Altitude and depth, have we been thinking of this wrong?

[broncobowsher]

Thinking about it, we are taught that 1 atmosphere of pressure is 33' or 10 meters. Yes, slight differences between fresh and salt water. But in general. But should we not use this at altitude?

So seal level is 14.7 PSI of pressure, lets keep the math easy and just call it 15. Lets say we find a nice lake at altitude, again to keep math easy to follow along call it about 10,000' elevation and that puts ambient pressure at 10 PSI.

In normal diving you want to go to 2 atmospheres of pressure, you go down 33' or 10 meters. But at altitude, that would only be 22' (6-2/3 meters).
Any of the computers will have a pressure sensor that will be accurate in depth. The old capillary guages (if they read in atmospheres) would be accurate in how many atmospheres you travel. Computers will be accurate in how many feet you travel.

If you ignore travel to and from altitude, you live there, you stay there. Would it be more correct in calculating decompression based on magnitudes of pressure and not actual depth?

The inverse of that. Stay in a habitat at 33' depth. Your 1 atmosphere surface pressure would be 30 PSI instead of 15. Would that expand depth potential, or just shift it?


I know this isn't the typical quick answer stuff. Actually some deeper thoughts of theory that probably hasn't been tested or tested very much. Maybe a couple really smart people here can add something to my thoughts. Not going to practice any of it. Just thinking outside the box(ed training).

 

[Alurpal]

I’m not an expert, but I can answer your question, please someone jump in if I’m saying anything incorrect.

In Rec diving, computers don’t really care about depth, they care about starting pressure, time at other pressures, and getting you back to the starting pressure with acceptable bubble formation. It’s a constant on gas/off gas dance that we theoretically model to keep us “safe”.

Max depth obviously has other factors like o2 toxicity and nitrogen narcosis but if you look at saturation divers, they live in a diving bell at pressure and can therefor make very long dives at depth since they don’t fully deco until the end of their multi day shift. Their starting and ending pressure remains within a smaller range even though they dive deep.

 

[tbone1004]

@broncobowsher the computers calculate in pressure, they just convert pressure to depth. The altitude settings are to "tare" the ambient pressure to 0ft and then the pressure sensors start at 0ft and move accordingly, this is why the salinity setting doesn't matter for decompression, only for accurate depth reporting.
Saturation diving is a bit different and I don't do that type of diving though it is all handled by surface support. Conceivably though the computers could "tare" to 0ft from the habitat, or you could ride your ceiling until it hits whatever the habitat is set to. I.e. you have your decompression calculated and 40ft is your deepest stop with the habitat at 30ft. Once the computer says you can move to your 30ft stop you just stay in the habitat and it hangs out. Getting accurate TTS readings would be complicated, but again this is why the dive plans are all run by surface support and the habitats are usually within a couple ATA's of the dive depths so you really have unlimited "NDL" so long as you are going back to the habitat.

 

[Akimbo]

It would be interesting to know to what extent the decompression algorithms take into account the saturation level when starting the dive. Technically, traveling to altitude causes outgassing/decompression unless you are there more than about 24 hours before descending.

 

[tursiops]

There is no problem. The altitude class introduces "theoretical depth," which takes into account the effect you are worrying about.

 

[inquis]

In normal diving you want to go to 2 atmospheres of pressure, you go down 33' or 10 meters. But at altitude, that would only be 22' (6-2/3 meters).

Gotta go deeper to get a desired pressure at altitude, as you're starting with less on the surface. Trying to get 30 psi when starting at 10 psi surface needs 20 psi * 33 ft / 15 psi = 44 ft.

The key to applying sea-level intuition to decompression at altitude is that tissues care about multiples of surface pressure. That works out the same as absolute pressure at sea level, so the distinction is glossed over in typical instruction.

That's why surfacing at altitude should be performed slower, as the rate of change of the ambient:surface pressure RATIO is much greater per foot at altitude compared to sea level.

Would it be more correct in calculating decompression based on magnitudes of pressure and not actual depth?

Your intuition is spot on. That's exactly how it is done. The principle behind "equivalent depth" taught in altitude class is to find the sea-level depth with the same pressure as the altitude depth so (depth-based) tables make sense. Computers sidestep the issue by working with pressure directly as others have said.

Would that [habitat] expand depth potential, or just shift it?

I haven't studied Sat diving, but I would expect that to expand it. NDL for a 60 m ocean dive when returning to a 30 m habitat: pressure RATIO = 7 atm / 4 atm = 1.75. That's equivalent to 7.5 m from the surface, so an NDL of 1102 minutes by the Navy air tables. Of course, that wouldn't be done on air, but hopefully you get my point.

 

[inquis]

I should point out it's only tissue decompression that cares about the depth-to-surface pressure ratio. To my knowledge, CNS, WOB, etc. are based on the actual (absolute) pressure. Fun fact: the O2 "MOD-Ceiling sandwich" is far more forgiving when decoing at altitude. MOD(1.6 atm) at 0.85 atm surface pressure (about 4700 ft altitude) is 25 ft.

 

[broncobowsher]

Gotta go deeper to get a desired pressure at altitude, as you're starting with less on the surface. Trying to get 30 psi when starting at 10 psi surface needs 20 psi * 33 ft / 15 psi = 44 ft.

That is what I am getting at. Still defining atmosphere as sea level pressure. If you go do a 100', 20 minute dive at sea level, you are doing a 4 atmosphere dive returning to 1 atmosphere. Now that I am typing this out it is making more sense then just thoughts in my head about equilivent depth. If my fuzzy math in my head is working right, 100' at altitude would be a dive to 6 atmosphere instead of 4 atmospheres at sea level. Now the equilivent depth correction is making a lot more sense

As for computers handling just pressure, I knew that, just wasn't thinking about it.

 

[dmaziuk]

It would be interesting to know to what extent the decompression algorithms take into account the saturation level when starting the dive. Technically, traveling to altitude causes outgassing/decompression unless you are there more than about 24 hours before descending.

One of Buhlmann's additions was recalibrating to absolute pressure for diving in Swiss mountain lakes. The issue is not the algorithm though, it's the implementation:

if your computer has an ambient pressure sensor and you wear it all the time, it will know exactly what our saturation level is at the start of the dive.

If you turn it on just before splashing, it doesn't know where you were before and what your current saturation level is. It would ass-u-me you're saturated at current ambient pressure. (Which is not unreasonable, just not guaranteed to be correct.)

If it doesn't have the sensor, you have to manually set the altitude (typically in fairly coarse increments) and it'll assume you're saturated to that.

Or it could not bother with saturation and just throw in some conservatism fudge factor.

 

[inquis]

If my fuzzy math in my head is working right, 100' at altitude would be a dive to 6 atmosphere instead of 4 atmospheres at sea level.

I would phrase it as 100' at altitude is 6x the surface pressure instead of 4x (as it would be at sea level), but yes you've got it. This is precisely why people get into trouble diving at altitude if they naively use a sea-level table without adjustments (or don't set the altitude on a computer that doesn't auto-set the surface pressure).