3.4: The Challenge of Pressure
- Page ID
- 31597
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Nearly every technology developed for ocean exploration beneath its surface confronts one inevitable question: How deep can it go without being crushed? Many a machine has been lost (some with lives aboard) because of the intense pressure exerted by the ocean. Pressure is the reason why it’s easier to explore outer space. A dozen humans went to the Moon half a century ago, traveling a distance of 238,855 miles (384,400 km), but until 2019, only three humans had been to the Challenger Deep, the deepest location in the world ocean, a mere 6.8 miles (10,925 m) deep. Though dozens of people have now descended to the Challenger Deep (see below), extreme water pressure remains a barrier to all but the most specialized technologies.
Physicists define pressure as the force exerted by a fluid (a gas or a liquid) on an object immersed within it. In the ocean, every 10 meters of water depth, about 33 feet, increases pressure by about 14.7 pounds per square inch, or 1 atmosphere (atm) at sea level. Earth’s atmosphere, on average, extends about 7.5 miles (9.17 km) above us. So the pressure exerted by about 33 feet of water equals the pressure exerted by about 7.5 miles of gases in our (one) atmosphere. For simplicity, oceanographers often express water pressure in units of atmospheres. The calculation is quite simple:
P (atm) = z (m) / 10 (m/atm) + 1 (atm)
(Eq. 3.1)
where P is water pressure, z is depth, and 1 atm represents the pressure at sea level (from the atmosphere pressing down on the ocean).
The simple math makes it apparent: you don’t have to go very deep to experience great pressure. At 800 times the density of air, water exerts a much greater pressure on objects immersed within it. At a depth of a hundred feet (roughly 30 m), a diver experiences three times the pressure they would experience at sea level. Free divers—persons who dive with no scuba gear and rely only on holding their breath—experience even greater pressure. The current no limits freediving record depth of 702 feet (214 m), held since 2007 by Austrian Herbert Nitsch (b. 1970), converts to a water pressure 22 times the pressure at sea level (e.g., The Salt Sirens 2021). Imagine a newborn baby elephant (about 300 lbs) balanced on a USB wall charger (about 1 square inch on each side). That’s what a free diver experiences at those depths.