Diving at altitude

[boulderjohn]

With that knowledge and common sense, you can use RD at altitude.

Please provide a simple example. Compare a specific ratio Deco dive at sea level with one at 2,000 meters and explain why you made the specific adjustments you made.

 

[Dan_P]

Please provide a simple example. Compare a specific ratio Deco dive at sea level with one at 2,000 meters and explain why you made the specific adjustments you made.

Personally?
At that altitude; half the NDL and double the shallows. It works pretty well in practice and isn't far off pretty much every table and practice I've seen (taking the liberty of a reminder of applicable context; Rec).

What would you do?

On another note, John, I hope you've taken note that I'm not dismissing what you're saying - I'm saying, and standing by, I'm in no position to say anything about it. That's a very different thing.
I've not said that your story is untrue, simply that UTD position as it stands is very clear, and directly contrary to what's being said by @tbone1004.

 

[boulderjohn]

On another note, John, I hope you've taken note that I'm not dismissing what you're saying - I'm saying, and standing by, I'm in no position to say anything about it. That's a very different thing.
I've not said that your story is untrue, simply that UTD position as it stands is very clear, and directly contrary to what's being said by @tbone1004.

If UTD is no longer saying that altitude does not have to be considered for decompression planning, I am glad of it. That is not what the issue is. The issue is that from what I recall of that very clear statement, UTD is further stating that it NEVER said that altitude was not a factor to be considered for decompression. I have provided ample evidence that this is not true, including statements made when I was a practicing UTD member and doing decompression dives at altitude, including direct conversations with Andrew Georgitsis when I challenged him on it. If UTD officials, including Andrew, are saying that none of that happened, then they are lying. Since you are the author of UTD's official statement on this, then either you have trusted the wrong people to give you accurate information, or you are lying.

Either way, since with all that I have posted regarding that history I could be neither mistaken nor delusional, all your earlier references to my being wrong about what was done during the years I was in UTD mean only one thing--you are calling me a liar.

I have done everything I can in my public life to maintain a reputation for honesty. I have made a number of SB posts over the years in which I have been wrong, and in each case I have corrected the misinformation and apologized immediately. When people read my posts, I want them to know they were made by someone with a steadfast reputation for honesty. Thus, your repeatedly calling me a liar simply outrages me, and it will forever color my perception of you.

 

[Dan_P]

If UTD is no longer saying that altitude does not have to be considered for decompression planning, I am glad of it. That is not what the issue is. The issue is that from what I recall of that very clear statement, UTD is further stating that it NEVER said that altitude was not a factor to be considered for decompression. I have provided ample evidence that this is not true, including statements made when I was a practicing UTD member and doing decompression dives at altitude, including direct conversations with Andrew Georgitsis when I challenged him on it. If UTD officials, including Andrew, are saying that none of that happened, then they are lying. Since you are the author of UTD's official statement on this, then either you have trusted the wrong people to give you accurate information, or you are lying.

Emphasis mine.

Look at what was actually written, here's the link, and tell me where it's even remotely hinting to what you're saying.

 

[lowviz]

i have done all my diving at sea level. I am about to do some diving in a lake at 2000 elevation. Any adjustments I need to make compared to diving at sea level? ...//...

Excellent, you have a concern and posted in Basic Scuba Discussions. I've had the exact same concern about a pond 1800 feet up.

A good first guess is better than nothing, so let's pick this apart a bit.
The weight of all the air above you at sea level is around 760 mmHg. This is called "one atmosphere". Water is denser than air so you only have to go 34 feet down in fresh water to experience a second atmosphere of pressure acting on you. We all got that from OW class.

The idea is to return along a profile that won't cause nitrogen bubbles to form. So everybody first runs to the US Navy air tables. OK, for your first dive with no prior nitrogen loading, you can approximate NDL's in the following way:
Divide 222,222 by depth in feet and then divide that result again by depth in feet. You now have a really good estimate of minutes of NDL at your chosen depth at sea level diving air.

So now bring in elevation. Air is compressible, so you can't use any linear approximation for an elevation correction, this is what flummoxes the agencies. (Well, not all. BSAC has it nailed. Buy a set of their tables if you want to get it right.)

