Diving at Altitude

I live inland near a lake that is 1,800 feet above sea level. Do I need to make special considerations when planning a dive there? Since I live at a similar altitude, am I acclimated and not required to make changes?

A: Many dives take place in inland lakes and rivers, some of which are at an altitude above sea level. When planning a dive, we need to consider two factors: The dive table and computer algorithms are based on being at sea level, which equals 1 atmosphere (atm), and are calculated for seawater, not fresh water. A dive at 2,000 feet above sea level will have a surface pressure of 0.93 atm. You will accumulate inert gas just as you would on any other dive, but when you surface the barometric pressure will be lower than at sea level.

The standard dive tables are designed for a particular altitude range, usually at 1,000 feet mean sea level or less. A variety of tables will allow you to convert your dive planner of choice to reflect an altitude dive, and some tables are designed for altitude use. Keep in mind that when ascending from a lower altitude to complete a dive at a higher altitude, you may need to treat your first dive as a repetitive dive.

Many dive computers have a barometer or pressure sensor that samples the pressure when you turn them on at the dive site. A drawback to this feature is it senses the pressure of only where you are. The computer does not consider where you may have travelled from and may not account for the ascent. If your computer does not have a sensor, you will need to manually change the altitude settings. You will also need to set your computer for fresh water or salt water.

All dive-planning devices use mathematical calculations and theory. Always plan a conservative dive profile, dive well within the limits of your planning device, and allow for a safety stop. A diver should prepare for variances of diving in fresh water or at altitude and follow the rules of the planning device, regardless of the altitude where they live or to which they may have acclimated. The instruction guide provided with your dive table or computer will help you better understand their limits and help you enjoy happy and safe diving. — Robert Soncini, NR-P, DMT
- © Alert Diver Magazine — Q2 2020