What is a part per million? If you’ve been in the pool industry for any length of time, you have certainly heard the term ppm. Chlorine should be kept between 2 and 4 ppm. Total alkalinity should be between 80 and 140 ppm. Cyanuric acid should be between 30 and 50 ppm, and not exceed 90 ppm.
But what is a ppm?
A ppm is a weight-to-weight comparison – but for practical purposes, it is simply a way of comparing apples to apples. It’s a single part in a million of other parts.
For example, a penny is 1 ppm of $10,000. Because there are 100 pennies in a dollar, then in $10,000, there are a million pennies.
A pound is 1 ppm of 500 tons. Because there are 2,000 pounds in a ton, then in 500 tons, there are a million pounds.
For the purposes of calculating the parts per million of chemicals that we add to water, it is necessary to know the weight of water, on a per-gallon basis.
It’s a good idea to simply memorize that a pound of water weighs 8.34 pounds.
So, a 20,000-gallon swimming pool contains 166,800 pounds.
20,000 gal. × 8.34 lbs./gal. = 166,800 lbs.
That means that adding 166,800 lbs. ÷ 1,000,000 = 0.1668 lbs. of anything to a 20,000-gallon pool would be adding a part per million of that thing.
It is necessary is to know the volume of water contained in the vessel to determine the ppm of chemical added.
Many times, however, the volume of water is not known. In this case, one can usually estimate the volume by multiplying the length, width, and average depth, times the number of gallons of water in a cubic foot.
There are 7.48 gallons in a cubic foot of water.
Consider the following example. A 15 by 20-foot pool has an average depth of 4.5 feet.
15 × 20 × 4.5 × 7.48 = 10,098 gallons 10,098 gal. × 8.34 lbs./gal. = 84,217 pounds of water.
84,217 ÷ 1,000,000 = 0.084217 lbs.
So, adding .084 pounds of anything to that water would result in adding a part per million.
Sometimes, you want to know how many ppm will result if you add an amount of something to water.
If we continue with the previous example, how many ppm would result if we added some amount of something to our 10,098-gallon pool.
For example, how much cyanuric acid would result (in ppm) after adding a 3-inch tab of trichlor to the 10,098-gallon pool?
An average 3-inch tab of trichlor weighs 8 ounces.
Because trichlor contains 54% cyanuric acid, 4.32 ounces of the tab is cyanuric acid.
54% × 8 = 4.32 oz.
Next, convert ounces to pounds.
4.32 oz. ÷ 16 oz./lb.
= 0.27 pounds.
Finally, because the 10,098-gallon pool takes 0.084 lbs per ppm, divide 0.27 by 0.084.
0.27 lbs. CYA ÷ .084 lbs./ppm = 3.2 ppm CYA
That means that every time we add a tab of trichlor, we’re adding about 3 ppm of cyanuric acid.
Currently, industry accepted guidelines place the ideal range of cyanuric acid between 30 and 50 ppm. You can see how quickly you can get to that number through regular use of trichlor tabs.