One of my favorite places to dive is Clear Lake in Linn County, Oregon. The water usually has at least 100 feet of visability and the water tempurature varies from 39 to 45 degrees Fahrenheit. (You did remember that being cold can contribute to decompression sickness, didn't you?) As you can see from the picture, the rowboat on the surface is clearly visible. One of my favorite things to see in the lake are the petrified trees that are over 3,000 years old. Thermal vents and algae take a close second. By the way, Clear Lake's altitude is 3,012 feet.
Altitude diving usually starts above 1,000 feet above sea level. While some divers use dive tables for altitude diving, many use dive computers that can make the calculations for you. One factor that most people never consider is their depth guage. Depending on how old it is and whether it is analog or digital, your actual depth may not be accurate unless recalibrated for the decrease in atmospheric pressure. What do you do if you're not sure if your depth guage or computer will compensate for a change in atmospheric pressure? If it cannot be recalibrated, are you actually deeper or shallower?
If we look at this chart, we can see that the atmospheric pressure for a dive at an altitude of 3,000 feet is .8962 atmospheres of that at sea level. In other words, about 13.17 psi, rather than the 14.7 psi that we would normally live in at sea level. Likewise, doing a 60 foot dive at this altitude is like doing a 65 foot dive at sea level (not at altitude). This means we take on more nitrogen. If you remember how to use tables, the diver would round to the next deepest depth to determine a maximum no decompression limit for that dive. (Did you remember that an atmosphere of fresh water is 34 feet and not 33 feet as it is in salt water.)
So... using NAUI dive tables, if we do a 70 foot dive (and if are you diving a square profile) on air at sea level our maximum no decompression limit is 45 minutes! NAUI dive tables are already pretty conservative... Have you ever spent 45 minutes in 40 degree water? What do you think would happen if you dove on nitrox?
Click HERE to go to an online "Air Pressure at Altitude Calculator" to calculate altitude from air pressure or air pressure at altitude.
