Altitude Diving

UWSojourner

Does anyone do significant diving above sea level?

What deco algorithms/ altitude procedures are you following and for what elevations? Any advice on adapting to altitude prior to dive? How long, etc.?

Thx

boomx5

I don't dive rebreathers yet, but I'm also not sure if this is really a rebreather question. Anyway, we dive Lake Tahoe quite a bit throughout the year and don't really consider the altitude (6,200') a real issue. Normally, we'll leave the house about 6:00 am and arrive at the divesite by 7:30-8:00 am. We usually kit up and are in the water by around 8:45-9:00 am. We'll then jump in and do a dive that may range from the recreational ranges of around 100' or so. Or do something in the ranges around 250', but nothing really deeper yet. After we get out of the water we clean up and head back over the summit (bout 7,000' or so) and back home to sea level by 2:00-5:00 pm...but it depends on where we stop to eat on the way home.

We use standard ratio deco for all our dives and may extend the ratio out a bit depending on fatigue, cold, hydration or just how we generally feel. This is a normal typical day for us when we go to Tahoe. We've never had an issue and have generally felt good after the dive. Again, I know this may not apply to the rebreathers since we're not diving them yet, but hopefully this helps a bit.

wookey

last year I did some exploration dives with my Kiss in swiss and italian lakes who where at an altitude between 2500m and 3000m above sea level.

I did the air and o2 calibration taking in account the modified ambient pressure in comparaison to sea level.
for the dives themselves I used the aladin nitrox computers who adapt themselves automatically to altitude (up to 4500m).

Lake_Tahoe_Diver

( ***** No flame intended, just a concerned local diver ******)
All I have to say Boomx5, is you are very lucky then, myself and other that dive and live around Lake Tahoe do take the altitude into account for various reasons.

First the surface ATM at Lake Tahoe is around .79 ATA so if you are pushing the tables, or computers without taking into account altitude you are going to get bent, you just have been lucky so far.

Also you need to take into account the amount of residual N2 in your body from were you live to where you are going to be diving. This is because if you live at a lower elevation than your dive site your body will have more disovled gas, since your body becomes saturated to the pressure that you live at. This is simple physics, so when we arrive at altitude you need to take this into account when planning your dive profiles. Look at the software GAP , Vplanner, and most other deco software, they try to take this into account. They credit you time for off gassing via the surface interval once you arrive. If you are diving tables without correction for altitude then you are asking for trouble.

If you are diving a computer that adjusts for the altitude then you have some degree of safety, but remember that most computer do not account the redsidual N2 from the drive to elevation. So use them with caution, or make sure you arrive at altitude and spend plenty of time off gassing before the dive.

The next time you are up here send me a PM and maybe we can compare dive sites at the lake. We are find new and interesting areas every weekend.

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Scuba Diving,the only sport where it is good to be in over your head....

Lake_Tahoe_Diver

Dive Site Lake Tahoe (6,225 feet or 1,897 meters elevation)


You should adjust your formulas to reflect the reduced pressure at the dive site. Remember that not only are you dive at altitude but you are also diving in fresh water in most cases. So 1 ATM is not 33 feet, but 34 feet. The surface pressure is not 1 ATM but some number less than that so if you push the tables and assume you are surfacing to 1 ATA you will have some issues. Here at Lake Tahoe you would be surfacing to .79 ATA not 1 ATA so you can see this causing some problems. You parital pressure calculation for the gas will also change since you are at less presure you can use a gas a deeper than you would at sea level since the Po2 is not as high, this is also a draw back since if you had planed on having a Po2 of 1.4 on OC but did not take into account the reduce ATM at the surface you will not be diving at 1.4 but a little less about 1.2 up here. When we get a new diver up here we spend about an hour going over how the adjust the various formulas and reminding them of the recomended accent rates 30ft a minute max. We perfer there not to be any problems up here since Lake Tahoe can be a little unforgiving (Cold and Deep around 39f. or 4c. at depth year around and a lttle over 1600 ffw or 488 mts. deep)

If you are ever up here I can give you the name a good dive store where to get all you OC and Rebreather need taken care of. They do custom blending, have have sorb in stock.

