Ascension after diving.

[Diver0001]

raybo once bubbled...


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I completely agree with the assertion that tables of different organizations shouldn't be used, but I'm curious as to why you consider (at least that is what the implication appears to me) that the Nay table swould be unsafe to use in this situation. I'll have to confirm my understanding, but I thought the Y tables were exactly the Navy tables except for a couple slight NDL & P-group changes in the +100 ft.

Another question for Walter?

I wouldn't say that the Navy tables are necessarily unsafe but the NDL's leave less margin, as you noticed. That's what I was getting at. There are also differences in calculating surface intervals which has an impact on repetative dives. In fact the Navy tables will force longer surface intervals than the PADI tables but then give you slightly less margin on your NDL's. It's just something to keep in mind. Personally I would feel comfortable using the Navy tables for planning (I did it for years - I still have the NDL's memorized) but I wouldn't push the limits.

What I can't see working is using the U.S. Navy's altitude table in conjunction with dive planners other than the Navy tables (or even worse, with a computer). I'm actually surprised at the number of people in the forums who refer to this table without making this footnote. The reason is that your group designatoin after your last dive (the one you should have gotten from the U.S. Navy dive planner) is the starting point on that table. If you don't know your group then I suppose you could use group Z to read this table but you'd be stuck with perhaps exceedingly long surface intervals.

R..

 

[raybo]

I'm aware that there are several models used with various different planners. We've been working through some of this on another thread. From what it looks like to me (I think you mention it above) Navy uses the 120 compartment, but there's a conversion method to be more conservative. don't have the book with me right now, but it's in there.

Just going to have to study it and get the formulas figured out. It's not that hard, I was just hopingit would be in a table form. But as active as I intend to be, I'm thinking it would be best (and safest) if I fully understood the physics & physiology so I can adjust the conservatism based on the individual cases.

Thanks

 

[GreenDiverDown]

Diver0001 once bubbled...
What I can't see working is using the U.S. Navy's altitude table in conjunction with dive planners other than the Navy tables (or even worse, with a computer). I'm actually surprised at the number of people in the forums who refer to this table without making this footnote.

Most excellent point! No mixing of tables allowed. Thanks for bringing that up. Most people would probably remember that dive groups from different tables are not interchangable but may not think about it when using a table like NOAA's "Required Surface Interval Before Ascent To Altitude After Diving" table.

SA

 

[roakey]

There’s altitude discussion going on in two places, so I apologize for duplicating this post.

First off I want to thank Stephen for the pointer to the NOAA ascent after diving table:

http://www.ndc.noaa.gov/pdfs/AscentToAltitudeTable.pdf

I had never seen that table before and it’s interesting, it probably represents what we’ve been doing for some time here in Colorado, though there’s nothing scientific about what we do except that qualitatively we have a couple thousand guinea pigs a year doing dives and then driving over mountain passes. Many guinea pigs are brand new OW students that are completely unaware that they’re guinea pigs; the instructors lay out the day such that they’re OK, well, in most cases.

One of the more popular OW checkout sites around here is Blue Hole in Santa Rosa, New Mexico. BH is about 4k feet, but to get there you pass over Raton Pass which is 9k feet (I’m rounding), so a 5k feet gain in altitude. The vast majority of shops do the checkout dives Saturday and Sunday and leave to head home on Sunday.

BH’s maximum depth is about 80’, which is a TOD of 110 as previously mentioned. The OW students don’t get to that depth (or let me say aren’t SUPPSED to go to that depth) but already certified divers are down there mucking around the bottom (sometimes literally) quite often.

Shops setup late checkout from the hotels for the students, and the students are encouraged to go back to the hotel, shower (another discussion), take their time and then head out as a group, usually stopping in Las Vegas (New Mexico) for lunch (Pizza Hut, yuck – there’s a fantastic Mexican food place there that they ignore). Theory being to kill time before heading over Raton pass, about three hours driving from Santa Rosa. What very few instructors realize is that there’s about a 3k gain in altitude from Santa Rosa to Las Vegas, which is only an hour away from Santa Rosa. Nevertheless, this seems to work fairly well, but it’s not perfect. Memorial Hospital in Colorado Springs sees about six DCS cases a year from divers returning from the ‘hole. Taking a WAG double or triple that to take into account the number of DCS cases that are treated in Denver. My theory is that these divers actually get bent on the drive between Santa Rosa and Las Vegas, and Raton Pass really does them in – Raton Pass being three hours away is pretty much a non-issue otherwise, despite the NOAA tables’ times (my guess).

So how does all this qualitative stuff translate to how we do out dives in Colorado, where sometimes we head over 11.2k passes on the way home? Answer: We guess a lot.

We dive the Navy tables and upon arrival do the 2 pressure groups for every 1k feet of ascension thing, as you’d learn in altitude class. If we’re going to have to leave fairly quickly and/or the pass is close, we’ll compute the dive at the altitude of the pass. This isn’t without error, since we’re not out gassing at the rate we’d be at that altitude, but as I said we do a lot of guessing. We also dive rich Nitrox mixes. At sea level I’d probably standardize on 32% Nitrox, but up here we use mostly 36%. Our lakes aren’t very deep, so we don’t have a lot of depth exposure (ignoring Blue Mesa, which is over 300’ at the dam face ) and they’re cold enough that we don’t stay very long. Those two factors work for us in limiting out exposure. After the dive we kill as much time as possible, usually stopping for something to eat before heading over the pass. I’d guess that we always put at least two hours between the dive and the start of the pass’ ascent. So far so good and no DCS from any of the die-hards I know of that are doing these dives and passes.

You’re not going to get a definitive answer in this area. Just look at the flying after diving rules that are STILL being figured out even though thousands of divers fly every day. A couple thousand divers a year driving over passes after diving isn’t even going to show up on DAN’s radar. Sorry I can’t give you a nice, encapsulated “this is what you need to do” answer. Just be conservative and kill lots of time between the dive and the drive. We’ve even thought about breathing O2 after the dive for some surface decompression, but so far it hasn’t been worth the bother.

Also be aware that at one end of the continuum, GUE believes that the flying after diving (and therefore driving after diving) restrictions are complete BS. They have some persuasive arguments as well as data, but they’re all flatlanders, so that’s beyond my comfort level.

Interesting aside – a couple of the lakes we dive are so high that in theory we could go on pure O2 at 30’ – it computes out to a 1.58 ATA of O2 at 30’ at Turquoise Lake, if memory serves.

Roak

Ps. There’s a truck weigh station at the very top of Raton Pass. Outside the office there’s a pay phone and on that pay phone there’s a DAN sticker – I put it there .