So how do we handle a wild guess for an NDL dive at elevation? Let's do yours. 2000 feet. What is my new ONE ATMOSPHERE? Barometric Pressure vs. Altitude Table

706.6 mmHg. So it stands to reason that 34 feet of fresh water is now too much, it gives you 760 mmHg.

So my new one atmosphere depth is ((706.6/760)x 34 feet of fresh water)) about 31.6 feet. So all I do is scale the intended depth of the dive, say 65' to get a new NDL.

65feet x (760/706.6) = about 70' at 2000' elevation. So seventy is the new sixtyfive. NDL? 222,222/70^2. 45 minutes.

Now the real gotcha in altitude diving is the location of the safety stop. You have to scale it shallower due to elevation.

15feet(706.6/760) = 14 feet.

So I concur with the others, fuggedaboutit.

 

[tbone1004]

Emphasis mine.

Look at what was actually written, here's the link, and tell me where it's even remotely hinting to what you're saying.

so how the **** does one adapt a strategy that comes out of AG's head based on his magical ability to offgas like a God when diving at altitude? You are literally saying "we don't know how to do it, so figure it out yourself and good luck". Any sane person is going to say use altitude tables and/or just dive a computer that can factor in altitude. You also better be VERY careful that whatever you're using for a bottom timer doesn't factor altitude in either or you're going to be real bloody confused and likely end up like the people @boulderjohn talked about and get bent like a pretzel

 

[lowviz]

...//... You are literally saying "we don't know how to do it, so figure it out yourself and good luck". ...//...

I have to agree. There is absolutely nothing of any substance in there.

Pressure at Elevation ~= 760 -(0.0265 x Elevation in Feet)

I can come up with MUCH better (and far more complex) equations but the above is as simple as I can possibly make it. Note that it is a linear approximation. Probably (maybe, who knows) good enough to keep you out of serious trouble up to about 5,000 feet. Don't know, totally untested. But most likely WAY better than just ignoring the physical reality of the situation and hoping to come back unbent while diving sea level rules at elevation.

 

[Dan_P]

so how the **** does one adapt a strategy that comes out of AG's head based on his magical ability to offgas like a God when diving at altitude? You are literally saying "we don't know how to do it, so figure it out yourself and good luck". Any sane person is going to say use altitude tables and/or just dive a computer that can factor in altitude. You also better be VERY careful that whatever you're using for a bottom timer doesn't factor altitude in either or you're going to be real bloody confused and likely end up like the people @boulderjohn talked about and get bent like a pretzel

Just try, you can do it if you want to.
But you do need to try. You see, anything beyond "moderate", isn't covered by "mainstream" organizations either - they cap at 3.000km on their altitude corriculi.
So probably, in the extreme altitudes covered prior in this thread (approx. 6km?), you'd need to look for solutions on your own anyway.

...Pressure at Elevation ~= 760 -(0.0265 x Elevation in Feet)

I can come up with MUCH better (and far more complex) equations but the above is as simple as I can possibly make it. Note that it is a linear approximation. Probably (maybe, who knows) good enough to keep you out of serious trouble up to about 5,000 feet. Don't know, totally untested. But most likely WAY better than just ignoring the physical reality of the situation and hoping to come back unbent while diving sea level rules at elevation.

Yeah, I mean, I haven't looked closely at your equation, but I agree with the bolded above - the problem I see here is that @tbone1004 says that's what UTD is doing, when it's perfectly clear that's not the case.
If one wants to dive at altitude, for whichever reason, then seek that training.
Loads of agencies offer altitude courses, UTD isn't one of them. No problem.
As I've said before, a GUE-diver who wishes to pursue sidemount will have had to seek courses in diving sidemount elsewhere. I don't think it's problematic unless one is desperate for sticks to beat GUE with (or, in this case, UTD).

-------

For "moderate" altitude Rec dives, I personally find that halfing the NDL and doubling the shallow stops seems pretty reasonable when held up next to the altitude corriculum of another agency that I've pursued previously.
Conservative, in many cases.

