What is Boyle’s Law?

The Shannon Portrait of the Hon Robert Boyle

The Shannon Portrait of the Hon Robert Boyle

What is Boyle’s Law?

Boyle’s Law is a relationship between the volume of a gas and its pressure. It states that as the pressure of gas decreases its volume will increase. It can be described by the following formula:

P1V1 = P2V2 with the temperature kept constant.

The pressure of the gas multiplied by its volume will equal the new pressure of the gas multiplied by its new volume, as long as the temperature remains constant.

How does this relate to diving?

We have all see the cartoon of what happens to a balloon, when it is filled with air at the surface and then taken to depth. The volume of the balloon shrinks.

Suppose we take a balloon of 120 litres, what will its volume be as we take it to depth. This is what happens:

Depth Absolute Pressure Volume
0m 1 ata 120 litres
10m 2 ata 60 litres
20m 3 ata 40 litres
30m 4 ata 30 litres
40m 5 ata 24 litres

Notice that the volume has decreased. If we are at 40m, where there is an absolute pressure of 5 ata, then the volume of the balloon is 5 times smallers or one fifth of the size compared to the balloon at the surface. The density of the air is also 5 times that as at the surface.

It is important to know this relationship. At depth your regulator delivers the air at the ambient pressure so this means that you will require 5 times the volume of air as you would at the surface to fill your lungs at 40m. The corollary of this is that you will consume your air 5 times faster than you would at the surface.

Understanding this means that you can take into account the increased consumption of gas at depth when you are planning a dive. Most recreational divers don’t take this into consideration when planning their dives. They either turn their dive when they run out of NDL or have reached the half way mark on their SPG. So gas planning isn’t the highest of priorities. Move on to technical diving where you may have a decompression obligation and quickly it becomes apparent that knowing how much gas you will have will mean if you can complete the dive safely or not. One doesn’t want to plan a gas management strategy too aggressively that there isn’t enough gas to complete the dive. It also means that you will require more gas to fill your BCD at depth.

The opposite of that table is true. Fill a balloon at depth and bring it up to the surface and it will expand until the volume of the gas can no longer be contained in the balloon. This is what happens when a panicked diver holds their breath and makes a rapid ascent to the surface. The air is trapped in the lungs forcing them to expand until injury is caused. This is why one of the most important rules of scuba diving is to always keep breathing, or at the very least exhaling when the regulator is not in your mouth.

So knowing a little science can go a long way to helping you have more successful dives.

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