Flying After Diving --- A Theoretical Question

[Kstnbike]

--This scenario is purely theoretical and is not about to be done.--


As many people know commercial aircraft are pressurized to up to 8000ft. Furthermore, many smaller prop-driven aircraft can be flown safely at altitudes above 1000ft. (of any obstacles, mountains, buildings, etc.) Having this knowledge would it be possible to dive based on altitude tables (say 6000ft) then immediately (or after a short 30 min/1hr surface interval) board a plane with a planned altitude of 6000ft or lower. I have been contemplating this question recently and to me it seems entirely possible unless I am forgetting to include some sort of gas release that would occur when ascending to altitute. Even if there was a gas release while ascending to altitute, if you followed the tables on a dive to 6000ft. ascending would be better than just getting out of the water at a 6000ft. I know that in order to get to Blue Hole in Belize some divers do use a float plane to shorten the 4 hour boat trip but I have a feeling that their altitude is kept under 1000-2000ft.

Although I know this isn't a good idea, if this were possible, wouldn't it be nice to dive your last day on a vacation and just dive very conservatively and follow altitude dive tables at 8000ft. (for commerical airliner pressurized cabins).

Just an Idea,
Kstnbike

 

[in_cavediver]

--This scenario is purely theoretical and is not about to be done.--


As many people know commercial aircraft are pressurized to up to 8000ft. Furthermore, many smaller prop-driven aircraft can be flown safely at altitudes above 1000ft. (of any obstacles, mountains, buildings, etc.) Having this knowledge would it be possible to dive based on altitude tables (say 6000ft) then immediately (or after a short 30 min/1hr surface interval) board a plane with a planned altitude of 6000ft or lower. I have been contemplating this question recently and to me it seems entirely possible unless I am forgetting to include some sort of gas release that would occur when ascending to altitute. Even if there was a gas release while ascending to altitute, if you followed the tables on a dive to 6000ft. ascending would be better than just getting out of the water at a 6000ft. I know that in order to get to Blue Hole in Belize some divers do use a float plane to shorten the 4 hour boat trip but I have a feeling that their altitude is kept under 1000-2000ft.

Although I know this isn't a good idea, if this were possible, wouldn't it be nice to dive your last day on a vacation and just dive very conservatively and follow altitude dive tables at 8000ft. (for commerical airliner pressurized cabins).

Just an Idea,
Kstnbike

If we look at this in a purely tissue model, what we have is two M0 values. The M0 for the actual altitude and the M0 for the 6000ft altitude. If you planned your dive such that you could surface at 6000ft, then via the tissue model, you have violated no rule regarding the time/fly rules. Its just like if you have been diving at 8000-9000 ft. the Time to Fly is 0 minutes because you will not be ascending during the plane ride.

If you look at this based on bubble mechanics - I'll let someone who understands this a bit better than I explain it.

 

[Charlie99]

The only hitch in planning your dives for the week as if you are at 8,000 foot altitude is that you NEVER SURFACE.

To accurately simulate diving at 8,000', you have to treat all of your surface interval time (even when sleeping overnight in your hotel) as if you are doing a very long extended safety stop at 6' or so.

You can indeed plot out a who series of dives (actually one week long dive) like this in many decompression programs, such as VPM.

================

Another way to say the same as above is to point out that the loading in your slow tissues will gtreater than someone who has been spending diving while at 8,000'. The very slow tissues/compartments are the limiting factors for fly after dive, and reflect the average ppN2 of your entire stay. The ppN2 at sea level is higher than that at 8,000' altitude.


Charlie Allen

 

[omar]

As Charlie pointed out your ppN2 in the “slow” tissues will be at or near the altitude you are actually diving at. The simulation of an extended stay at depth will work. However for an 8,000 ft simulation the depth you need to use would be 8.7 ft.

OR

To simulate a surface altitude of 8,000 ft you need to reduce the ambient pressure that you are at or reduce the partial pressure of nitrogen. How to do this? You get into a chamber and reduce the pressure, use a reduced pressure sleeping system (like this: http://www.hypoxico.com/home_sleep.htm) or breath elevated levels of oxygen while at the surface. You would need to breath pure O2 for an extended time to gain any real benefit. The best practice is to do a proper dive profile with decompression stops and then fly out after a 6hr or longer SI.

 

[Far_X]

I may not be following this entirely but the difference between following tables at 6,000ft and what you are actually diving is that you are not diving at 6,000 feet. You have the gas in your body that was absorbed at pressures 6,000 feet lower.

If a cabin is pressurised to 8,000ft, does that actually mean once you fly higher than 8,000 feet, the pressure of the air inside the cabin decreases? If that is the case, then in theory, if you made it to the surface, and as long as the aeroplane stays lower than 8,000 feet, it shouldn't make any difference as to which tables you followed because you will be okay. I seem to remember that island hopping is permissible following diving because you don't ascend that high.

 

[Kstnbike]

The only hitch in planning your dives for the week as if you are at 8,000 foot altitude is that you NEVER SURFACE.

To accurately simulate diving at 8,000', you have to treat all of your surface interval time (even when sleeping overnight in your hotel) as if you are doing a very long extended safety stop at 6' or so.

You can indeed plot out a who series of dives (actually one week long dive) like this in many decompression programs, such as VPM.

