ABQdiver once bubbled...
This has always been a source of interest to me. Should the dive be calculated on the deepest dive depth or the average depth? If your using the tables its the deepest point, plus any altitude adjustments. What if a computer is used and the "average" depth is known?
First off- we've got to get together and buddy before the DIR-F class. I'm sure you'd like a little brush up as much as me. Looks like it got changed to April which is better for me. And what was that other post about no Quattros in that class? Oh, well!
Back on topic of altitude:
I think you metioned in another post that you use a Cobra. I just got one, and have a Stinger that I have about 35 dives on. Although they're good for essentially taking a time/depth logging and running a "continuos" calculation against the deco algorithm, I can't imagine they have any way of "knowing" that you are essentially continuing to ascend above what it registered as the starting point of zero at the dive site surface. It's someting they could probbably do, but such a limited necessity I can' timagine an economic justification for it. It also has no way of knowing how much fiurther you're going to ascend.
Ironically, Bert says he has had a situation where as he "descended" from Blue Hole to somewhere in Texas right after diving, and his computer started another dive and tracked it, apparently based on the increase in ambient atmospheric pressure. Thought that was strange, but I have no reason to doubt his claim.
Someone with more backgound with SSI (or anybody else) jump in here if this doesn't sound right.
I think you indicated you were certified with SSI. Not familiar with their tables, and how they adjust for muti-level dives. PADI has what they call the wheel, and gives credt for decreasing depth over time, and lightens up the pressure group you end up in. With a computer, I don't think there is any way to know what your final group is because it's based on a "model" and not a specific table, so there's no way to make a determination of an SI to get to a safe group before ascending.
Absent a multi-level type table, I was taught in both the Y and PADI courses that you have to use max depth, with total time from the time you leave the surface until you start your final ascent to the surface. "Average has no bearing in determining you group at end of dive.
When we did our OW the instructors taught that a 4% increase in depth should be added to the actual depth dived for every 1000' of altitude and should be worked into the table calculations. So for 5000' the addition would be an additional 20% to the dive depth.
I have an altitude table, and that adjustment looks right for the spot checks I did against it.
According to the Bruce Weinke book, it's a fairly linear change up to 10,000 ft, but then it gets exponential (it's actually exponential below that, but apparently close enough to be linear for simplicity) Still working through all that stuff. I'm trying to wade through the formulas and undersand where all the variables are coming from. The book is very good from a technical standpoint, but has very few practically applied problems. At some point, the light's going to come on, and it'll all make sense, but right now it's still a little foggy. The text states that it is a relativley simple and sttraighforward progression to determine, but doesn't give an example.
We're doing a "class" session in preparation for our ice trip. I'll see if I can get some clarification on it. We went over it, but it apparently didn't stick too well. I'd feel a lot more comfortable traveling if I understood it a little better.
I mentioned in a previous post that I try to dive a "conservative" profile. Someone jump right in here an contradict me if I have an erroneous interpretation of that:
I try to dive the deepest part first, with time at max depth at about 50%-60% of that depth's NDL. Maybe a little more time if it's something interesting, but rarely past 75%. Ascend to about 2/3 that depth for about another 50% of remaining gas (not including end of dive reserve), hit about 1/3 of max depth for balance of gas, safety stop (almost always unless buddy is having issue and needs to get out) and out. Try to do something along these lines all the time. Always try to avoid increased depth (barring something that I just have to see) once I've started into that ascent transition. Varies a little based on conditions, but if so, almost always more conservatively. I'm getting older with a family, and want to come home from every dive, and really not up for trying out my DAN insurance policy.
