Reading on-line scuba forums and groups is both a blessing and a curse. I think that the curse (and cursing) has been exacerbated by Covid-19. Everyone is frustrated, and patience, tolerance, and kindness are often neglected. The blessing comes when you hear real questions from divers—novice and experienced—from all over the world. One recent question was about Altitude Diving: Is a course on Altitude Diving worth paying for? You can imagine the answers in Facebook’s “Scuba Divers Uncensored.” I private messaged my answer to the diver in Peru who lived at altitude and needed a good answer. It seemed better than taking abuse!
Recreational divers began to consider the need to adjust the Navy Dive Tables for the effects of altitude at least as early as 1966. (See the link below for Skin Diver Magazine). We’ve pondered and experimented and discussed diving at altitude for years and years. And we’ve discovered that instruments built for sea water at sea level aren’t necessarily accurate for fresh water at altitude. For example, “mechanical depth gauges that use a bourdon tube will read shallower than actual depth, while mechanical depth gauges that use a capillary tube will read deeper than actual depth” (See Todd Stedl’s article cited below). So what does a newer diver need to know? Is it worth paying for a course on Altitude Diving? Are PADI, SDI, SSI, PSAI, and other agencies ripping newer divers off? I don’t think so!
Why am I passionate about this? I live, train, dive, and lead training dives in Utah. Our lowest altitude dive (Sand Hollow State Park, near St. George, Utah) is at about 3,000 feet above sea level. Blue Lake, near Wendover, Nevada, is approximately 4,300 feet above sea level. We certify many students at The Crater, in Midway, Utah at an altitude of approximately 5,700 feet. We do fun dives at Bear Lake, near Logan, Utah and at Flaming Gorge Reservoir, both at approximately 6,000 feet, and at Strawberry Reservoir (~7,600 feet). Occasionally, dry suit divers venture up to Fish Lake at 8,848 feet. Maybe all of this pales in comparison with Jacque Cousteau’s 1968 dives in Bolivia and Peru’s Lake Titicaca at 3,812 meters (12,500 feet). But still, our divers need to know about Altitude Diving because we live at altitude!
Here is the gist of it: If you live and dive at altitude—above 300 meters/1000 feet—you need to know three things about diving at altitude. Of course the higher you live and dive, the more you need to know these three things!
Your no-decompression limit is shorter for dives at altitude. When you begin your dive, and especially when you surface, the lesser weight of the atmosphere becomes critically important. With less ambient pressure on the surface at altitude, your no-decompression limit is shorter, and if your dive plan and computer do not account for this, you may not have off-gassed enough nitrogen to be safe and you may get DCS (aka “the bends”). Don’t push the limits of depth and time!
You need to know what your dive computer is doing at altitude. You may actually have to read part of the manual! It is probably displaying an accurate depth in the water column. Hopefully it is adjusting for fresh water instead of sea water. And probably, it is adjusting for altitude and reducing your no-decompression limit. Almost all computers put into service since 2015 do this. But ask this question: When does it read for altitude? You may need to manually turn it on before you submerge, even if it turns itself on at a meter / 3 feet or so underwater (See the DAN article below by Tyler Coen).
You need to descend and ascend more slowly. A 60 feet per minute ascent might work at sea level. At altitude, slow to 30 feet per minute. MAX! With less ambient pressure on the surface at altitude, your body needs time to adjust, both in terms of off-gassing nitrogen and in terms of your ears and sinuses. Why do new divers have more difficulty equalizing at altitude than at sea level? The change is amplified by the lower initial ambient pressure. Said another way, the change in ambient pressure in the first 5 feet is larger at altitude! Why do so many dive instructors at altitude have hearing problems? Slow the last 5 feet of that CESA and give your ears a break!
Your safety stop may need to be a bit shallower and perhaps a bit longer. At 5700 feet, a 15 foot fresh water safety stop has a theoretical depth of 18 fsw. That falls within the 15-20 foot recommendation, but it may be preferable to stay in the 12-15 fsw range.
You may find that Nitrox is more helpful at altitude than at sea level. The higher percentage of oxygen helps compensate for the lower ambient pressure when diving at altitude. Theoretically, your MOD will be slightly deeper at altitude. Safety says we don’t take advantage of this, because we are still surfacing at a lower ambient pressure. So where is the gain to using Nitrox? You won’t need to off-gas as much nitrogen and hopefully, the extra oxygen will help you keep a clear head and help you make better decisions. In addition, Nitrox can offset the mild hypoxia some divers experience as they surface at altitude.Just saying… (See the analysis by Peter Buzzacott in the last DAN article listed below).
Here is the question people aren’t asking (out loud): If the Recreational Scuba Training Council limits students and OW divers to 60 feet maximum depth, are instructors and divemasters breaking their agency’s standards if they take students below a theoretical depth of 60 feet when training at altitude? Hmmm.
All of this is just the tip of the iceberg! I’m not saying everybody needs the 180 page manual published by PSAI, but if you live at altitude, please do your homework! Get schooled! Take the course! Until then, here is the first place I would go to begin to plan my next altitude dive:
http://www.8thelementdiving.com/scuba_math/altitude.php
I love this guy! Todd Stedl earned his Ph.D. in computational chemistry in 2003. He notes that the 1970 Altitude Tables need to be corrected for fresh water lakes, and he includes “a quick calculator to determine your Theoretical Ocean Depth.” Here are computations for some popular destinations:
3,000 feet (Sand Hollow State Park, Utah)
The surface air pressure is 0.8962 atm.
At 50 feet, your Theoretical Ocean Depth is 54 fsw.
The depth for your safety stop is 14 ffw.
4,300 feet (Blue Lake, Utah)
The surface air pressure is 0.8541 atm.
At 62 feet, your Theoretical Ocean Depth is 71 fsw.
The depth for your safety stop is 13 ffw.
~5,700 feet (The Crater, at Midway, Utah)
The surface air pressure is 0.8105 atm.
At 64 feet, your Maximum Theoretical Ocean Depth is 77 fsw. This is a huge reason to keep students off the bottom.
The depth for your safety stop is 12 ffw.
6,000 feet (Bear Lake & Flaming Gorge, Utah)
The surface air pressure is 0.8014 atm.
At 60 feet, your Theoretical Ocean Depth is 73 fsw.
The depth for your safety stop is 12 ffw.
6,225 feet (Lake Tahoe, California)
The surface air pressure is 0.7946 atm.
At 60 feet, your Theoretical Ocean Depth is 74 fsw.
The depth for your safety stop is 12 ffw.
12,500 feet (Lake Titicaca, Bolivia and Peru)
The surface air pressure is 0.6236 atm.
At 60 feet, your Theoretical Ocean Depth is 94 fsw.
The depth for your safety stop is 10 ffw.
Additional Resources:
PSAI Altitude Diving Manual
Gary Taylor authored a 180 page Altitude Diving Manual for the Professional Scuba Association International. Probably the gold standard at this point!
https://www.scubadiving.com/altitude-diving-differences
Altitude Diving: The Challenges and Benefits for Scuba Divers: How getting high affects your dive routine. By Eric Michael, July 19, 2019.
http://www.alertdiver.com/Establishing_a_Baseline
Establishing a Baseline: How dive computers determine surface pressure
Alert Diver, Q4 Fall 2015 • By Tyler Coen
https://www.diversalertnetwork.org/diving-incidents/high-altitude-dive-landed-in-chamber Analysis by Peter Buzzacott, MPH, PhD