I don't think that methodology is radical or reasonably described as anything to the effect of voodoo or the like. It's just looking at what's available and noticing actionable patterns across e.g. altitude and adjustments to NDL (that's one method), or altitude and ascend adjustments.

Pretty basic stuff, really.

That said, in fairness, I don't have much interest in diving beyond about 3km - In all of Europe, you won't find anything higher than 4,5km and there's no diving possible at that altitude there. I'd have to go to a different continent, and even so, I find that extreme altitude dive sites are very far inbetween.

 

[tbone1004]

Just try, you can do it if you want to.
But you do need to try. You see, anything beyond "moderate", isn't covered by "mainstream" organizations either - they cap at 3.000km on their altitude corriculi.
So probably, in the extreme altitudes covered prior in this thread (approx. 6km?), you'd need to look for solutions on your own anyway.

that equates to "we have no idea, but when you get bent, you can't blame us, but you're probably going to be bent, but you should still use our fully scalable ratio deco for altitude, but we can't tell you how to do it. It's proprietary though so no one else can teach you how to use it at altitude either, so you're going to have to abandon ratio deco *which we say is the only way to dive because tables are stupid and computers are unreliable and don't know "you", because that's the only way anyone is going to teach you to dive at altitude"

I don't recall this going to 6km in altitude, but most manuals go to 4km/10kft.

You said you'd half the ndl and double the shallows for a 2km altitude dive, what does that even mean?

Do you even understand the issues with altitude diving? It's not the actual diving at altitude, it's the discrepancy between actual depth, and gauge depth. If your computer auto-calibrates the depth gauge, which most do, then you can't go back to using adjusted tables because you are getting actual depth not gauge depth. The problem is you looking at a depth and trying to do a 20ft deco stop so you go up to 20ft on the gauge but if you're at 2km then you're actually around 15ft and are at risk of getting bent. Halving the NDL continues to show that whatever RD is, it isn't science, it's just fly by the seat of your pants nonsense when it goes outside of any sort of correlation to established tables or algorithms

 

[Dan_P]

that equates to "we have no idea, but when you get bent, you can't blame us, but you're probably going to be bent, but you should still use our fully scalable ratio deco for altitude, but we can't tell you how to do it. It's proprietary though so no one else can teach you how to use it at altitude either, so you're going to have to abandon ratio deco *which we say is the only way to dive because tables are stupid and computers are unreliable and don't know "you", because that's the only way anyone is going to teach you to dive at altitude"

I don't recall this going to 6km in altitude, but most manuals go to 4km/10kft.

You said you'd half the ndl and double the shallows for a 2km altitude dive, what does that even mean?

Do you even understand the issues with altitude diving? It's not the actual diving at altitude, it's the discrepancy between actual depth, and gauge depth. If your computer auto-calibrates the depth gauge, which most do, then you can't go back to using adjusted tables because you are getting actual depth not gauge depth. The problem is you looking at a depth and trying to do a 20ft deco stop so you go up to 20ft on the gauge but if you're at 2km then you're actually around 15ft and are at risk of getting bent. Halving the NDL continues to show that whatever RD is, it isn't science, it's just fly by the seat of your pants nonsense when it goes outside of any sort of correlation to established tables or algorithms

You know what NDL is, right? And what a shallow stop is, right?
I can't believe I need to help you through the door here - but, half of the NDL-time of a sea level dive, and double the stop time on the shallow stop, i.e. for a rec. dive, 2 mins rather than 1, at 3m. Or it could be an extended Safety Stop at 5m/slower ascend after it, if one prefers that approach.

You think the issue with altitude diving revolves primarily around instrument adjustments????????????

The tricky bit is that the relative pressure drop over each metre of ascend, is higher on an altitude dive, than at sea level. This drives a steeper gradient over the same ascend, thus increasing supersaturation, and thus, off-gassing.
That's the "problem" - not adjustment of the gauge.
EDIT (correcting calculation error);
For a 2km dive, if you assume the instrument is not adjusted to altitude, it will show you around 4,8m when you're really at 6m, and 2,4m when you're really at 3m. You're stopping deeper.


And no, I don't agree at all with your interpretation of UTDs position either.