Diving at altitude

[Trace Malinowski]

@Dan_P yes, let's continue to advocate deep stops which have been disproven by science since they actually increase the decompression stress your body goes through, brilliant idea

Yes, I'm trying to break that habit myself. AG was my first tech instructor. I've been doing ratio deco on 100% of my trimix dives for as long as I can remember. Playing with GF's. 30/70 has been the most "aggressive" ascent I've run. If diving at altitude, I definitely wouldn't run ratio deco despite 17 years experience with it and a background with altitude training and diving.

 

[Dan_P]

I think people are missing the most important aspect of diving at altitude. Please read my full article to get it all, but the mistake people are making here now is emphasizing some of the least important aspects of it. Here is the quickest possible summary of why diving at the altitudes we are likely to encounter is important.

1. Pressure during a dive is created by the combination of the weight of the water plus the weight of the atmosphere.
2. The atmosphere weighs less at altitude, but the water weighs the same.
3. At depth during the dive and during the first 2/3 of the ascent, the overwhelming majority of the total pressure comes from the weight of the water, so a diver will ongas during the dive and begin to offgas during the initial part of the ascent at a rate very similar to sea level. That means that when the diver approaches the surface, his or her gas loading situation will be similar to sea level.
4. As the diver approaches the surface, it is critical to keep tissue pressure within a safe gradient compared to ambient pressure, and normal decompression algorithms are designed to do that at sea level.
5. At altitude, though, the ambient pressure in the shallowest portion of the dive and at the surface is less than at sea level because the weight of the atmosphere becomes a more significant fraction of the total, so the algorithm that kept a safe gradient at sea level may not keep a safe level at altitude. At 2,000 meters, the difference in ambient pressure is 20%.
6. Similarly, bubbles formed at depth will increase in volume according to Boyle's Law. At altitude, bubbles will increase in volume greater than at sea level.

All of the above contributes to an increase in the likelihood of DCS. The higher the altitude, the greater the likelihood.

What John said.

The closer you get to the surface, the greater the effect - hence, "double the shallows".

@Dan_P yes, let's continue to advocate deep stops which have been disproven by science since they actually increase the decompression stress your body goes through, brilliant idea

bubbles formed at depth will increase in volume according to Boyle's Law. At altitude, bubbles will increase in volumegreater than at sea level.

 

[tbone1004]

@boulderjohn do you teach that same technique if someone has to get on a plane relatively quickly after diving? We were taught how to get the deco software and manipulate tables for a relatively immediate ascent to altitude basically using it as an extra "stop depth" to put the ceiling @.75ata/8000ft instead of 1ata/surface

said instructor was a frogman in the Navy though so they had all sorts of weird tricks for getting into airplanes/helicopters relatively quickly after being sneaky sneaky

 

[Dan_P]

What is the science behind this? How did you calculate it?

What would you do at 1,500 meters? How about 2,500 meters?

At 1.500m, be slightly more conservative, at 2.500m slightly less. It's how fixpoints work.
But at 2.000m, I'm 3 minutes to the conservative side of the ascend @tbone1004 offered.

When you wrote the definitive, official guide on this matter, you said you conferred with UTD headquarters. Why not confer with them again?

Why don't you?
It's you who are angry at them because you feel they're saying something you say happened, didn't.
So tell them. I've sent you their email address, have at it.

 

[boulderjohn]

What John said.

The closer you get to the surface, the greater the effect - hence, "double the shallows".

Umm--you did not answer my question.

Are you saying you make exactly the same adjustment for diving at any altitude, no matter what that altitude would be?

 

[Dan_P]

Umm--you did not answer my question.

Are you saying you make exactly the same adjustment for diving at any altitude, no matter what that altitude would be?

What? Why would I have the same adjustment for all altitudes?
No, what I'm saying is I have fix points.
Look at Trace's chart in post #17, for instance. It has the same adjustment on a "100ft" dive (30m) regardless if it's at "5.000 feet" or "7.000 feet" (Scuba math 2,5km and 3,5km).
When I'm right smack in the middle of that, 3km I'm 3 minutes conservative of @tbone1004 - and when we're at slightly greater altitude, those two ascends will probably align pretty well in terms of time to surface.

Do you think that's an accident?

You see a pattern and you deduct a rule of thumb. For instance, 2km - half the NDL and double the shallows.
Same tables, same studies, same algorithms, just a different interpretation.

 

[tbone1004]

What? Why would I have the same adjustment for all altitudes?
No, what I'm saying is I have fix points.
Look at Trace's chart in post #17, for instance. It has the same adjustment on a "100ft" dive (30m) regardless if it's at "5.000 feet" or "7.000 feet" (Scuba math 2,5km and 3,5km).
When I'm right smack in the middle of that, 3km I'm 3 minutes conservative of @tbone1004 - and when we're at slightly greater altitude, those two ascends will probably align pretty well in terms of time to surface.

