The cornerstone of scuba diving is understanding the relationship between pressure and volume. This relationship dictates the science behind dive planning and provides guidelines for flying after diving and altitude-diving procedures. Rewinding to core open-water training, we revisit two dive-planning considerations when it comes to scuba diving and altitude:
Flying after diving
According to the PADI/DSAT RDP table, if you’re flying after a single no-decompression dive, wait a minimum of 12 hours. For repetitive and/or multi-day diving, leave a minimum pre-flight surface interval of 18 hours. SSI recommends that you always wait at least 24 hours after diving before flying to avoid decompression problems.
According to both PADI and SSI, special procedures apply for diving at an altitude of greater than 1,000 feet (300 m).
So, when it comes to scuba diving and altitude, it’s clear that something is happening that may adversely impact our bodies. But what exactly is going on?
Changing ambient pressure
The answer has to do with relative compression and the rate and which our bodies absorb and release nitrogen. At sea level, we’re exposed to one bar of ambient pressure. However, as altitude increases, such as when in a mountainous region or flying, the relative ambient pressure drops.
Standard dive tables and most computer algorithms calculate dive times based on a hypothetical diver returning to the surface at sea level. Here, the partial pressure of nitrogen (ppN2) is 0.79 bar.
Now, imagine we dive to 33 feet (10 m). We are now under two bars of pressure and the ppN2 is 1.58 bar (2 x 0.79). The difference is 0.79 bar, which is called the pressure gradient. If the diver stayed at 33 feet (10 m) for long enough, the body would eventually become saturated with nitrogen in accordance with the ambient pressure. The diver could, hypothetically, remain at that depth indefinitely without absorbing additional nitrogen.
However, when the diver begins to ascend, the pressure gradient changes. The partial pressure of the gases decreases and the saturation process reverses. The inert gas diffuses from the body’s tissues into the bloodstream. From the blood the gas goes into the lungs and, eventually, out of the body with each exhalation.
Historically, Captain Robert Workman of the U.S. Navy Experimental Diving Unit (NEDU) made fundamental progress in decompression theory when he discovered that there was a maximum amount of nitrogen pressure gradient that any tissue could contain upon surfacing. Captain Workman called this calculated maximum amount the “m-value.”
The pressure gradient of dissolved nitrogen in the body in relation to the partial pressure of nitrogen at the current depth (or on returning to the surface) is the key factor. Managing this gradient is the art of managing potential decompression sickness risk with regard to safe ascent rates and — as we’re discussing here — managing that risk of DCS when diving at altitude or flying after diving.
Diving at altitude or flying after diving plays havoc with the nitrogen exposure and pressure-gradient calculations that standard dive tables and computers models use.
For example, if you’re diving in a mountain lake there’s lower atmospheric pressure. The relative difference (and corresponding pressure gradient) between the atmospheric pressure and the pressure underwater increases. Therefore, the impact of scuba diving to any given depth is proportionally greater than it would on the same dive at sea level. Because of this, altitude dives have shorter no-decompression/no-stop times.
Divers can use simple altitude tables with typical ambient pressure at various elevations when conducting these types of dives. Also, many modern dive computers allow users to adjust and re-calibrate the unit for the different ambient pressure. Altitude-diver specialty training courses teach the correct procedures and planning considerations.
You must also take precautions if you need to drive to altitude after a dive at sea level. For example, driving back over a mountain range after a day of diving at sea would require an extended surface interval before safely undertaking the journey. Failure to do so creates an increased pressure gradient between the nitrogen in your tissues and your surroundings. This forces your body to dispel the nitrogen faster — perhaps too fast to do so safely. Doing so can trigger decompression sickness.
Altitude and flying
The same is true for flying. Aircraft cabins are pressurized to the equivalent of approximately 7,800 feet, or 2,400 meters of altitude. This, again, means that flying directly after diving heightens the pressure gradient of the absorbed gas in comparison to the surrounding pressure. It may reach a point where the body can’t keep up. Bubbles form and decompression sickness occurs.
If you’re planning to dive at an altitude of greater than 1,000 feet (300 m), obtain the proper training to plan and execute your dives safely. And, similarly, if you’re planning a dive trip, allow for a sufficient surface interval before returning to an altitude of higher than 1,000 feet or flying home. Scuba diving and altitude needn’t be at odds, as long as you plan well and take the proper precautions.