Diving at altitude

[tbone1004]

@Dan_P yes I do, which is why I'm curious how you can double a stop that doesn't exist because it's NDL diving

The 15ft was from a table, I didn't actually do the math because I couldn't be bothered. Point is we agree that when you think you're at 20ft, you're actually shallower. By how much depends on your depth, but hey, if you pre-plan your dive with a computer *which does all sorts of math for you*, and then follow that ascent strategy, you don't have to worry about it.
I know I have a 24 minute dive plan at 100ft actual depth using EAN32. If I run that at sea level, it's no stop. If I run it at 6500ft, it's a 1min stop at 20ft and a 4 min stop at 10ft. Done.

18min NDL diving at altitude, vs 24min NDL for that dive. Hardly half the NDL

How do you make an adjustment if the diver lives at sea level, drives up to altitude, then goes diving and comes back? How does all of that factor into RD? You can't tell someone how to do that because you guys don't know how to make it work so you come up with some nonsense about going to ask an outside source to adapt your custom wacky deco strategy to altitude and no one in their right mind would do that. They're going to tell them to use known, tested, proven, decompression strategies. UTD's RD is not one of them

 

[Trace Malinowski]

“There was only one catch and that was Catch-22, which specified that a concern for one's safety in the face of dangers that were real and immediate was the process of a rational mind. A diver not using ratio deco was crazy. All he had to do was ask how to use ratio deco at altitude; and as soon as he did, he would no longer be crazy and would be able to dive at altitude. A diver would be crazy to dive ratio deco at altitude and sane if he didn't, but if he was sane he should use ratio deco. If he dove ratio deco at altitude he was crazy and didn't have to; but if he didn't want to he was sane, and therefore, should employ ratio deco. The diving community was moved very deeply by the absolute simplicity of this clause of Catch-22 and each ScubaBoard.com member let out a collective respectful whistle.

"That's some catch, that Catch-22," they observed.

"It's the best there is."

 

[rjack321]

I don't agree with the UTD position because its like jello. It changes shape to conform to whatever container it's in today.

Some of us have known AG from way before UTD ever existed and probably a decade longer than you've been a diver. And he absolutely has claimed that RD worked fine at altitude, that its only a meter or so difference in depth (or 20cm in your posts), and that no adjustments were necessary. That far far predates your affiliation with UTD Dan. Yes at some point "altitude is nothing to worry about" changed to "you are on your own with altitude adjustments, but adjust based on how you feel" which is a completely bogus way to plan.

Do you even dive at altitude? Looking at the UTD website you aren't even a technical instructor and don't teach anything beyond OW and a few specialties. Profile of Daniel Pinderup · UTD Scuba Diving

You're a bit out of your element here both historically and technically.

 

[lowviz]

@Dan_P ...//... How do you make an adjustment if the diver lives at sea level, drives up to altitude, then goes diving ...

Stay there for a day before diving so that your slowest tissues can degas to that altitude. Or, one could further complicate things by estimating nitrogen load in one's slowest tissues.

I see 3km altitude as nothing to dismiss lightly for NDL diving or deco diving. The one-atmosphere depth has shrunk to about 23 feet. That is a substantial error from what you would expect at sea level.

Disclaimer, I do not follow RD at all. However, I consider myself to be 'DIR friendly' but certainly not a subscriber. They have their way of doing things and that is all well and fine by me. As I see it, RD is a pattern that accommodates time and depth at sea level by comparing their pattern to much more generally accepted ascent profiles.

If DIR followers would both shrink the distances between 'stops' (while adding to the bottom) in step with altitude and raise the whole series (shallower in depth) in step with altitude, then I might be more accepting of their altitude protocol. But it just doesn't make sense to go through all that trouble when a PDC will do it for you. I use my calculations just as a sanity check to see if something looks really stupid. It has happened, always the same reason. Improperly setting something in my PDC.

 

[Dan_P]

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

3km. PADI and SSI, anyway, but I guess those two make up the "mainstream" anyway.

@Dan_P yes I do, which is why I'm curious how you can double a stop that doesn't exist because it's NDL diving

What do you mean? You wouldn't do any stops on an NDL dive?
You can call it a "safety stop" if you like, and do it whichever way, but you'd do something, surely?
Mine from a 30m dive is 15m 1min, 12m 1 min, 9m 1 min, 6m 1 min, 3m 2 min. That's my "safety stop", if you will, on a 30m dive at 2km.
6 minutes ascend from a 30m dive. Let's put a pin in that.

Point is we agree that when you think you're at 20ft, you're actually shallower.

If the malfunction is that the user is misinterpreting his PDC, then that could be an issue, yes.
E.g. you're unaware if the software/hardware you're using is working the way you want it to. What do you do?
What do you do every time you go on a dive, anyway?
It'll read 0.8 or 1.0 and there you go.

I know I have a 24 minute dive plan at 100ft actual depth using EAN32. If I run that at sea level, it's no stop. If I run it at 6500ft, it's a 1min stop at 20ft and a 4 min stop at 10ft. Done.

18min NDL diving at altitude, vs 24min NDL for that dive. Hardly half the NDL

I know I have 30min on EAN32 at 30m at sea level.
Conservatively, I reduce it to half at 2km altitude, making it 15 min.

You land on 18min NDL-time. I on 15min NDL-time. That 3 minutes of conservatism is what this is about?
Bigger fish to fry.

How do you make an adjustment if the diver lives at sea level, drives up to altitude, then goes diving and comes back? How does all of that factor into RD? You can't tell someone how to do that because you guys don't know how to make it work so you come up with some nonsense about going to ask an outside source to adapt your custom wacky deco strategy to altitude and no one in their right mind would do that. They're going to tell them to use known, tested, proven, decompression strategies. UTD's RD is not one of them

I think it'd land on something to the effect of "double the shallows".

 

[tbone1004]

@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

 

[boulderjohn]

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.

 

[Dan_P]

That far far predates your affiliation with UTD Dan. Yes at some point "altitude is nothing to worry about" changed to "you are on your own with altitude adjustments, but adjust based on how you feel" which is a completely bogus way to plan.

I know it outdates my affiliation with UTD. It's why I keep saying things like this:

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.

Do you even dive at altitude?

Yes, occationally.

Looking at the UTD website you aren't even a technical instructor

Indeed, I am not. Problem?

and don't teach anything beyond OW and a few specialties

Thanks for the heads up, we'll have that updated.

 

[boulderjohn]

I know I have 30min on EAN32 at 30m at sea level.
Conservatively, I reduce it to half at 2km altitude, making it 15 min.

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

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

 

[boulderjohn]

I know it outdates my affiliation with UTD.

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