Partial pressure

When a diver descends, the ambient pressure surrounding them increases. This increase in ambient pressure directly affects the partial pressure of each gas within their breathing mixture. For example, if a diver breathes air (approximately 21% oxygen and 79% nitrogen) at the surface at 1 bar (1 ata), the partial pressure of oxygen (PO₂) is 0.21 bar (0.21 ata) and nitrogen (PN₂) is 0.79 bar (0.79 ata).

Upon descending to 10 metres (33 feet), the ambient pressure doubles to 2 bar (2 ata). Consequently, the partial pressure of oxygen doubles to 0.42 bar (0.42 ata) and nitrogen to 1.58 bar (1.58 ata). This linear increase continues with depth. The physiological effects of breathing gases are determined by their partial pressures, not just their percentage in the mix. For instance, high partial pressures of oxygen can lead to oxygen toxicity, while high partial pressures of nitrogen can cause nitrogen narcosis.

Divers, especially those using enriched air nitrox, must calculate partial pressures to ensure they stay within safe limits for specific gases. The maximum allowable partial pressure for oxygen (PO₂ max) is typically 1.4 bar (1.4 ata) for recreational diving, though 1.6 bar (1.6 ata) is used for decompression stops. Exceeding these limits can result in central nervous system oxygen toxicity, which can manifest as convulsions underwater.

Understanding partial pressure is critical for liveaboard divers, particularly for those planning multiple dives over several days or using enriched air nitrox. Many liveaboards offer nitrox, which allows for extended bottom times and reduced surface intervals compared to air. However, nitrox contains a higher percentage of oxygen, making partial pressure calculations essential to avoid exceeding safe oxygen limits.

Before each dive, especially with nitrox, divers on a liveaboard should calculate their maximum operating depth (MOD) based on the oxygen percentage in their tank and the accepted maximum partial pressure of oxygen (typically 1.4 bar or 1.4 ata). This ensures the diver does not descend to a depth where the oxygen partial pressure becomes dangerously high, which could lead to a convulsion underwater.

Liveaboard crew and dive guides are proficient in these calculations and will often assist divers. Nevertheless, divers are ultimately responsible for their own safety. A solid grasp of partial pressure allows for more informed decision-making, enabling divers to choose appropriate gas mixes and depths for their liveaboard itinerary, thereby enhancing safety and enjoyment throughout the trip.

Myth: Only the percentage of oxygen in my tank matters for safety. Fact: While the percentage is crucial, it's the partial pressure of oxygen that determines its physiological effect. As you go deeper, the same percentage of oxygen creates a higher partial pressure, increasing the risk of oxygen toxicity, even with a standard air mix if deep enough. Divers must consider both the percentage and the depth.

Myth: Partial pressure only applies to Nitrox diving. Fact: Partial pressure applies to all breathing gases, including standard air. Oxygen and nitrogen in air also have partial pressures that increase with depth, causing oxygen toxicity and nitrogen narcosis, respectively. Nitrox merely highlights the need for calculation due to its higher oxygen content and expanded depth limits.

Myth: If my dive computer shows my MOD, I don't need to understand partial pressure. Fact: Dive computers calculate MOD based on partial pressure limits, but understanding the underlying principle empowers divers to make informed decisions and verify settings. It also helps troubleshoot issues or plan dives without a computer. Relying solely on a device without conceptual understanding can be risky.

Myth: High partial pressure of nitrogen is directly toxic like oxygen. Fact: High partial pressure of nitrogen causes nitrogen narcosis, which impairs judgment and motor skills, but it's not a direct chemical toxicity in the same way high oxygen partial pressure is. While dangerous, narcosis is generally reversible upon ascent, whereas oxygen toxicity can lead to immediate, severe symptoms like convulsions.