Boyle's Law & Water Pressure - Scuba Physics #1

There's tonnes of science that goes into scuba diving, some of which still isn't fully understood. The chances are when you signed up for your first PADI Open Water course (or equivalent) you didn't do it for a science lesson.. Well, it's critical that you understand the science of the sport in order to stay safe and to progress your diving career. Those diving veterans at the club that dive all sorts of gasses and run times - they'll know the diving science like the back of their hand. The Dive Masters and Instructors - They'll be able to regurgitate most of the course book if you ask them to. Basically, knowing the science is the most important part of scuba diving. Help is at hand though! Active Elements UK will be debunking and breaking down the science of the sport over several articles to help you learn, or refresh, your memory.

Boyle's Law

One of the very first theories you will have learnt in your Open Water Course (OWC) will be Boyle's Law. It's also one of the most critical but overlooked theories going. It is the fundamental to all following diving training and is the reason for the golden rule: "Never hold your breath." Despite this, its

The Concept of Water Density

Before we get into Boyle's law, let me explain the concept of the weight of water. It's pretty common knowledge that if you had three buckets, filling one completely with water, one half full and one 'empty' (it's actually filled with air) you'll know that picking up the completely full one will be heavier than the half-full one, which is heavier than the bucket filled with air. This illustrates the extra weight water carries over air. In fact water weighs around 800 times more than air at surface level. It stands to reason then that the more water above you, the heavier it pushes down on you. This is where pressure at depth and atmostpheres come into play.

Water Pressure and Atmosphere's (ATM)

So now we know the concept of water density, let's look at atmosphere's. As you are probably already aware, on the surface we are surrounded by our atmosphere. That atmosphere equates quite nicely as 1 ATM or 1 Bar.

1 ATM = 1.01325bar (10.3m)

For every 10.3m of water, another atmosphere (1 ATM) of pressure builds due to the weight of the water (owing to the concept of water density we've just learned). To make things simple, 1 ATM equates to 10 meters. Simply put, the more water acting on an object, the more pressure that object feels.

If a diver was to descend to 30 meters, they would therefore be subject to the pressure of 4 ATM (atmospheres) due to the three from the water, as well as the atmosphere from the surface. We will see in just a moment what this means for the diver.

What is Boyle's Law?

Boyle's Law is simply the concept of an increase in pressure as the volume of the container decreases. Sounds simple enough right? Even the formula looks pretty straight forward.

Formula: P = V

For those of you that are still a little lost - don't worry! As simple as the concept sounds, it's hard initially to understand what is actually meant by pressure and volume.

If we take an example of an empty plastic bottle with the lid screwed on tight. Right now if that was sat on your table at home, it would look like a normal empty bottle. The pressure acting on the bottle is equal to that inside it, of 1 ATM.

If you were to take the exact same bottle underwater, say to 10 meters, you'd notice that the bottle will have crumpled up and shrunk. Why - Boyle's Law. What you have just seen is the pressure on the outside of the bottle (at 10 meters = 2 ATM) is acting on the 'container' and as the air inside the bottle is still 1 ATM the bottle has been pressed inwards and reducing the volume inside the bottle.

The atoms and particles inside the bottle don't disappear though. Instead the particles are compressed and pushed closer together as there is less room for them inside the container. This causes the pressure increase of the contents of the bottle.

This is exactly what Boyle's Law explains, describing that the pressure of the inside of the bottle is directly correlated to the volume of the container (which is affected by external pressure). Effectively, the higher the pressure around the container, the smaller the container becomes, reducing the volume and increasing the pressure internally.

The Balloon Experiment

A perfect example of this is the Balloon Experiment. It's also a sobering reminder of the golden rule in scuba diving 'Never hold your breath'. So what is it?

The balloon experiment demonstrates the effects of pressure and volume as described by Boyle's law.

You'll need two balloons, keep on in a pocket, this will be used a bit later. Now blow up the other balloon as normal and tie a knot in the end. On the surface the balloon will be whatever size you've just blown it up to. Now take it with you underwater (10 meters if you can) and you'll see that the balloon shrinks in size despite the same amount of air inside it. This is the result of the pressure outside of the balloon (2 ATM at 10 meters) pushing against the 1 ATM of the air inside. Upon resurfacing, the balloon will return to its 'normal' size.

Now for that second balloon. Use your alternate regulator to inflate the second balloon underwater and tie a knot in it. Now surface with the balloon and watch just how much it expands. Physics is amazing right?!

What does it mean to divers?

As we have just seen from the balloon experiment, a container filled at depth will expand as its surrounding pressure is reduced. Now imagine that the balloon is your lungs. Every time you take a breath at depth, you're filling your lungs (containers) at pressure which will expand as you resurface. This is the reason for the rule of Never hold your breath* - if you do, your lungs could very well burst.

The principle isn't just applied to lungs though. Your BCD, whether its a jacket or wing, is also a container which will be subject to pressure changes - hence why you need to add more air as you descend. Sinuses too are the same - which is why you need to equalise often.

Your PADI instructors advice is starting to make a lot more sense now right...?

Disclaimer: Please note that this article cannot be used as a replacement for professional instruction. You should never attempt to scuba dive without adequate training and supervision.

* Obviously if you're free diving we recommend you hold your breath!

Have you got a story to share with us? Please feel free to get in touch, we would love to hear from you!

Image credit: Unsplash

#Scuba #Diving #physics #BoylesLaw #Pressure

40 views0 comments

© 2020 by Active Elements UK. All rights reserved.