Boyle's Law and Scuba Diving

This law relating pressure, depth, and volume affects every aspect of diving.

scuba diver
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One of the fantastic consequences of enrolling in a recreational scuba diving course is being able to learn some basic physics concepts and apply them to the underwater environment. Boyle's law is one of these concepts.

Boyle's Law explains how the volume of a gas varies with the surrounding pressure. Many aspects of scuba diving physics and dive theory become clear once you understand this simple gas law.

Boyle's Law is 

PV = c

In this equation, “P” represents pressure, “V” signifies volume and “c” represents a constant (fixed) number.

If you are not a math person, this may sound really confusing—don't despair! This equation simply states that for a given gas (such as air in a scuba diver's BCD), if you multiply the pressure surrounding a gas by the volume of gas you will always end up with the same number.

Because the answer to the equation can not change (that's why it is called a constant), we know that if we increase the pressure surrounding a gas (P), the volume of the gas (V) must get smaller. Conversely, if we decrease the pressure surrounding a gas, the volume of the gas will become greater. That's it! That's Boyle's entire law.

Almost. The only other aspect of Boyle's Law that you need to know is that the law only applies at a constant temperature. If you increase or decrease the temperature of a gas, the equation doesn't work anymore.

Applying Boyle's Law 

Boyle's Law describes the role of water pressure in the dive environment. It applies and affects many aspects of scuba diving. Consider the following examples:

  • Descent - As a diver descends, the water pressure around him increases, causing air in his scuba equipment and body to occupy a smaller volume (compress).
  • Ascent - As a diver ascends, water pressure decreases, so Boyle's Law states that the air in his gear and body expand to occupy a greater volume.

Many of the safety rules and protocols in scuba diving were created to help a diver compensate for the compression and expansion of air due to changes in water pressure. For example, the compression and expansion of gas leads to the need to equalize you ears, adjust your BCD, and make safety stops.

Examples of Boyle's Law in the Dive Environment

Those who have been scuba diving have experienced Boyle's Law first hand. For example:

  • Ascent - As a diver ascends, water pressure around him decreases and the air in his BCD expands. This is why he has to release excess air from his BCD as he ascends—otherwise the expanding air will cause him to lose control of his buoyancy.
  • Descent - As a diver descends, the water pressure around him increases, compressing the air in his ears. He must equalize the pressure in his ears to avoid pain and a possible ear injury called an ear barotrauma.

Scuba Diving Safety Rules Derived From Boyle's Law

Boyle's law explains some of the most important safety rules in scuba diving. Here are two examples:

  • Don't hold your breath underwater. According to dive training organizations, a diver should never hold his breath underwater because if he ascends (even a few feet) to an area of lesser water pressure, the air trapped in his lungs will expand according to Boyle's Law. The expanding air can stretch the diver's lungs and lead to a pulmonary barotrauma. Of course, this only occurs if you ascend while holding your breath, and many technical diving organizations modify this rule to "Don't hold you breath and go up."
  • Ascend slowly. A diver's body absorbs compressed nitrogen gas while he dives. As he ascends to a depth with less water pressure, this nitrogen gas expands according to Boyle's Law. If a diver does not ascend slowly enough for his body to eliminate this expanding nitrogen gas, it can form tiny bubbles in his blood and tissue and cause decompression sickness.

Why Is a Constant Temperature Necessary to Use Boyle's Law?

As mentioned above, Boyle's Law only applies to gases at a constant temperature. Heating a gas causes it to expand, and cooling a gas causes it to compress.

A diver can witness this phenomenon when they submerge a warm scuba tank in cooler water. The pressure gauge reading of a warm tank will drop when the tank is submerged in cool water as the gas inside the tank compresses.

Gasses that are undergoing a temperature change as well as a depth change will have to have the change in gas volume due to the temperature change accounted for, and Boyle's simple law must be modified to account for temperature.

Boyle's law enables divers to anticipate how air will behave during a dive. This law helps divers to understand the reasons behind many of scuba diving's safety guidelines.

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