One of the first rules taught in the open water diver course is to never hold your breath, especially during an ascent.
Boyle’s Law tells us that as a diver ascends and the pressure decreases, the volume of gases in airspaces will increase in direct proportion to the decrease in pressure. Breathing normally during an ascent enables the expanding air in the lungs to escape normally, preventing a potential pulmonary overexpansion injury. Overexpansion injuries are discussed at length in the SNSI Rescue Diver course.
Tissue compartments and relative loading time were previously discussed as they relate to Henry’s Law. Actual tissue loading times vary based on the tissue type and composition. Because of this, many decompression models take advantage of theorized tissue compartments that are divided into fast, slow and multiple intermediate levels.
Fast and slow refers to the velocity at which a tissue absorbs and eliminates nitrogen. A fast tissue, like the brain and blood, will absorb and eliminate nitrogen quickly. Conversely, a slow tissue, like cartilage and fatty tissue, absorb and eliminate nitrogen relatively slowly. Tissues classified as intermediate fall somewhere between fast and slow, and their number varies based on the model being discussed. Compartment classification is largely based on the tissue’s vascular properties; how much blood flows to and from the tissue.
During ascent, ambient pressure decreases, resulting in a decrease in the pressure of the inhaled gas, and consequently, decreased partial pressures of the component gases. This decrease in pressure creates pressure gradients opposite of those observed on descent. In this situation, dissolved gas particles move to where the pressure is lower in the blood and lungs. Assuming that tissue saturation and desaturation times are equivalent, we can say that the limiting factors on deep or short dives is represented by fast tissue compartments; while the limiting factors on shallow, or long dives is represented by slow tissue compartments.
When external pressure decreases slowly, the nitrogen will move from high concentration in the tissues to the lower concentrations in the blood, which will then move into the lungs for elimination.
When the external pressure decreases rapidly, a much greater pressure gradient is created. In this situation, a larger amount of the nitrogen that was in solution in the blood and tissue returns to a gaseous state. As a result, the nitrogen molecules and asymptomatic microscopic nitrogen bubbles combine, creating larger (yet still relatively small) bubbles capable of obstructing blood flow. An obstruction in circulation prevents regional elimination of excess nitrogen, only making the problem worse. This is referred to as Decompression Sickness (DCS).
The human body’s ability to release excess nitrogen without causing damage or injury is dependant on the speed of ascent, and the combined quantity of nitrogen that has been dissolved into the tissues by the end of the dive. This quantity is dependant on the deepest depth of the dive, the total dive time prior to ascending and any residual nitrogen still in the body from dives completed in the previous 24 hours. Based on previous research and experience, we know that the human body can tolerate a specific amount of excess nitrogen without forming symptomatic gaseous bubbles in the tissues. This allows divers to ascend at a specific rate, 9 meters/30 feet per minute, and greatly reduce, if not eliminate the risk of DCS.
The SNSI Rescue Diver course covers DCS in greater detail, including the treatment of a diver suspected of having DCS.
To create reliable models and accurately judge gas elimination times, tissue compartments are assigned theoretical number values that indicate tissue half time; the amount of time it takes for the tissue compartment to become half saturated. Knowing this, we are able to theoretically determine the approximate nitrogen absorption and elimination times of tissues. Based on years of research, it has been determined that as long as divers maintain a saturation ratio of 1.48:1 or less they are theoretically able to complete a dive and resurface without having to make a decompression stop. Integrating the tissue half times of the various tissue compartments so that the pressure ratio never goes above the theoretical limit led to the development of the no-stop curve, in which the maximum theoretical time and depth limits, without a required decompression stop are represented.
Dives that are deeper and, or longer than recommended no-decompression limits require scheduled, precise decompression stops during ascent in order to avoid decompression sickness. This is referred to as decompression diving. While SNSI does not advocate decompression diving at the SNSI Advanced Open Water Diver level, it is important for the Advanced Diver to understand what it means.
Adequate training is required to perform dives that require decompression stops, training that you can receive by participating in the SNSI Recreational Deco Diver course or in the SNSI Tek courses, much more demanding in terms of equipment and time dedicated to more in depth training. In fact, while the SNSI Recreational Deco Diver course enables you to dive within 45 meters / 150 feet of depth with a maximum decompression time of 15 minutes, the SNSI Tek courses are Technical Diving courses, far beyond the limits of recreational diving.
Decompression stops are prescribed stops during the ascent, at specific depths for a predetermined amount of time before continuing with the ascent. These stops provide additional time, while under a smaller amount of ambient pressure, for the higher partial pressure of nitrogen inside of the body to return to the blood and be exhaled. If you’re interested in taking this next step, you can get the required training in a SNSI Recreational Deco Diver course.
Deep diving within the no-decompression limits requires that additional attention be placed on gas consumption, and calculating gas requirements. Even so, it is highly recommended to place a spare cylinder at the depth of the safety stop, usually 3 to 5 meters / 10 to 15 feet. The spare cylinder should be used in the event you or another diver reaches the safety stop with less than the recommended minimum cylinder pressure.