During your SNSI Open Water Diver course you learned that it is necessary to compensate for the increased pressure on gases and gas spaces. The Irish chemist, Robert Boyle (1627-1691) noted that at a constant temperature, the volume of a gas is inversely proportional to the pressure it is under. This is known as Boyle’s Law.
Based on Boyle’s Law, we can deduce that by changing the pressure of a gas, and consequently its density, we can fill a static capacity container with a greater amount of gas under pressure. For example, a scuba cylinder can contain varying amounts of gas, even though it has a fixed capacity.
The constant factor with regard to Boyle’s Law is temperature. However, there is a direct relationship between volume and temperature. French physicist Jacques Charles observed that at a given pressure, temperature increases proportionately as volume increases.
Guy-Lussac, another French physicist, later observed that pressure is also directly proportional to temperature. Together, these laws explain why a scuba cylinder’s pressure will, when left in the sun, increase and why a cylinder warms when it is filled.
Because scuba cylinders are rigid, they do not visibly expand with increasing pressure, they simply get warm, as the molecules are forced closer together. Conversely, as a cylinder is depleted, and the pressure and volume decrease, it cools. These volumetric variations of gases in relation to the pressure they are under are the reason divers must follow strict depth and time guidelines when scuba diving.
To illustrate these gas laws, we can observe the changes in a plastic bottle as it is put under increasing water pressure. Your SNSI Instructor may demonstrate this for you using a clear, plastic bottle.
On the surface, with the lid off and holding the bottle in a vertical, upside-down position, your instructor enters the water and immerses the bottle. Due to the increasing water pressure during the descent, water gradually enters the bottle as the air inside is compressed. At a depth of 10 meters / 33 feet, the water line will be at one-half of the bottle’s total volume. This is because at 10 meters / 33 feet, exactly double the amount of pressure is being exerted on the bottle.
According to Boyle’s Law, volume is inversely proportional to pressure. In this case, the pressure is two times normal; therefore, volume inside the bottle will be cut in half, and the pressure inside the bottle will be equal to the pressure outside of the bottle.
As your instructor continues to descend, the water level in the bottle will continue to rise. At a depth of 20 meters / 66 feet, the bottle will be under three times the amount of pressure it was at the surface. As such, we can expect the water level to have increased to fill two-thirds of the space, leaving only one-third for the gas inside. Continuing to a depth of 30 meters / 99 feet, the bottle will be filled three-quarters with water, compressing the gas into one-quarter of the total space.
When the instructor ascends with the bottle; again, keeping it upside-down and vertical, we see the water level decrease, as the pressure is relieved and the gas inside expands.
At a depth of 20 meters / 66 feet, the instructor replaces the cap on the bottle, and continues his ascent. As he ascends, the bottle becomes very rigid, and may slightly expand. If the bottle has not burst by the time the instructor reaches the surface, the gas inside the bottle will be under three times the amount of pressure that it was when the instructor sealed it at 20 meters (66 feet), three atmospheres.
A less rigid container under the same circumstances would expand to twice its original size, while the pressure returned to normal.
Another practical demonstration of volumetric variation can be conducted using a tennis ball. Because the ball is a sealed, semi-elastic container, it will compress in proportion to the pressure that it is exposed to.
During this exercise, at approximately 20 meters / 66 feet the ball will suddenly crush, and at 25 meters / 85 feet it will appear completely deflated; though, the air inside is really just compressed.
Upon ascending to the surface, the opposite takes place. The pressure is relieved and the gas inside expands. Once back on the surface, the ball will have returned to its original size.
Understanding density; how temperature, pressure and volume are interrelated with it, and their affects on rigid and non-rigid containers is vital for divers. It forms the basis of how direct and indirect pressure affects a diver, and dictates the steps necessary in order to counteract the physiological changes.