Deco dive plan sheet

[Rick Murchison]

...News flash, its not relevant on paper or in the water.

An interesting concept, precision... or, better, relevant precision... Take depth, for example. Just how deep am I if I'm 100' deep? Depending on body position, I can occupy nearly six feet of "depth" all at the same time, so any precision beyond about the nearest meter or yard is hardly worth messing with. That's why I liken much of our dive planning, especially when dealing with decompression algorithms, to measuring with a micrometer, marking with chalk and cutting with an ax.
On the other hand, changes of just a foot or two are physiologically significant in bubble dynamics; when the deco model assumes a steady stop those changes can have profound consequences in the model's efficacy, so depth control may need to be more precise than depth measurement... but... I can't control my depth precisely unless I have a way to measure it precisely - more precisely than is relevant to a discussion of "how deep am I?" alone.
And so it is with PO₂, EAD, END, gas analysis, consumption rate, etc.
But...
Let me use a military metaphor to illustrate my next point.
The ballistic dispersion of a dumb bomb is about four mils. That is, if you drop a whole bunch of bombs from precisely the same point and under precisely the same conditions, their trajectories won't be precisely the same, but will vary (disperse) as much as four feet per thousand feet of travel. So, if a dive bomber drops a bomb at a slant range of 5000 feet from the target, no matter how precisely the bomb is aimed, the bomb can only be counted on to come within about 20 feet of the target, which makes the aim point "imprecise" or only "relevant" within that 40' diameter circle. However... if my objective is to actually hit the target, and avoid having to come back and get shot at tomorrow, my chances of success are best with very precise aim - far more precise than the ballistic dispersion I know is a physical property of the weapon's trajectory.
ergo...
making calculations a bit more precise than is relevant may not matter much in a given instance, but it does help define the bullseye.
In other words, calculating (and measuring) my FO₂ to the nearest hundredth of a percent in a +/- 1% world may not be relevant, but it's harmless so long as I bear in mind my calculation is just a center point on a curve of probability that stretches a whole percentage point either side of my calculation.
Now, is it reasonable to ask for a two decimal answer for FO₂(%.xx) in a class? Not in my class, at least, though if someone does it I'll usually say something along the lines of "that's fine, just remember that it's far finer than is really meaningful..."
I like to use terms like "about 21%" to emphasize the range of reality.
Rick

 

[TSandM]

I've been reading a lot of this discussion with a fair amount of incredulity, Trace.

There is simply nothing in diving that is known to three significant figures; there is probably little or nothing that is known to two. Criticizing people for answering 80 versus 81 feet is simply being picky. I get your point that the 80 foot people would use the tables differently from the 81 foot people, but that's just an example of how ridiculous tables ARE -- and we both know that, were the two groups to dive together, there would be a lot more things than which table box that would have to be ironed out before a significant dive would come off seamlessly.

Me? I'll stick with "battlefield math" I can do in my head, to a degree of accuracy and precision that is all that is required for decompression, which is far from an exact science. I may come up with something different from your table folks, but people using DecoPlanner and VPlanner are going to come up with different things, too -- the fact is, you just have to make sure the team is on the same page for any deco dive.

 

[Blackwood]

.... column 3 is Max Depth in ft, shouldn't column 4 be Max Depth in meters? Last time I checked ATA is not a measure of distance like ft or m :depressed:

Atmospheres Absolute is often considered a depth dimension although it has units of pressure.

 

[netmage]

Glad you went with PSIA.

Who...?

 

[Trace Malinowski]

I've been reading a lot of this discussion with a fair amount of incredulity, Trace.

There is simply nothing in diving that is known to three significant figures; there is probably little or nothing that is known to two. Criticizing people for answering 80 versus 81 feet is simply being picky. I get your point that the 80 foot people would use the tables differently from the 81 foot people, but that's just an example of how ridiculous tables ARE -- and we both know that, were the two groups to dive together, there would be a lot more things than which table box that would have to be ironed out before a significant dive would come off seamlessly.

Me? I'll stick with "battlefield math" I can do in my head, to a degree of accuracy and precision that is all that is required for decompression, which is far from an exact science. I may come up with something different from your table folks, but people using DecoPlanner and VPlanner are going to come up with different things, too -- the fact is, you just have to make sure the team is on the same page for any deco dive.

Lynne,

No table, nor any decompression software, is ridiculous. They are tools that attempt to define a tighter bullseye of safety as Rick mentioned in his great post. Tables and software are periodically updated. The US Navy just published Version 6 of their tables and many agencies that teach these tables as a basis for understanding are updating their versions as well. My first version of V-Planner is different than my current version. The GUE ratio deco I learned in 2001 was different in 2008 and that is different today.

All of these are attempts to improve the margin of safety.