================

Another way to say the same as above is to point out that the loading in your slow tissues will gtreater than someone who has been spending diving while at 8,000'. The very slow tissues/compartments are the limiting factors for fly after dive, and reflect the average ppN2 of your entire stay. The ppN2 at sea level is higher than that at 8,000' altitude.


Charlie Allen

I can understand that the ppN2 at sea level is much higher than at 8000ft., but if I plan my dive based on altitude tables and then ascend to that altitude after the dive why does that matter. The pressure in the water is going to be the same partial pressures whether you are at sea level or at altitude it's just the air consistency that changes. I know that from driving to sea level to 6000ft (over 30 minutes or so) puts you around an N diver (gotta double check that) so if you calculated the extra nitrogen that would be in your system and planned accordingly you would be fine right? I'm sitting here w/ my DM book opened right now looking at the Haldaen compartment models. Would I really max out the slow compartments (fatty tissue) once I gained altitude? I understand what was meant by the safety stop needing to be at 8.7ft. in order to simulate the 8000ft. altitude.


I am gonna "plan" a dive like this using the RDP and Altitude RDP below just to illustrate what I'm saying. This is going to be even more conservative then what would actually happen (imo) because I am using the RDP and not my computer.

After dive flying altitude 4000ft. (more than reasonable altitude if you were flying just off the coast or over high rise buildings or small coastal ranges considering you aren't breaking any FAA regulations).

Dive 40ft @ 4000ft. adjusted to 46 (50) feet actual. for 39 minutes
Ending pressure group immediately after the dive "L"
Small surface interval of 30 minutes getting yourself and gear undressed and to the aircraft.
Pressure Group after SI. "G"
Add 2 pressure groups per thousand feet while ascending to altitude
4000ft. "O" pressure group.

I know this is just a basic table related calculation and it doesn't take full models into consideration but based on this the dive could be done.

Kstnbike

 

[Charlie99]

To simulate a surface altitude of 8,000 ft you need to reduce the ambient pressure that you are at or reduce the partial pressure of nitrogen. How to do this? You get into a chamber and reduce the pressure, use a reduced pressure sleeping system (like this: http://www.hypoxico.com/home_sleep.htm).

Actually, if you look closer at the "high altitude" training systems like this, they don't change the pressure. What they really are is a membrane O2 generator, but instead of feeding you the O2 enriched air, you get fed the air from which some of the O2 has been removed. The lower FO2 is what simulates the altitude.

So if enriched oxygen air is called nitrox, what's the name for the partially de-oxygenated air?

 

[Charlie99]

. The simulation of an extended stay at depth will work. However for an 8,000 ft simulation the depth you need to use would be 8.7 ft.

Agreed. 8,000' altitude is about 0.75ata, so the "missing" 0.25ata is 8.x fsw rather than the 6' of my earlier post.

I know that from driving to sea level to 6000ft (over 30 minutes or so) puts you around an N diver (gotta double check that) so if you calculated the extra nitrogen that would be in your system and planned accordingly you would be fine right? ......... I understand what was meant by the safety stop needing to be at 8.7ft. in order to simulate the 8000ft. altitude.

Just to make sure that you really do understand where the 8.7' comes from here's the logic ........

Let's say I do a series of repetitive dives to 40' for 1 hour, then up to the surface for 1 hour. Working that out on a dive table is pretty straightforward for sea level. OK, now lets assume that you have acclimatized at 8,000' and do the same series of dives. You just adjust for equivalent sea level depth. Nothing else.

OK, now lets look at doing the same series of dives at sea level and pretending that we are at altitude. The big difference is that unlike that series of dives at 8,000' altitude, during your 1 hour SI you are NOT at 0.75ata. You are at 1.0ata. At 8,000' you only reach 1.0ata at somewhere between 8' and 9' of depth. So to accurately simulate your sea level series of dives on an altitude table, you need to treat the whole series of dives as a multilevel of dive that bounces back and forth --- 1 hour at 40', 1 hour at 8.7', 1 hour at 40', 1 hour at 8.7'. Clearly, this leaves a different loading in your slow compartments.

That's why "Just plan the last day as if you were diving at altitude" isn't a good way to plan.

============

NOAA does have an time-to-ascend table that tells you how long you need to wait to ascend various multiples of 1,000' according to which repetitive dive group (of the USN/NOAA table) you have reached.

 

[MikeFerrara]

If I remember right "Technical diving in depth" has some tables giving surface intervals for ascents to altitude after diving based on the navy tables I think. Either way, if I'm understanding the question, the goal is to keep all the theoretical compartments below their critical tension. That sounds more straight forward to me than pretending you just did a real long dive to 8 ft or whatever. Or am I missing something?

 

[shurite7]

NOAA does have an time-to-ascend table that tells you how long you need to wait to ascend various multiples of 1,000' according to which repetitive dive group (of the USN/NOAA table) you have reached.

Is the NOAA time-to-ascend table accessible online?

I've ran across a similar scenario when I first became an instructor. I made a deal with a skydive instructor, his o/w cert for my class A license. The skydiver wanted to fly down to the coast (we both lived in Dallas) in his personal plane and fly back the same day. He informed me he could fly the plane at whatever altitude needed.

This never happened because I was offered a job in the Cayman Islands and moved a week after we spoke about the barter.

I wonder if anyone on this forum has flown in a small aircraft at low altitude several hours after diving at least 2 dives?

Salaam

Chris