Do you think that's an accident?

You see a pattern and you deduct a rule of thumb. For instance, 2km - half the NDL and double the shallows.
Same tables, same studies, same algorithms, just a different interpretation.

yeah except not the same tables, same studies, or same algorithms. None of the ones currently accepted by experts in decompression theory acknowledge deep stops, and since you are talking about doubling mandatory decompression stops, you're also no longer NDL diving. So no, we are using actual studies and algorithms generated by scientists with PhD's and MD's that study decompression for a living. You are taking directives from a computer science major who is being different to make himself marketable and make money. Totally different scenario. You can't correlate UTD's RD to an algorithm or a table because it doesn't actually make any sense....
Hell, even VPM doesn't do that ****. VPM 0 is 2 mins at 10ft for a 30min 100ft dive and at 6500ft it only drops to 25 mins to get the same 2 mins at 10ft. So try again on you interpreting this all you want, but don't tell me it actually correlates to anything done by science because you're just going to continue to make a fool of yourself and your agency.

 

[Trace Malinowski]

I think the game-changer is the US Navy deep stop study. On the surface, one would think that you can just adjust for depth and play with the strategy here and there. Chances are you'd come out okay. But, even our own personal experiences are just a small sampling. How many deep dives would one need to do at altitude before he or she could claim something worked for them? Over in the accidents and incidents forum there is a thread about 4 instructors who ran OOG at 40m and made rapid ascents. Two were asymptomatic. One experienced a hit that was treatable. The fourth suffered permanent spinal cord damage. The two that were okay couldn't assume they'd always be okay if making rapid ascents from 130 feet. How many times can they get away with it? I've successfully used ratio deco in both UTD and GUE forms without any major issues since day 1 of breathing helium. I once came up above a mid-range stop depth to unfoul a line and returned to depth to add time for extra safety. Despite adding stop time, I had some "decompression stress" as DAN called it. They actually told me to take 2 aspirin, hydrate, breathe O2, and call them in the morning if symptoms persisted or worsened. Fortunately, I was okay ... at least okay enough not to require a chamber. Altitude diving with computers and US Navy tables has more man hours due to the number of dives made by a larger population over the years employing altitude tables, dive tables, and computers. If making a dive at altitude, I'd personally go with the "bend and mend" tried and true most of the time strategy rather than employ what has worked for me at sea level time and time again, but may fail at higher elevations. Then again, I'm a humanities major rather than a STEM guy. I'm pretty confident that I can find a way to work Joseph Heller into a decompression discussion and less confident second guessing DAN and NEDU researchers.

 

[lowviz]

I'm solidly a STEM guy but I don't trust myself either. My calcs are just enough for me to know when my PDC is obviously bullsh****g me.

My money and -personal well-being are always on 1) statistics and 2) the protocol with the most convincing scientific evidence behind it. I'm not a Dive God and never will be, so I need a real-world PHYSICAL approach to something that can either enrich my life or compromise it.

 

[Dan_P]

yeah except not the same tables, same studies, or same algorithms. None of the ones currently accepted by experts in decompression theory acknowledge deep stops, and since you are talking about doubling mandatory decompression stops, you're also no longer NDL diving.

I'm talking about how altitude impacts dive parametres on those same tables you're probably using or algorithms you're using, detecting fix-points in those, and using them to plan your dive.
There's nothing radical about that, at all - and as we've seen from the example we discussed before, I´m not far off (on the conservative side of yourself, might I add).

You're taking that, and twisting it to look like it's voodoo.
Which I think would have to fall on either childish, daft or intentionally instigative.

Same thing with extending the stops, and how you're trying to make it look like it can't be NDL-diving, then.
Dude. Everybody else is on 3-minute stops every day, I'm extending my last stop to 2 minutes.
What's the problem there?
It's still a "Safety Stop" on an NDL-dive.

To me, it's really starting to look like you just want a problem regardless if there is one.

And yes, to answer your previous question, if memory serves, the case(s) you brought up, from Boulder, are 6km-range dives, from what I've been told.
Which no diving agency I know of will certify you for, anyway (and PADI/SSI, which pretty much IS the mainstream, don't certify beyond 3km).
So no, UTD won't certify you to do the dive that you brought up - nor will anyone else.
And as for the adaptations I personally use when diving at altitude, I got the initial input towards those, from a PADI-course. Same altitude adaptations that have been, I believe "tried and tested" is the term used, just not in a PDC or a table. I.e. "different interpretation".

Altitude diving with computers and US Navy tables has more man hours due to the number of dives made by a larger population over the years employing altitude tables, dive tables, and computers. If making a dive at altitude, I'd personally go with the "bend and mend" tried and true most of the time strategy rather than employ what has worked for me at sea level time and time again, but may fail at higher elevations.

I agree in regards adjustment of NDL-times for altitude. Those'd be among the main inputs I've used as the basis of my altitude adaptations (rec).