How Big Is That Pool?

By Kim Skinner, Que Hales & Doug Latta

Almost anyone who’s spent much time around swimming pools is familiar with the classic method for measuring their water volume – that is, by multiplying length by width by average depth and then multiplying that result by 7.5, which is approximately the number of gallons in a cubic foot.

That’s all well and good for basic rectangular watershapes, but what do you do when the pool in question has a truly irregular shape? The answer, as we’ll discuss here, has to do with the pool’s water chemistry and a little trick that involves an alkalinity test kit, some sodium bicarbonate or acid – and the formulas we’ll share here.

Most who have spent their time around pools are also familiar with dosage charts showing the amount of acid or sodium bicarbonate you need to induce a specific shift in total alkalinity in a pool of a given size. Turning that information a bit and using it to calculate a pool’s volume is as simple as adding a specific amount of acid or base to the pool water, observing the result and plugging both the dosage and the resulting change in alkalinity into these formulas.

Once you do the math, you’ll know with some precision how many gallons of water are in the pool.

GETTING STARTED

Before we begin, please note: To get an accurate volume calculation, it is important to alter your testing methodology to get your test kit to give you a reading to within one or two parts per million. To accomplish that with a standard kit, here’s what you do, step by step.

[ ] Take a water sample from the pool and test it for total alkalinity. With most test kits, each drop of titrant represents 10 ppm. In calculating volume, however, you need to be more precise, so you need a larger sample – in fact, a test volume that is ten times the size of the usual volume for testing alkalinity. For this, you need a small cup, and each drop of titrant will represent just 1 part per million of alkalinity instead of ten.

[ ] If the alkalinity is on the low side, use sodium bicarbonate to raise it. For pools known to hold between about 15,000 and 30,000 gallons of water, add precisely 5 pounds of sodium bicarbonate. If the pool is thought to be larger than that, add 10 pounds.

[ ] With the circulation system running, wait for the sodium bicarbonate to disperse throughout the water – a process that should take something less than an hour in most residential pools.

[ ] Take another water sample from the pool and test the alkalinity a second time, using the same larger volume of sample water.

[ ] Insert the resulting reading into this formula: 71,400 (the formula number representing the sodium bicarbonate) multiplied by 5 (the pounds of sodium bicarbonate added) divided by the ppm change in alkalinity. The resulting number is the pool’s volume in gallons.

Here’s an example: Let’s say the starting alkalinity is 100 ppm, that 5 pounds of sodium bicarbonate are added and that the ending alkalinity is 116 ppm – a change of 16 ppm. So: 71,400 times 5 divided by 16 equals 22,312 gallons. You can round that number up or down to the nearest 1,000 – that is, 22,000 or 23,000 gallons, whichever you prefer. The margin of error should be less than five percent either way.

THE FLIP SIDE

Let’s say the initial total alkalinity reading was high – upwards of 160 ppm. Here, you want to lower the total alkalinity with muriatic acid (31.45%) instead of raising it with sodium bicarbonate. In such a case, the testing and calculations proceed as follows.

You start with 125,000 (the formula number for 31.45% muriatic acid), multiply it by the quarts of acid added, then divide it by the ppm reduction in alkalinity. In going this way, you must (of course) be careful not to add too much acid at any one time – no more than two quarts of acid per 20,000 gallons of pool water when the TA is above 160 ppm and the pH is above 7.7. Two quarts of acid will lower the TA by 12.5 ppm for each 20,000 gallons.

The accuracy of this method is directly dependent on the accuracy of the chemical dosing and the method used in testing for alkalinity.

Obviously, using ten times more water in the test sample means that if the alkalinity level is 100, you will use 100 drops of titrant. That may seem a lot, but the result is an extremely accurate volume calculation for a pool where only guesswork would be possible using the conventional width-by-length-by-depth approach.

But please: Do not go overboard on either volume or dosage precision. Since a one-inch change in a pool’s water level can result in hundreds of gallons of water-volume difference, simply round off the calculated volume to the nearest thousand and pronounce it “good.”

Please note as well that the method you might use to add acid does not affect the amount that the alkalinity will change, so don’t worry about “columns” or “slugs” or any other potentials for introducing acid to the pool. That said, we believe it is best to dilute the acid by trickling it around the pool or by introducing it over the return lines while the system is on.

The key in all of this is making certain the sodium bicarbonate (or acid) disperses uniformly throughout the pool before the testing is done and the second sample is taken. Our experience with residential pools shows that, with brushing and the circulation system running, that time is usually from 30 minutes to 45 minutes.

Kim Skinner, Que Hales and Doug Latta are the principals behind onBalance, a swimming pool research/consulting partnership that investigates water chemistry and pool plaster – and studies the relationship between the two.