Modern scuba diving isn’t the result of the efforts of one or two individuals. Rather, it was the collective effort of many pioneering individuals, who taught us how to dive safely through pushing boundaries and experimentation. Their discoveries were fascinating, but sometimes the individuals themselves were even more so. In this series of articles, let’s investigate the lives and contributions of some of dive pioneers, even if they weren’t thought of thusly in their own time. Here, we’ll take a look at Robert Boyle.
We all learn about Boyle’s law in our open-water course, and then most of us promptly forget it. Robert Boyle was born in 1627, and is widely regarded as one of the founders of modern chemistry. He was a polymath, but his studies concentrated mainly on physics, chemistry and, as was the fashion at the time, alchemy.
Robert Boyle investigated and made observations on all manner of natural processes, but as divers we are mainly concerned with the law named after him. Boyle’s law states that in a closed system, and as long as temperature remains constant, that pressure and volume are inversely proportionate. This leads on to pressure and density being directly proportionate.
So what does this mean in English? Inversely proportionate means “opposite,” so if volume is high, pressure will be low. Basically, if you add one liter of air to your BCD at the surface, and then descend to 30 feet (10 m), the air’s volume will fall to ½-liter due to the water’s increased pressure. The water pressure falls as you ascend and the air’s volume increases to one liter as you reach the surface. This happens because your BCD is a flexible container. Your air cylinder is not, so outside pressure (at scuba depths) does not affect its internal pressure.
Pressure and density
The latter bit about pressure and density is a little more straightforward. If I descend to 100 feet (30 m), the air in my lungs is at the same ambient pressure as the surrounding water — four atmospheres. This is due to the weight of water above me, plus the air at the surface. So the air in my lungs is four times denser than at the surface. This also means that I am breathing four times the number of atoms per breath. Therefore, my gas consumption is now four times higher than it would be at the surface.
Diving and Boyle’s law
We use Boyle’s law every time we dive. We intuitively add more air to our BCD as we descend deeper, and we equalize our ears on descent. As the air in our BCD expands during our ascent, we vent just enough to maintain control and ascend slowly. We must also breathe at all times. If we hold our breath, our lungs will expand on ascent and we could cause a barotrauma. We also use Boyle’s law to calculate gas volumes. We can work out either how much gas we will need for a given dive profile or have a fixed amount of gas available, and plan a dive profile based on that amount.
Divers use Robert Boyle’s discoveries on every dive, but these weren’t his only contribution to our sport. Boyle was also the first person to observe decompression sickness. While testing the effects of pressure change on a snake in a vacuum jar, he removed the air from the jar. When he rapidly reduced the pressure inside, he observed a bubble move across the snake’s eye. He described the snake to be writhing as if in pain, but had no idea what had happened and didn’t put forward a theory. On increasing the pressure, the bubble disappeared. The snake seemed as happy as it could be while encased in a small jar. But further discoveries on decompression sickness would have to wait two centuries for an explanation by French physiologist Paul Bert.