The Weight of Water for the Clueless

Workings of a water tower

Of all the offenses against basic knowledge created by wannabe freelancers, perhaps the worst is the scientifically ignorant freelancer who tries to answer a technical question and gets it wrong. Second, we suspect, is the "fact-checker" who fails to catch and correct the error; usually because the fact-checker is equally ignorant. Yes: we're talking about the eHow.com family of contributors and the J-school graduates who pretended to be "content editors." Today's candidate is "professional journalist" W. L. Adkins, and his topic (at Sciencing.com) is "How to Calculate Pounds Per Square Inch in Elevated Water Storage Tanks."¹

Adkins proposed measuring the (vertical) pressure at the bottom of a tank containing 10 feet of water at an elevation of 25 feet. We took advantage of an online calculator to determine that the correct answer is approximately 15.14 psi. W. D., on the other hand, came up with a figure of 15.17; a difference attributable to the fact that we know that water's density changes with temperature and Adkins doesn't.

Adkins doesn't seem to know a lot of things, or perhaps better, seems to be confused about some things. Take, for instance, his introduction (which should have been corrected, Ms CE):

"The elevation [of the tank] means that the weight of gravity presses down on a tall column of water,"

Silly W. D., gravity doesn't have "weight," gravity is a force. With a faux pas like that right up front, it's a wonder that Adkins even came close to the correct answer. But he did – although his path was tortuous. Here's what he wants his readers to do:

1. Measure the height of the bottom of the tank above the reference point
2. Measure the depth of the water in the tank
3. Convert each measurement from feet to inches
4. Calculate the pressure at the bottom of the tank using a "weight" of 62.43 lb/ft³ and convert that from psf to psi by dividing by 1728 (in³ per ft³)
5. Calculate the pressure attributable to the depth in the tank plus column of water in the outlet pipe using the same clumsy method.

Adkins' formula looks something like this: ((h/12) * 62.43) / 1728 . That's four computations.

W. D.'s method, typical of a liberal arts major, involves multiple calculations and conversions. A scientist, on the other hand, might well streamline the method:

1. Determine height of the water column in feet
2. Multiply by mass of water (corrected for temperature, of course, something Adkins didn't address).
3. Divide by 144 - the number of in² per ft²

No conversion of feet to inches, no conversion of cubic feet to cubic inches; just two computational steps using the formula  (h * 62.43) / 144 ... but that's what a scientist would do, not what a Dumbass of the Day journalist would do.

¹ The original has been rewritten by Leaf Group's cleanup team, but can still be accessed using the Wayback machine at archive.org. Its URL was   ehow.com/how_5858171_calculate-elevated-water-storage-tanks.html

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