Scuba Diving and Gas Laws

One of the the first things that scuba divers learn is Boyle’s Law. In a nutshell, Boyle’s Law states that pressure and volume are inversely proportional.

This relationship is typically illustrated with a balloon. As you take this theoretical balloon deeper and deeper, the pressure on the gas increases and the volume of the balloon decreases. On your ascent, the pressure on the gas decreases and the balloon gradually returns to its starting volume at the surface.

I have seen this demonstrated with a plastic water bottle. Sadly, these demonstrations involve plastic, whether plastic bottles or plastic balloons. However, my instructor had taken the plastic bottle out of a trash receptacle (improperly discarded) and after our dive I submitted the bottle for recycling.

The bottle was sealed at the surface and brought down to 40 meters (130 feet). My instructor then showed me that the bottle was completely crushed under the pressure, because the gas volume was only 20% of its volume at the surface. Because the bottle was sealed, the shrinking gas volume creates a vacuum, sucking in all sides of the bottle.

He then refilled the bottle with air while still at depth.

When we were back on the boat, the volume of the gas wanted to be 500% of what it was at 40 meters. Because the bottle was sealed, there was a lot of pressure inside the bottle. Suddenly, there was a loud POP, and the cap was off. Fortunately, the bottle was still in my instructor’s buoyancy compensator device (BCD) pocket, so no one was hurt and we didn’t lose the cap in the ocean.

My instructor then handed me the bottle and cap, knowing that I would ensure proper disposal.

If you can find improperly discarded plastic (I hope not), this is a fun experiment to try.

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