Ascent to altitude

[aquaregia]

The road to Las Vegas from Los Angeles has a 10% grade that lasts for 6 miles -- that's 3000' in five minutes. I know that's a far cry from 8000' in one or two minutes but yeah. I'm planning to go back to Italy within the month and I'll probably wait a bit longer before leaving, have a less aggressive last dive and pad my last stops a bit longer, just in case.

 

[ajduplessis]

You know, if you think about it -- if you get symptomatic while DRIVING over a pass, you were bent before you went.

Airplanes cause trouble because their ascent rates are very rapid -- sea level to 8000 feet in a couple of minutes. It just isn't like that with a car. You have the surface interval of packing the gear, and then you have to drive. A 6% grade is an extremely steep highway hill, and few roads maintain that kind of climb for very long. Usually, you are talking about time in hours to get to 3000 or 3500 feet (for me, sea level to 3500 is an hour, if there's no traffic ). If you translate that into ascent rate in water, you realize how tiny a rate that is (it's only about 60 fpm in AIR).

Interesting opinion from someone who has no idea of the area, elevation change or the diver in question. A 6% grade incline here is a little bump in the road!!! Did he make a judgment error?? Definitely yes

 

[Hatul]

There are places in CA where you can go from sea level to 6000+ feet in a matter of about an hour and a half, if you drive just barely over the posted speed limits. For those of us that don't mind zooming along, it's not really that difficult to get there even faster. If I dive at Folsom lake, Sacramento, for example, and then drive to Tahoe, I've got an elevation gain of about 6000 feet. I have done that drive in just about an hour.

That's not to say I disagree with you, TSandM, just wanted to make it clear that some places it's much easier to get more drastic altitude changes without much effort. In some places in Europe where the roads are older and less consistent in their gradation I'd be extremely careful while driving after diving close the the limits.

If you're going to making rapid altitude change after diving, one conservative way to deal with this may be to set your computer to altitude diving at the max altitude you'll be driving to. So in the above example set your computer to altitude diving from 6000 feet. So now your dive is in two stages, first to sea level, and then an hour or so later to the final 6000 feet. I think this may work with some computers, that have manual altitude settings.

Adam

 

[raftingtigger]

If you're going to making rapid altitude change after diving, one conservative way to deal with this may be to set your computer to altitude diving at the max altitude you'll be driving to. So in the above example set your computer to altitude diving from 6000 feet. So now your dive is in two stages, first to sea level, and then an hour or so later to the final 6000 feet. I think this may work with some computers, that have manual altitude settings.

Adam

I've thought about that idea. Do my dive using tables for my highest return altitude. Wish I could manually set my computer. Brings up another altitude computer question. When you drive (or fly) up to altitude to do a dive unless you have an adequate SI you are doing a repetitive dive. I've wondered if the computer that came up on the drive with you takes this into consideration. After doing a simple experiment I don't think it does.

I carried my computer, a Veo 100, with me to an altitude of 6000'. I noted the NDL times and then rechecked them over 6 hours later. In this one example there was no change. If the computer was calculating the drive as a 'dive' there should have been some change in the NDLs. I picked 6 hours since according to the PADI RDP my pressure group from a 'dive' from SL to 6000' MSL would have returned to baseline by 6 hours.

My current altitude diving protocol is to start my SI when I first get to my dive altitude, even if I go higher and then back down. Subtract the residual nitrogen time from my computer's NDLs as calculated by the RDP and the "2 pressure groups per 1000' ascent" rule. and use that as my max NDLs. Then write these down on my slate. It's cumbersome, but since I haven't come anywhere close to the NDLs it hasn't been a problem.

Does anyone see a flaw with this thinking? An easier way to do it - without waiting 6 hours to dive? How about the return drive? In my case it is a dive at 6200' (Tahoe) with a drive over either Donner Pass (7000') or Echo Summit (7400').

I could use the table for a dive at the pass altitude but that loses all the benefits of using a computer.

Lastly, does anyone know if there have been any experiments or studies on this - or is it all just educated extrapolation?

 

[scubamiller]

To answer your question about research, it has been done, but the numbers are not neat and easy like you would want them to be for recreational diving. For starters, I work in an Aviation & Dive Medicine training facility, so this is not just my opinions:


A couple of things to remember:

1. Your NDL limits do NOT change due to driving from a lower altitude mimicking a dive; it changes (decreases) due to two reasons; one, that your body has a greater partial pressure of Nitrogen than the atmosphere around it so you are off gassing Nitrogen (this is why you can acclimatize after 6 hours before diving), and two, that once you surface at altitude you are surface-ing into an environment which is less than 1 atmosphere absolute. So the pressure difference is greater between sub surface and surface pressure (~34 ffw still equals 1 atmosphere (gauge)). The NDL times do increase over time (the 6 hours) since while you are underwater you are not at the same pressure (absolute) that you would be at if diving at sea level (you are under less pressure absolute).

2. Driving after diving is generally safe since by the time you do any of this, the most dangerous Nitrogen compartments have already off-gassed much of their excess nitrogen. UNless you are going to the top of pikes peak (14000) after diving at the base of it in Colorado SPrings (5,000) you should be good since it take extra time to doff gear and load vehicles and get where you are going. The 12 hours rule for 8,000 (pressurized aircraft) is similar to this since it takes into account the 6 most dangerous nitrogen compartments. Airplanes are more problematic due to their rapid ascent RATE.

3. Ascent RATE is the most dangerous factor that you can control in any diving; MUCH more dangerous than total/residual Nitrogen TIME. Remember that the Ascent rate while using tables at altitude assumes an ascent rate half that at sea level (30 feet/minute or less maximum) and your 3 min. safety stop that is optional at sea level becomes mandatory during altitude dives.

4. The safest method is to off-gas as much as possible before heading to a significantly higher altitude and be conservative when planning your dives if you know you will be driving to a much higher altitude after diving. The few hundred feet scenario is not significant, and if you experienced signs/symptoms of DCS, then you were likely bent during the dive, not because of the Drive.


***If you remember nothing else from this physics lesson, remember SLOW ASCENT RATE, SLOW ASCENT RATE, SLOW ASCENT RATE***