Altitude Diving Reference Notes
At 4600 feet, Blue Hole is an altitude dive. In practice, this means to ascend more conservatively than usual (ascending at 30 ft/min rather than 60 ft/min) and that No-decompression limits are shorter than sea level. Use altitude tables or a computer set to altitude mode. If you're taking a class, you don't have to worry about this- your instructor will tell you everything you need to know. But, if you're interested in the background, keep reading!
The basics
Interesting impacts
The biggest effect that you need to know is the reduced time at depth. However- there are a few other interesting impacts of reduced atmospheric pressure that are usually not considered:
Buoyancy- Altitude reduces the amount your wetsuit compresses, meaning you'll need a pound or two more weight than you would at sea level
Maximum Operating Depth- If you've a nitrox diver, maximum operating depths of your gas blend will be deeper than at sea level
Air Consumption- Your air consumption rates will be slightly lower than at sea level because the absolute pressure is lower
The reason for differences at altitude is that the atmospheric pressure in Santa Rosa is only 85% of normal sea level pressure, making water comparatively heavier and causing your tissues to on-gas faster. You can calculate something called equivalent sea level depth (ESLD) in feet like this:
where Ps is the pressure in atmospheres (for Blue Hole it's 0.84) and 34 is the weight of 1 atm of freshwater (for saltwater, you'd use 33) For example, a dive to 80 feet in Blue Hole would have an equivalent sea level depth of:
so- a dive to 80 feet would be equivalent to a dive to almost 102 feet at sea level, and your no decompression limit would be much shorter as a result.
That being said: If you’re using a dive computer with altitude compensation, trust it (and verify it’s set correctly). If you’re using sea-level tables, use cross-corrections / an altitude table (NOAA/U.S. Navy-style) rather than ad-hoc formulas.