Tips for SCUBA diving at altitude

I think a lot of divers get a natural ‘high’ when they go underwater even though it’s nowhere near what we define as ‘at altitude’. But why do it anyway?

Almost invariably, diving at altitude is in freshwater and the underwater terrain and aquatic life is quite different from what we expect to see at sea. Common comments I have heard are that it’s dead in comparison and colder but it’s often quicker to get to a pub afterwards. Fair enough, but apart from the adventure of diving in freshwater there are an incredible array of learning experiences related to diving in it at altitude – often linked to the history and geography of the specific location including such stuff as what buildings/structures were/are close to the lake and what those who used them did with their old bottles and alarm clocks that didn’t work when we weren’t so eco-friendly. Who knows what you will find down there?

Even so, apart from often interesting history and spectacular settings, diving at altitude does give rise to concerns to what we should do to dive there safely. The science can get quite interesting, complicated and something rather to avoid for those who prefer to dodge that and just get wet. But we can’t close our eyes to facts and there are some issues to consider and recommended rules to follow.

I’ll lay out a few of these below but for those wanting more information, I would suggest doing a course with someone whois an expert through qualification as well as experience (and/or google the subject).

11 tips for diving at altitude

1. Use a computer that you can adjust for the altitude at which you intend to dive. Failing this, if you can use the PADI RDP, add two pressure groups for every 300 metres of altitude, rounding up for an ‘in-between’ height. This will give you an idea of how much extra nitrogen you have in your tissues after having ascended from sea level. For instance, a dive at 750 metres above sea level falling between 600 and 900 metres would make you choose 900 metres; this would make you an F diver. Then proceed to account for your surface interval, final pressure group and no-decompression time limit prior to commencement of the dive.
   
2. Use appropriate altitude correction tables for establishing depth corrections.
   
3. Depth gauges will often read incorrectly at altitude. Generally, bourdon tube types of depth gauges will read about 0.3 metres/300 metres of altitude shallower than actual depth. Take this in to account with your depth adjustment calculations. Capillary depth gauges are often discussed in this context and lauded for their ability to automatically adjust for altitude. I would suggest that using a computer that automatically compensates for lower ambient pressure is the way to go. Possibly use the other stuff as backup and not as primary measuring devices.
   
4. If you are intending to dive different locations at different altitudes, dive the highest altitude location first.
   
5. At altitudes greater than 2400 metres wait six hours before diving.
   
6. Ascend at a rate slower than your tiny exhaust bubbles. Many computers will give you a sonic alarm if you beat the correct rate of ascent but if you are able to measure it well with timer/watch and depth gauge, the recommended maximum speed is no faster than nine metres/minute. Good luck with that.
   
7. Perform safety stops at depths prescribed on appropriate depth correction tables.
   
8. Do no more than two dives a day.
   
9. Buoyancy in freshwater is different and you may require less lead than you use at sea. However, wetsuits are made up of gazillions (or googles) of tiny gas bubbles that will expand because of the lower pressure at altitude. This may give you more buoyancy. So check your buoyancy characteristics out before going too far from the car and those spare weights! – things might turn out opposite to what you might expect.
   
10. A lower ambient pressure at altitude means a lower partial pressure of oxygen and that may mean you will probably get out of breath quicker at the surface. Take your time doing things.
    
11. Just because water bodies at altitude are closer than the sea is to the sun doesn’t make them any warmer! It gets cooler when you climb up hills with many lakes and rivers also being glacially fed. This offers a much chillier environment than what you may be used to at sea. With that in mind, give serious thought to getting a drysuit if you don’t already own one. Certainly a better way of keeping warm in the water - but don’t neglect a jumper or three for after the dive. A good thermos of hot soup is a good idea also. And a working heater in your car (or whatever)!
    

Diving at altitude offers some interesting adventures and it’s not only the underwater topography and aquatic life that can give us a different ‘high’ underwater but so too can the history and geographical location we find these sometimes unusual diving sites in. Treat yourself to something different this weekend, follow the diving at altitude tips and have a high old time!

Test your knowledge.

1. The ideal dive suit is a what?
   
2. What is the pressure group designation on arriving at altitude of 450 metres above sea level?
   
3. What condition is attributed to lower partial pressure of oxygen?
   
4. Should you start by diving the highest location or the lowest?
   
5. What characteristic of a diver varies with altitude and is vital to check before starting the dive?
   
6. What is the common analogue depth gauge mechanism mentioned above?
   
7. What sort of gauge automatically adjusts for change in ambient pressure?
   
8. What potential health problem could be exacerbated by diving below a thermocline?
   
9. How long should you wait before diving about 2400 metres?
   

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