I can tell you that you can ascend from a 60 foot dive after 60 minutes of bottom time in just 1 minute safely. Would you do that? Or, does that seem too aggressive since you began diving when safety stops and a 30 foot per minute ascent rate were already advocated. I've done 60 minute ascent rates and max bottom times. Many others have too. But, would I do that today? No, because the safety guidelines have changed. I'm sure most every diver here certified before 1980 is no longer ascending at 60 feet per minute and most are also using dive computers rather than tables. What you can do and get away with in reality and what you should do are different things. And, yes, because decompression is such an inexact science, you can get away with a lot and get away with a lot of averaging.

But, that is not the point. This thread is about dive planning sheets. General Dwight D. Eisenhower once remarked that, "Plans are useless, but planning is indispensable." There are advantages to using dive planning sheets during the educational process. As I said, in my experience as a student and as an instructor, I found these to be helpful. Chris Wright and Terrence Tysall both employed such sheets in cave instruction. Terrence's sheets were great and students really got into the logic behind deco planning software by calculating everything by hand. However, deco planning sheets are time-consuming. I got away from them in favor of teaching how to use software and ratio deco simply because few divers will use sheets after class.

But, then I ran into a problem. That is that I get really great students in class such as those trained by GUE, who have fairly decent in-water skills, but who then nearly fail the written examinations for the courses they take when it comes to understanding how to properly plan dives according to current industry safety guidelines for dive table use. It doesn't matter if I'm teaching a PDIC class, a TDI class or a PSAI class. Each of these agencies uses US Navy tables as well as others such as DCIEM and Buhlmann as part of the learning and testing process. I don't know a single tech instructor for these agencies, nor for others such as NACD or NSS-CDS, who plans dives using tables alone. Most use software, computers, or even on the fly planning. But, to test the student's understanding of the concepts of conservative dive planning, the basic use of dive tables, and as a contingency, dive tables serve as useful tools.

Instructors evaluate diving skills in the water and academic understanding in the classroom. Knowing how to safely use a liftbag is just as important as knowing how to safely use a table. Yes, you can deploy a liftbag without looking up and you can count an 81 foot dive as an 80 foot dive, but you are taught to look up when launching a bag and you are taught to calculate formulas with certain rules for rounding up or down and what to do if your depth or time exceeds a tabled depth or time. You probably won't get hurt if you don't look up when shooting a DSMB and you probably won't get hurt considering an 81 foot dive an 80 foot dive.

But, in class you will be corrected if you fail to look up and you will be corrected if you fail to advance to the next greater depth and time when using tables. Students pay hard earned money to instructors for the information on how to perform correctly and safely. I didn't invent the tables nor the procedures for their use. Instructors teach students to round up and use next greater depths and times in the interest of safety.

I just spoke to two instructor friends from two agencies while discussing business and asked them that question. Both chose 81 feet as their response. Both would have rounded up to 82 feet using calculators. The point of the question tests the student's knowledge of the guideline to advance to the next greater depth and time.

The guideline is just a safety step much like a safety stop. Divers should know they are both recommended. Whether to do either of them or not is up to the individual.

 

[Randy43068]

JAX, your thread took on a life of it's own

 

[TSandM]

Well, Trace, if your students are routinely failing the written exam, then are you making it clear to them during the class, how that class expects them to answer the question? I mean, if I took a class that used tables, I wouldn't expect to give RD answers on the written exam for that class.

And, given that depth gauges will often differ by several feet among a team, I do think that rounding 81 feet up to 90 -- and insisting that's important -- verges upon ridiculous. Yes, it's what the tables tell you you have to do; I doubt that anybody did validation dives to prove that you have to do it to remain within the probabilities upon which the tables are based. Somewhere in the interval, it becomes important, but I doubt one foot is where that line is drawn.

 

[netmage]

I really don't know who's worse.... Brevity people... brevity...

 

[Blackwood]

Well, Trace, if your students are routinely failing the written exam, then are you making it clear to them during the class, how that class expects them to answer the question? I mean, if I took a class that used tables, I wouldn't expect to give RD answers on the written exam for that class.

And, given that depth gauges will often differ by several feet among a team, I do think that rounding 81 feet up to 90 -- and insisting that's important -- verges upon ridiculous. Yes, it's what the tables tell you you have to do; I doubt that anybody did validation dives to prove that you have to do it to remain within the probabilities upon which the tables are based. Somewhere in the interval, it becomes important, but I doubt one foot is where that line is drawn.

Would I use the 90 foot table for an 81 foot EAD? No. But if I were an instructor teaching tables, I wouldn't be very keen on teaching practices which violate table/agency instructions.

Trace is discussing planning, not execution. The precision of dive gauges isn't relevant. Right?

Now, if you want to get MFR data about the accuracy range of your gauges and then during the dive deco off whichever level is closest to your depth envelope, that's great. Not sure I'd teach it though.

 

[PfcAJ]

Come on guys...