Mid-altitude diving

[Erku]

Hi! My wife and I live in Calgary, Alberta, at 1080m above sea level. We learned to dive in cold water, so that isn't a problem, but we learned at sea level; we want to get out and do some diving without driving all the way to the ocean, but I've heard high altitude diving can be a whole different ballgame. I get the impression from Wikipedia that it's mostly a matter of being more careful about NDLs, which I'd assume to be due to the greater disparity between surface pressure and water pressure. We're very conservative divers at this point: we're mostly just interested in splashing around between about 3-15m of depth and getting a feel for our equipment, buoyancy, and such. Should we be looking into a high altitude course before diving locally? Are there other dangers I'm not aware of?

Thanks!

 

[Marek K]

If you have dive computers, they should automatically or manually adjust for altitude. Check the user manual.

If you don't, then it gets a lot more complicated. There are tables to adjust your "theoretical depth" -- the depth you should be using on your dive tables, versus your actual depth. The amount you adjust depends on the altitude -- at 1,200 meters, it's almost 20% more than actual depth.

Your analog depth gauge will also not be showing an accurate actual depth, and that needs to be accounted for.

There are also other considerations, such as always doing safety stops.

You can do an altitude dive as an option in the PADI Advanced OW Course. Even without doing that dive, the course would give you the Adventures in Diving manual, which has the adjustment tables and other altitude information.

All this from a guy who's never done an altitude dive in his life. But I'm planning some.

 

[Erku]

If you have dive computers, they should automatically or manually adjust for altitude. Check the user manual.

Comps are going to be our second investment after mask/fins/snorkel; it hadn't occurred to me to make sure that we get a comp that handles altitude, but of course that makes sense. Easy enough to hold off on the local diving until we are properly equipped, I suppose, although it does mean waiting longer

There are also other considerations, such as always doing safety stops.

That's reasonable; with the rest of what you said I'd assume the safety stop depth changes at altitude, though.

You can do an altitude dive as an option in the PADI Advanced OW Course. Even without doing that dive, the course would give you the Adventures in Diving manual, which has the adjustment tables and other altitude information.

We'll get to that soon enough, but I'm kind of interested in just doing some basic puttering around dives before we start investing in a lot more training.

Thanks for the information!

 

[ianr33]

This might be useful to you:
Altitude

Easiest thing to do is use a computer. If not using a computer,and not going deeper than 100 feet just add 10 feet onto your actual depth. e.g for a 45 foot dive (that would be rounded to 50 feet at sea level) Plan it as if it was a 60 foot dive.

The safety stop depth will change but its not enough to worry about. Anywhere between 20 and 10 feet is fine.

 

[Crush]

I dive in Waterton all the time. I used to use tables, which you adjust with an altitude penalty. For example, when calculating your NDL, treat 100 feet depth as 120 feet. PM me if you are interested in the altitude penalties and I will hunt them down and email them to you. Alternatively, if you get a computer most computers will automatically account for elevation penalties. There are four very important things to note:

1. Your computer must be activated while dry in order to gauge your elevation. Most computers will automatically activate when plunged into the water. Unfortunately, if you use a wet activation your computer will default to the last elevation at which it was used. If you were at sea level for your last dive, your computer will be giving you dangerously incorrect information;

2. You must acclimatize to higher altitudes for your computer or elevation-adjusted tables to be correct. If you zip out from Calgary to Lake Minnewanka and jump into the water immediately, you will still have some residual N2 (from the higher pressure of Calgary) in your system. You will continue to off-gas while your body adjusts to the lower pressures at Lake Minnewanka. Your tables and computer will assume that you spent 12-24 hours at your current elevation, which is likely not the case. The difference is likely small (given the length of the drive and the time required to suit up), but you should bear this in mind;

3. Plan your post-dive travels carefully. You should avoid substantial increases in elevation after diving, especially in the first few hours; and

4. Cold water diving can complicate decompression and off-gassing. Starting a dive cold and ending it warm is not a problem. Starting a dive warm and ending it cold can be a problem (search for this topic on SB) as your body will off-gas more slowly than predicted by tables or your computer. Try not to red-line your NDL on cold dives.

Finally, buy a drysuit for local diving - you will love it.

A few articles for you:

http://www.scubaboard.com/forums/4882051-post25.html

http://www.scubaboard.com/forums/ne...ving/328008-i-know-another-gear-question.html

 

[GrumpyOldGuy]

...
1. Your computer must be activated while dry in order to gauge your elevation. Most computers will automatically activate when plunged into the water. Unfortunately, if you use a wet activation your computer will default to the last elevation at which it was used. If you were at sea level for your last dive, your computer will be giving you dangerously incorrect information;

While computers will not sense altitude when wet, the aeris / oceanic / pelagic computers (very common) sense the altitude every 30 minutes even when the computer is not activated, and every 15 minutes when activated. Thus you can let the computer be water activated at altitude. You are safe unless you activate the computer, get the contacts wet and they remain wet while making significant changes in altitude.

 

[Crush]

While computers will not sense altitude when wet, the aeris / oceanic / pelagic computers (very common) sense the altitude every 30 minutes even when the computer is not activated...

Thanks for the correction, GrumpyOldGuy. My Sherwoods don't do this - I have to activate them dry, or they default to the last altitude. Good feature on the aeris / oceanic / pelagic (an possibly other) computers.

 

[Erku]

For a noob like me, I'd say it's best just to turn it on dry anyway; it sounds like a smart practice. Good to know that many computers still track altitude when off though.

Would a computer ever do the math for the out-of-water altitude changes and associated nitrogen loading? It seems to me that they could, but of course the question is if they do rather than if they can.

 

[Crush]

Would a computer ever do the math for the out-of-water altitude changes and associated nitrogen loading? It seems to me that they could, but of course the question is if they do rather than if they can.

Altitude sensors can't function when wet - you would have to shake the heck out of your computer and dry it off pronto (with a hair dryer?) if you hope for it to register changes in altitude that same day.

 

[Erku]

Altitude sensors can't function when wet - you would have to shake the heck out of your computer and dry it off pronto (with a hair dryer?) if you hope for it to register changes in altitude that same day.

Oh, sorry, I was unclear... I meant on the way up. The drive from cowtown to just about anywhere with diving adds +200m or so which may need to be accounted for like you said earlier. I'd think coming back down should be more or less safe, and we're not at a level yet where we're going to be going from site to site changing altitude in between.