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Understand the physics of SCUBA diving

This is an excerpt from Advanced Environmental Exercise Physiology-2nd Edition by Stephen S. Cheung & Philip N. Ainslie.

Physics of Diving

The physics addressed in Boyle’s law and how it plays out in the pressurized environment underwater are covered in chapter 7. Figure 8.1 highlights how it can be a danger during underwater diving if divers hold their breath. Several additional physical laws are critical in understanding the hyperbaric environment.

Dalton’s Law

Named after John Dalton, this law is summarized as: The total pressure of a gas mixture is the sum of the partial pressures of each component gas. In normal room air at sea level, total ambient pressure is the sum of the partial pressures of nitrogen, oxygen, carbon dioxide, and other trace gases.

P_total = P₁ + P₂ + P₃ + … + P_n 

(Eqn. 8.1)

Where this plays out in diving is in calculating the partial pressure of an individual gas such as nitrogen or oxygen, which is critical for understanding issues such as air usage at depth, oxygen toxicity, inert gas narcosis, and decompression sickness (DCS). Breaking this law down further, the pressure of each gas within the total can be calculated as a function of the ambient pressure and the fraction of that gas within the environment:

P_gas = P_ambient × F_gas 

(Eqn. 8.2)

where P is pressure in mmHg and F is the fraction of that gas in the inspired mixture.

Remember that 1 atmosphere (ATA) of pressure, the standard pressure at sea level, is defined as 760 mmHg. The next examples calculate the partial pressures of N₂ and O₂ using Dalton’s law.

Example 1

When breathing compressed air at 2 ATA, or 10 m of seawater (msw), the partial pressure of oxygen would be:

PO₂ = 2 × 760 mmHg × 0.2093 = 318 mmHg

Example 2

For N₂, its partial pressure at 2 ATA would be:

PN₂ = 2 × 760 mmHg × 0.79 = 1,201 mmHg

We will return to the importance of partial pressures throughout this chapter, so it is essential that you understand Dalton’s law and how to calculate the partial pressures of gases at different depths before we proceed further. Note that the preceding partial pressure calculations are only for using compressed air, with the same gas fractions as at sea level. While the vast majority of recreational diving uses compressed air, commercial and military diving may use specialized gas mixtures.

Henry’s Law

William Henry was responsible for proposing another gas law with high relevance to hyperbaric physiology (see also chapter 7). Henry’s law can be summarized as follows: At a given temperature, the amount of gas dissolved into a fluid is directly proportional to the pressure. In simple terms, the higher the ambient pressure, the greater the amount of gas that can be dissolved into a liquid. Additionally, the amount of gas dissolved is also dependent on the solubility of the gas in the liquid. In basic respiratory physiology, carbon dioxide has a much higher rate of solubility in water than oxygen, which explains why the arteriovenous pressure differential is much smaller for carbon dioxide than for oxygen. In combination with Boyle’s law, Henry’s law and the dissolution of nitrogen at high pressures into the body tissues become the primary mechanism behind DCS (see later section on DCS) when there are overly abrupt changes in pressure.

Diving Systems

One of the requirements of recreational diving and most commercial diving is the use of a self-contained underwater breathing apparatus (SCUBA) and a mouthpiece to provide a regulated flow of air to the lungs. For recreational diving, the dominant system is the open-circuit SCUBA, where exhaled air is released out of the regulator and into the water. For commercial diving, closed-circuit SCUBA—where exhaled air is recycled and recirculated rather than released into the water—may also be used. As we will see, this can be useful, but it can present additional challenges and risks. The design of the two systems is outlined and compared in figure 8.2.

Open-Circuit SCUBA

Recreational diving uses an open-circuit SCUBA, consisting of a pressurized tank of air along with a system of hoses, valves, and a regulator mouthpiece. It is categorized as open because the diver’s exhaled breath passes out of the regulator and into the surrounding environment. This form of SCUBA allows the diver to breathe normal concentrations of air (i.e., the same relative percentage of 79% N₂ and 20.93% O₂ as at sea level) but at ambient pressure, so the partial pressure of each inspired gas depends on the dive depth. The main components of an open-circuit SCUBA are:

1. One or more diving cylinders containing compressed air pressurized to about 4,000 pounds per square inch (psi). How long a diving cylinder lasts depends on both the ventilation rate and the ambient pressure; a single breath at depth contains more gas molecules than the same breath at a shallower depth.
2. Hoses and an initial regulator that reduce pressure from the very high levels within the diving cylinders to slightly above ambient pressure.
3. A diving regulator built into the breathing mouthpiece. This regulator reduces gas pressure to ambient pressure as the diver inhales, and it is called a demand regulator because it only releases air upon demand during inhalation.
4. An exhaust valve in the mouthpiece. Exhaled air is released through this valve into the surrounding water. Notably, none of the exhaled air is recovered, and thus one of the limitations of open-circuit SCUBA is the requirement for much larger and heavier air cylinders. While buoyancy negates much of the weight disadvantage underwater, the large tanks can make it difficult to maneuver within confined spaces such as shipwrecks and caves.

Closed-Circuit SCUBA

In military special forces diving, one key goal is to avoid detection by the enemy during missions. This can be an unavoidable hazard with the use of open-circuit SCUBA, with the exhaled air forming bubbles at the water’s surface. Therefore, another goal in diving technology has been the development of closed-circuit SCUBA breathing systems, in which the exhaled air is recycled and rebreathed.

This design requirement results in a number of changes from open-circuit SCUBA. The main components of a closed-circuit SCUBA are:

1. A diving cylinder of pure oxygen rather than compressed air. Because the exhaled breath is filtered and reused, a much smaller and lighter tank can be used. Also, because typically only oxygen is inhaled, the risks of nitrogen narcosis and decompression sickness (see below for more detail) are removed.
2. A dual-stage regulator similar to those in an open-circuit SCUBA.
3. A scrubbing system to remove the carbon dioxide in exhaled breath that is produced during metabolism.

While the advantages of a closed-circuit SCUBA make it seem vastly superior to an open-circuit system, there are several fundamental drawbacks that require additional training and caution for sport and technical divers. The first and most important is that oxygen at high pressures can be toxic (Wingelaar et al. 2017). Acute or sustained exposure to high levels of oxygen can be marked by central nervous system, pulmonary, and ocular toxicity, with symptoms including seizures, tinnitus, nausea, dizziness and disorientation, impaired vision, coughing, and difficulty breathing. Long-term, overly high oxygen pressures may also cause increased oxidative damage from reactive oxygen species. Therefore, specific dive tables for oxygen diving have been created, and dive depths are typically limited to 6 m (20 ft) or less. To minimize the risk of oxygen toxicity, closed-circuit systems may also use helium or nitrogen within the gas mixture to decrease the partial pressure of inspired oxygen. A second serious risk is from a depleted or defective CO₂ scrubbing system, which can also elicit central nervous system issues. An epidemiological survey of French military divers reported 153 incidents from 1979 to 2009, or a rate of 1 for every 3,000 to 4,000 dives, with 3 lethal drownings (Gempp et al. 2011). This observation suggests that, with tight adherence to safety protocols, closed-circuit SCUBA can be deployed effectively with minimal risk.

Respiratory Demands From SCUBA

The respiratory consequences of diving revolve around two important changes: (a) the requirement to breathe through a regulator and (b) the higher density of gases and changes in hydrostatic forces with depth and pressure.

An inevitable consequence of a supplemental air source is an increase in the work of breathing. This increase arises from the increased anatomical dead space in the air system (regulator, hoses) and also the higher inspiratory pressure required to overcome the pressure within the regulator. The higher inspiratory pressure is compounded by the hydrostatic pressure from the surrounding water and also the diving harness increasing resistance against the respiratory muscles. As a result, the energy required to exercise at the surface with SCUBA is more than that needed to swim without supplemental air at the surface. This increase in the work of breathing and in metabolic rate also causes a greater rate of air usage, thus decreasing dive duration.

As shown by research on firefighters exercising in air with similar self-contained breathing apparatus (SCBA), the magnitude of the additional metabolic cost of exercising with a regulator system can be substantial. This increase in energy demands exists apart from the effects of the tank and harness weight or changes in ambient pressures or temperatures; it comes from the increased work of breathing required to overcome the pressure seals in the respirator. For a given exercise intensity, the difference in ventilation rates between breathing ambient air through a low-resistance valve and breathing compressed air through a SCBA regulator are significant, especially at higher exercise levels. However, there is an additional benefit in technical diving with regulators using special gas mixtures like helium-oxygen (heliox). For example, the lower density of the heliox mix can decrease the ventilatory rate and the work of breathing to levels comparable to those of breathing with a low-resistance valve.

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