Top Scuba Diving Laws of Physics May 10, 2019 by lukesheppard

Back to school guys, when I became a Dive Instructor I never really expected that I’d become a chemistry, biology and physics teacher at the same time. As children we scoffed at all of the classes we used to take and what we learned as if we’d never need to know these things as an adult but guess what; some things we learned in school directly affect us when scuba diving. I mean probably not pottery or how to use a semicolon properly but science and maths we definitely use.

In today’s class, we’ll be looking about the smart people from history that defined what is happening to you when you go scuba diving and the laws of physics that we are currently at mercy to while diving.

Top Scuba Diving Laws of Physics.

Boyles Law

Robert Boyle was an Irish physicist and chemist from the 1600s and he worked out the correlation between pressure and volume of a gas at a constant temperature. This is why your mask sucks into your face as you swim down and why you can’t hold your breathe and ascend.

A volume of gas in an air space is inversely proportional to the pressure so the deeper down you go the smaller the volume, think back to that balloon we all learned about, at 10m the volume has halved even though we haven’t taken any gas from it. Boyles law explains why we have to equalise our ears, equalise our mask, adjust our BCD as we change depth and can’t hold our breath and ascend.

Gay Lussac’s 2nd Law

Joseph Louise Gay-Lussac was a french smart boi from the 17-1800s and his second law is all about pressure and temperature. And this is all about why we wait for our cylinders to cool down when filling them up. Basically; if you have a hot cylinder at 200bar on the surface because it’s been sat in the sun for a while and then jump into the cool water then your tank pressure will go from 200 down to about 180 just from the change in temperature.

So, this is why we need to keep our cylinders cool. That way we know more accurately how much gas we have in our tanks and keep them out of hot places when full so that 200bar doesn’t turn into 250bar and test the strength of your cylinder…

Henry’s Law

William Henry was another 17-1800 smart guy from Manchester England and his law basically explains pressure gradients and ultimately why we get narked, can’t breathe too much oxygen at depth and decompression illness. As the pressure of a gas increases outside of your body, the more is absorbed into your tissues, the higher pressure is sort of pushing more gas into your tissues.

The usual analogy is the fizzy drinks bottle; kept at relatively high pressure the carbon dioxide is kept in solution inside the drink but when you release the pressure the gas expands out of solution. At depth, if you open that same bottle not much will happen because the pressure hasn’t changed very much and the gas will stay in solution. Send it up to the surface and those bubbles will start showing up, if you follow the bottle up too fast then you’ll have some bubbles of your own, because of decompression sickness.

Daltons Law

Dalton’s Law is for the Nitrox and mixed gas people out there. John Dalton was also from the 17-1800s and looked into the composition of gases and basically came up with partial pressures as a representation of a gases total pressure. Air at sea level is about 20.9% Oxygen and about 78% Nitrogen.

As you dive down the ratio doesn’t change but thanks to Henry’s Law, see above, the increase in pressure changes the amount we absorb. Partial pressures make it easy for divers to understand the concentration or amount of a certain gas, such as oxygen, in a breathing mix at a given depth so they know if it’s breathable. At sea level, Dalton’s total pressure is one atm. 21% of that, 0.21, is the partial pressure of Oxygen at sea level. At 10m the pressure is double so that 1.0 becomes a 2.0 and that 0.21 becomes 0.42 so it’s a bit like breathing 42% air on the surface, if that 0.21 gets much higher than 1.6 then bad things happen. 

Archimedes Principle

Archimedes worked out why heavy boats float and the difference between weight and buoyancy. This is why we need lead weights to sink and how our BCDs adjust our buoyancy. It’s all about displacing the weight of water that you weigh, balancing the equation by displacing more or less water. Imagine the space that you and your gear takes up in the water and imagine what that amount of water weighs.

That weight of water is the buoyant force pushing you up and you need to equal that force with gear and lead. As you descend and the volume of the airspace in your BCD decreases you’re displacing less water so you sink, that’s why it’s important to use as little lead and therefore little air in your BCD as possible. A lot of gear is made to be as neutrally buoyant as possible to make your weight calculations as easy as possible.

Snell’s Law

Snell’s Law is why you can’t catch things and judge distance when you start diving and why we need to wear masks to see. Named for Dutch Oubichapter Snellius, Snell’s Law looks at how light acts through water compared to air. Light changes direction when it leaves water into air so while it may look like something is in one place, it’s actually somewhere else because the light isn’t going in a perfectly straight line.

It’s why Tom Hanks couldn’t catch fish when he first landed on the island in Cast Away, he was throwing the spears and where the fish looked like it was not where it actually was. And through generations of evolution, our human eyes have adjusted so that we can only focus properly in air. We don’t live underwater so our eyes haven’t adjusted to focus properly surrounded by water and that’s why it’s all fuzzy when you open your eyes in the pool.