13,000 free form pool it took 277 gallons to raise 1 inch - about 30 minutes with a garden hose. Being off ( of raising the water 1 inch) by 1/16th equals a 6.25% error - about 800 gallons. You can increase your accuracy by adding 2 inches, or 6 and adjusting the calculation. Don't know if its possible or desirable to lower the pool below the wading area then add 1 inch.