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Monthly Science: Motel Water Pressure vs. Height

Being a sucker for infrastructure and numbers, the fire sprinkler system pressure gauges in the motel stairwell proved irresistible.

The first floor gauge shows a nice round 100 psi:

Hotel water pressure - floor 1

Hotel water pressure – floor 1

Up on the second floor, it’s 90 psi:

Hotel water pressure - floor 2

Hotel water pressure – floor 2

With a different brand of gauge, it’s also 90 psi on the third floor:

Hotel water pressure - floor 3

Hotel water pressure – floor 3

Maybe 85 psi on the fourth:

Hotel water pressure - floor 4

Hotel water pressure – floor 4

Squinting at the parallax, call it 80 psi on the fifth:

Hotel water pressure - floor 5

Hotel water pressure – floor 5

At the top of the vertical pipe on the fifth, on the other side of a valve, we return to the original valve company at 78 psi:

Hotel water pressure - floor 5 - top

Hotel water pressure – floor 5 – top

Water weighs just over 62 lb/ft³ at room temperature, which works out to 0.43 lb/in² per vertical foot. Not having packed my laser distance widget, I’ll guesstimate 12 feet and 5 psi per floor.

A quick graph with an eyeballometric straight-line fit:

Motel sprinkler water pressures

Motel sprinkler water pressures

Call it 0.42 psi/ft, which is pretty close to the right answer.

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  1. #1 by MishaBurnett on 2018-12-01 - 08:20

    My hat’s off to you. I don’t often get outgeeked on fire/life safety issues.

    • #2 by Ed on 2018-12-01 - 19:21

      It’s a nasty job, but somebody’s gotta do it … [grin]

  2. #3 by scruss2 on 2018-12-16 - 14:32

    Or in other countries, take the pressure difference between floors and divide by 10 — and there’s your storey height in metres, near enough. Divide by your preferred value of g instead if you want to be all fancy and accurate like.

    • #4 by Ed on 2018-12-16 - 17:46

      Heck, even we USA-ians can divide by ten!

      Now, if only we’d make the change and be done with it, life would be so much easier. [heavy sigh]