OT: imperial vs. metric units [Study As­sess­ment]

posted by Helmut Homepage – Vienna, Austria, 2016-05-16 16:26  – Posting: # 16316
Views: 14,704

Hi Angus,

» Yes; we are still in 1/16" units here. They are difficult to work with.

All scientific journals I know demand SI (i.e., metric) units. Imperial units are a mess.

One story from my past: When I was already a diving instructor I went through my cave-diving courses in Mexico according to a US-system (NACD).
To calculate the maximum dive time (leaving air consumption for descent/ascent aside) in the metric system is bloody easy. Say the tank’s volume is 12 L and pressurized to 200 bar. The volume of expanded air is 12 × 200 = 2,400 L. If the breathing rate at the surface is 20 L/min, the tank’s air lasts for 2,400 / 20 = 120 min (ambient pressure 1 bar). If you dive, every 10 meters the pressure increases by 1 bar. Thus at 10 m the tank’s air will last for 2,400 / [20 × (1 + 1)] = 60 min, at 20 m for 2,400 / [20 × (1 + 2)] = 40 min, and so on. No pocket calculator needed.
How is this stuff done in the US? To start the confusion tanks are not classified by their volume, but by the volume of expanded air if the tank is filled to its rated pressure (which commonly is 3,000 psi). A standard tank is called “80 ft3”. A common “surface breathing rate” is 0.7 ft3/min. The surface pressure is 14.5 psi and increases by 14.5 psi every additional 33 ft. Good luck in calculating the dive time.
In cave diving tanks are regularly “overfilled”, e.g., to 3,333 psi. Then your 80 ft3-tank contains 89 ft3. There is also special equipment (compressors, regulators, pressure gauges) rated for 300 bar (~4,350 psi). In the metric system the 12 L-tank is still a 12 L-tank (what else). In the imperial system it’s a 116 ft3-tank despite the dimension are the same. Crazy. I wonder why not more US-divers drown. Maybe they are better in math than me.

Helmut Schütz

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