I have been surprised at our profession’s lack of basic understanding regarding pressure and vacuum. I think most people of the general public have no idea that we live under 14.7 pounds of pressure per square inch on our bodies. We say this is what it is at SEA LEVEL, which means a lot. As with the recent catastrophic implosion of the submersible, it was well BELOW “sea level,” meaning there was considerably MORE pressure per square inch on its surface/shell. When we look at a pressure gauge that reads 0 psi, that is what we call PSIG (G stands for gage). If the gauge measured absolute pressure (and we were standing on a beach at sea level), it would read 14.7. So a gauge on a vessel reading 0 psig tells us there is actually 14.7 psi (at sea level) inside the vessel. When we see a vessel that is “caved in,” we will hear people state, “It was sucked in,” which is technically incorrect. What happened was we removed the pressure from within (created a vacuum), and the external pressure pushed the walls inward.
I had to explain that so the rest of this makes sense…
So the vessel sitting there in our process is @ 0.0 psig. It is equalized with the atmospheric pressure (14.7 psi/1 atm) exerted on us every second.
What happens when we remove that 14.7 psi from within the vessel?
Well, now we have 14.7 psi atmospheric pressure per square inch on the walls of that vessel, pushing against the outer walls of that vessel. And if this vessel is NOT designed for this vacuum or pressure differential between the outside pressure (14.7) psi and inside pressure (something less than 14.7 psi), it will begin to implode/collapse.
Think about the amount of energy 14.7 pounds per square inch (psi) represents. Take your computer screen and measure its size. Then divide that into square inches. Then imagine a 14.7-pound weight sitting on each one of those square inches. That is 14.7 pounds per square inch.
Now imagine 6,000 pounds per square inch. What would happen to a vessel that had 14.7 psi inside it but 6,000 psi pushing on its walls from the outside? That is a serious pressure differential, and any flaw, even something as small as a “hairline crack,” could cause the vessel to fail, and it would fail in the blink of an eye.
So at sea level, the atmospheric pressure is 14.7 psia (absolute). But what does water weigh? Denser than air? For sure. So as we go deeper underwater, the pressure increases are substantial. This is why our ears pop when we swim underwater; the deeper we go, the more pressure is exerted against our ear’s membrane. Every 33 feet we go down underwater, the pressure increases by 1 atm or 14.7 psi. And we can certainly feel it on our bodies and in our ears. The mask even gets tighter on our faces!
NOTE: salt water and fresh water do not weigh the same, so there will be differences depending on the FLUID you are surrounded by.
At the depths of the Titanic, the pressure would be around 6,000 psi. This says a lot about the Titanic ship – it still stands at the bottom of the ocean, but a submersible built in the 21st century catastrophically imploded at a depth much less.