Chemistry for the Dummy Swimming Pool Owner (Math Week 5)

Cement pond

The next time you watch an episode of "The Big Bang Theory," check out the whiteboards in the apartment shared by Leonard Hofstadter and Sheldon Cooper. They aren't covered by diagrams and cartoons (except when Sheldon's taping an episode of  "Fun with Flags"); they're filled with equations from top to bottom. As any scientist will tell you, math is the common language of all sciences. That's why it sneaks up on people all the time, even when they think they're just they're talking about landscaping or the environment. This time out, we spotted a case of innumeracy in SFGate.com's Ruth de Jauregui (affectionately known as razzberry-jam), who got confused, at best, while trying to explain the "Amount of Chlorine to Use Per Gallon of Pool Water."

Never mind that de Jauregui was unable, for some reason, to leverage her degrees in art to write a primer on concentration and solutions, we're here to figure out how anyone could use Ruth's instructions for determining the number of gallons in a swimming pool. Right off the top we knew something was amiss: the original question asked about the concentration of the chlorine, so the volume of the pool doesn't actually enter into the calculation -- that's for a later step. 

In her useless padding (which should have come at the end, not the beginning), de Jauregui "explains" how to calculate the volume of a swimming pool:

"A round ring pool, 48 inches tall and 15 feet across, holds 4,978 gallons. An in-ground pool can hold 31,000 gallons or more. To find the number of gallons in a pool, measure and multiply together the length, width and average depth, then multiply by 7.5 for a rectangular pool, 6.7 for an oval pool or 5.9 for a round pool..."

Errata: A round pool with a diameter of 15 feet and a depth of 4 feet holds 707 cubic feet of water or almost exactly 5300 gallons; not 4978. Pretty precise, Ruth, but not very accurate... WTF are the "length" and "width" of a round pool? Where on earth did she get that fudge factor for an oval pool? It's not in her references (such as they are) -- though we did note that the fudge factor for round pools is relatively accurate. Math questions aside, de Jauregui truly screws the pooch in her instructions for calculating how much chlorine to add. In her introduction she states, "The basic balance of chlorine to water is between 1 and 3 ppm, or 0.00013 ounces of chlorine per gallon of water."

Question, Ruth: what is that number 0.00013? Does it represent 1 ppm, 3 ppm, or somewhere in between? You cite a range but only provide one measurement. You then go on to instruct your readers:

"...if the chlorine level is already 1 ppm and you want it higher, add 0.00013 ounces per gallon to raise the chlorine by 1 ppm..."

The problem, as anyone with a pool already knows, is that there are different "kinds" of chlorine. Are you adding liquid sodium hypochlorite (household bleach)? chlorine tablets? shock? Had you read your main reference more closely – or perhaps had you understood your main reference – you'd have known that products differ in the amount of free chlorine they add when used.  For demonstrating both scientific illiteracy and mathematical illiteracy (innumeracy) in a single article, Ruth wins our coveted Dumbass of the Day award. Mazel tov, Ruth.

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