Now if you are diving a Rebreather like a lot of here at altitude, we dive various units KISS, Prism, Evolution, I think I saw a meg diver the other day. Each of these are calibrated differently at altitude. When calibrating the KISS and the secondary for the Prism you need to set it to what the actual ATA is so you can have an accurate idea of your Po2. Some of these unit expect the surface to be at 1 ATM so you may need a pressure pot to put the cells in to calibrate it to 1 ATM or know by what percentage the set point controller will be off. The Evo and the Meg I think take it in to account upon start up (I have not used those units nor watched the calibration, so I can not say for sure.)

When you do your dil flush to check your sensors at depth you will need to take into account actual ATA, if not things may appear to be a little strange. For example 34 depth at Lake Tahoe is about 1.79 ATA not 2 ATA since the surface is only at .79 ATM so yu'll need to either adjust what you expect to see for you Po2 or adjust the depth where you do the flush to get the Po2 you expect to test the sensors.

__________________
Scuba Diving,the only sport where it is good to be in over your head....

boomx5

Thanks for the invite, I may take you up on that sometime. As far as the altitude discussion is concerned, we've been having this discussion for years. Sometimes it gets a bit heated, but we have all managed to remain friends. I fully understand the math and arguments regarding altitude correction, however we take a different approach to the way we decompress. Our approach is not as cavalier as it may seem, and forgive me if that's the way it came off. It is also something we have not taken lightly or have pulled out of our hats and ran with because we have gotten lucky a few times.

We simply believe, that if we can decompress more efficiently that the altitude issue becomes a moot point within a certain range of diving. This is something that many of us have been proving for the last few years within the ranges that we have been diving. I don't recommend others do this without the proper training. The funny thing is we've taken friends on dives where they have been using their computers, and the computers have given them some sort of rediculous profile and way more decompression; yet they get out just fine and are blown away that there is a different approach then the way they have been doing things.

Let me be clear on one thing. I'm not a dare devil or a huge risk taker. I'm more conservative in the way i approach things than a lot of people I know. Heck, I'm so conservative that I'm still not sure if a rebreather is right for me. So when I approach something I take the approach of caution.

Anyway, this is huge subject and probably one that we may not fully agree on. However, I'm still open to diving some of those secret spots you may have sometime.

Sutty

I always find this an odd concern as we measure our depth as a pressure - so if your computer or guage is measuring in feet of sea water (metric is better) then if it shows 33 feet but you are in freshwater, you'll be in 34 feet (assuming your quoted fudge-factor is right). So what? its the pressure that matters not the depth measured with a tape-measure.

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Never forget that life is a finite resource.

UWSojourner

Can you provide some profiles for your approach? And are you starting at sea level that day and returning that day? Thx

boomx5

Yes. I live at sea level. When I get a second, I'll PM you an average profile. In reality, they are not much of a big deal.

Lake_Tahoe_Diver

(Not meant to start a fire here just trying to get people to think a little here)

I will agree with you about it all being about the pressure. So every formula that has a messurement of pressure needs to be evaluated when diving at altitude, be it in Feet, Mts, Bars or ATM. (I know there is some debate between the purist on the differents between a 1 BAR (1000 mBar) vs 1 ATM (1013.25 mBar) but I am not going to open that can of worm here.


I also agree that a unit of messure is not important just remember that when diving at Altitude you will surface to a lower atompheric pressure than what most standard tables are based on. So you may want to adjust you depths of your stops or extend out the time at those stops, and slow the rate of accent to keep the pressure gradent within the realm of the deco model you are following. So the question becomes at what depth do you want to do your last deco stop since you are surfacing to .79 ATA?
(If using a computer generate custom table from GAP or V Planner, etc this is a mute point since it adjusted the times at the stops for you to account for this.)


On a side note: It is funny to watch people's faces when they come up here to take an altitude class and we pull out the old non altitude adjustable depth guage, a capillary guage, and of course a computer and adjustable guage. The non adjustable style of guage will not even move until you are about 7ft. or aprox 2.1 mts of depth due to the lack of pressure, they assume 1 ATM on the surface, but Lake Tahoe is only .79 ATM and is fresh water so we get quite a differnce in the depth on that guage.

I'll bring people to about 66 feet and have them compare an altutde adjustable guage, computer, capillary guage, and of course the non-adjustable depth guage. The only two that read the correct depth are the computer and the altitude adjustable depth guage. The capillary guage will of course read deeper than actual depth. It will show about 78 feet. While the non adjustable guage will read about 59 feet. So you end up with a range from 59 - 78 feet depending on what guage you wish to look at.

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Scuba Diving,the only sport where it is good to be in over your head....