Altitude questions

[Omb]

OK...trying to wrap my head around the physiology. Say I start my dive at an altitude of 7000 feet. Then dive down an atmosphere or two. I get nitrogen in my tissues and then come up. It seems to me that the pressure at 35-45 feet down at 7000 feet is going to be similar (within a few to several feet) of 35-45 feet down at sea level. So when I come up, I am essentially rapidly transported to 7000 foot pressures. If that is the case, why do we have rules like "don't drive over a 5000 foot pass immediately after diving at sea level"

Can someone set me straight as to what is really going on?

 

[Outbound]

Regular dive tables and many computers calculate NDL based on surfacing at sea level. If you surface at a higher elevation (lower pressure), you increase your risk of DCS. Similarly, if you surface at or near sea level but then quickly ascend to a higher elevation, whether that be in a commercial airplane pressurized to 8,000 ft. elevation or a high mountain pass, the ambient pressure drops and you run the risk of bubbles coming out of solution too rapidly.

 

[boulderjohn]

I am not sure I understood your question precisely.

Yes, it is true that up until you hit a depth of 35 to 40 feet, there is not much difference between your dive at 7000 feet altitude and they dive at sea level. Most of the difference comes during the rest of the ascent. It is important that your tissue pressure not be too great in relation to ambient pressure, and ambient pressure is significantly less at 7000 feet than it is at sea level. That means that having roughly the same tissue pressure when surfacing as you would at sea level makes surfacing more dangerous.

This is similar to ascending to altitude after diving. You need to keep your tissue pressure within a certain range relative to ambient pressure and you you need to do that for a while as your tissues off-gas. If you ascend to a higher altitude with lesser pressure too quickly, then you have a higher pressure gradient between tissue and ambient, and it may be too high.

 

[inquis]

The goal is to avoid tissue loading (partial pressure of inert gas) being some factor higher than ambient. Say that's 1.7x for simplicity. At sea level then, with tissues loaded around 1.7 atm, you can surface with acceptable risk. However, 1.7x ambient pressure at an 8000 ft dive is 1.3 atm, so those 1.7 atm tissues would be a problem. That's why NDL times are shorter at altitude for a given depth -- the shorter times only load to 1.3 atm, and you can therefore surface without a mandatory decompression stop. Similarly, after the sea level dive, the required wait before going to altitude is effectively a mandatory decompression stop at 0 ft depth before you are safe to ascend further.

 

[EFX]

OK...trying to wrap my head around the physiology. Say I start my dive at an altitude of 7000 feet. Then dive down an atmosphere or two. I get nitrogen in my tissues and then come up. It seems to me that the pressure at 35-45 feet down at 7000 feet is going to be similar (within a few to several feet) of 35-45 feet down at sea level. So when I come up, I am essentially rapidly transported to 7000 foot pressures. If that is the case, why do we have rules like "don't drive over a 5000 foot pass immediately after diving at sea level"

Can someone set me straight as to what is really going on?

I think the rule of "don't drive over a 5,000 ft mountain pass" after a dive has to do with the lack of tracking the pressures in your tissues as you ascend the mountain. Whereas, on a dive at 7,000 ft altitude your dive computer is tracking your tissues and will adjust the NDL time accordingly or give you the proper deco ascent schedule to keep you safe.

 

[boulderjohn]

I think the rule of "don't drive over a 5,000 ft mountain pass" after a dive has to do with the lack of tracking the pressures in your tissues as you ascend the mountain. Whereas, on a dive at 7,000 ft altitude your dive computer is tracking your tissues and will adjust the NDL time accordingly or give you the proper deco ascent schedule to keep you safe.

....and that's a shame.

Those of us who regularly ascend to altitude wish we had more guidance. The problem is that there are many variables that have not been adequately studied. Obviously, the rate at which you ascend to that 5,000 foot pass, including pauses at intermediate altitudes, is a major fact, but there is not enough research for dive computer manufacturers to add the simple programming it would take. When I wrote to Shearwater about such options, I was simply told to refer to flying after diving guidelines.

 

[EFX]

I added the altitude capability to my spreadsheet. You can enter the final destination altitude and the time it takes to get there and it will happily calculate all the tissue compartment pressures, CurGF, SurGF, and more. It adds an altitude segment after the surface interval (air and O2 if selected). From what I remember the changes in GF and TCP's were modest after arrival at altitude.

I don't understand what the big issue is. At 7,000 ft altitude the surface pressure is 0.77 atm. All the tissues "see" is a drop in inspired inert gas pressure and therefore off gassing until the TCP's reach saturation. The tissues don't understand altitude just pressure differences. Interestingly, on a flight at cruising altitude I turned my Perdix on and it showed the decrease in cabin pressure. I remember seeing about 860 mbar. The tissue compartment display actually showed some compartments pressure above the ambient pressure line and therefore off gassing. Since the computer was not in dive mode the NDL and dive time stayed at 0.

I understand the dangers of commercial flying. Typically, the cabin pressure is roughly at the same pressure as being at an altitude of 8,000 ft. The ascent to cruising altitude is quick (~20 minutes) placing greater stress on the tissues as opposed to driving there. Also, if a window blows out the sudden decrease in cabin pressure could be devastating to someone with elevated TP's above surface pressure.

 

[inquis]

I don't understand what the big issue is. At 7,000 ft altitude the surface pressure is 0.77 atm. All the tissues "see" is a drop in inspired inert gas pressure

They also have an increase in supersaturation. However, that increase in supersaturation doesn't cover the entirety of the issue. If it did, then waiting to fly until you have a GF of, say, 80% relative to the 8000 ft ambient pressure would be safe. Testing shows it's NOT.

I believe the issue is that it takes longer for bubbles to be eliminated once formed than the time to off-gas to GF(8000 ft) of 80%. And we know bubbles form, even when the surfacing GF is as we've dictated.

After following the Navy ascent guidelines to go to 8000 ft after a sea level dive, you wind up with an average GF(8000 ft) of about 13%, pretty consistently across the various ending pressure groups. I personally use that as my time-to-fly indication. Obviously, less altitude gain can be done sooner, with a corresponding increase in GF present after the required time.

In other words, I feel that controlling the supersaturation results in asymptomatic bubbles. Bubble growth due to ascent after the dive is a different problem not addressed by controlling the tissue supersaturation (GF). Doing the latter only prevents more bubbles from being formed.

 

[inquis]

The ascent to cruising altitude is quick (~20 minutes) placing greater stress on the tissues as opposed to driving there.

The rate of pressure increase during the airplane climb is FAR below that experienced with the 1 atm per minute pressure reduction during the dive (30 fpm ascent rate). As we all know, that's considered to be a safe rate of pressure reduction.

In my view, we have to wait long enough for the (asymptomatic) bubbles that have already formed to reduce in size such that a volume increase of 1.3x (=1/0.77) won't be an issue. If a really slow drive allows that, great. If you wait long enough before the plane (or car) leaves/gains altitude, that's great too.

 

[Omb]

I guess my question is: Is there a significant difference between diving 45 feet at 7000 feet vs diving 45 feet at sea level and then taking and elevator up to 7000 feet in a matter of minutes. It seems like the tissue loading from being 45 feet underwater would be pretty close to the same at sea level vs 7k feet. Is that correct? Just trying to reconcile guidance to not drive over passes after diving, yet it’s ok to dive at altitude if you just ascend extra slow to the top