Posted by MHK on September 25, 2001 at 12:04:04:
In Reply to: Re: Dive Computers-Really Long posted by Jon D on September 25, 2001 at 11:21:41:
I put out a post a few months back that discusses some of the issues that you raised so add this to Kendall's excellent analysis and I hope that clears up any confusion..
The *hot topic* of the moment when I wrote this post was about *computing on the fly*, but it serves well to address the overall issue.
Hope it helps.
I have been reluctant to put out a post that *instructs* people how to compute on the fly. I'll disclaim up front and then I'll add my thoughts and you are free to do with them as you see fit.
1) I am not an instructor and this post is NOT intended to replace quality training in this regard. I recommend John Walker for training in this regard.
2) Decompression theory is a very inexact science and it is therefore suggested that while you are learning this process that you continue to use a computer as a back up, but that you begin the transition from blind reliance on the device to an overall understanding of your profiles, surroundings and projected M-values.
3) We believe that dive computers discourage proper planning and preclude divers from studying/understanding the impact of various gases.
Accordingly, divers have three primary methods by which to calculate NDL limits for a dive; namely tables, wrist (console) mounted computers and personal computer generated programs. Using tables and/or computer generated diving profiles mandate that a diver use pre-planning as opposed to a diver that uses blind reliance on an underwater computer that allows the diver to blindly follow the computer. The obvious problem with that is that if your computer fails the diver is left without necessary information. Whereas a diver that has been tracking and/or planning his dives will have the required information to continue his/her diving day, weekend or live aboard. The advocates of in water computer use point out that by using a computer they will achieve added bottom times.
By in large that seems to be the biggest obstacle in addition to the issue that has been raised about * how to do it*.
So let me take each of these items and discuss them independently.
1) Does computer's provide for added BT's????
We were presented with the *challenge* (for lack of a better word) to discuss saw tooth profiles. We were, and still are prepared to do that, but I was surprised yesterday when Seahunt presented us with what he calls a typical dive day. I didn't save the exact profiles he generated and I invite him to post them here because I think it speaks to a VERY common misconception that was clear to me, and other's, that there was a significant lack of understanding of basic decompression theory. I don't want to single out Seahunt so my comments will be presented as broad in nature. The profiles presented were in the order of:
29 minutes 85' SIT for x number of minutes
25 minutes 80' SIT for x number of minutes
25 minutes 80' SIT for x number of minutes
28 minutes 75'
Seahunt can be more specific but it may not be necessary.
We were then asked how would we compute these dives, which is VERY VERY different from how would we complete these dives. It's important to note that we would approach these dives from a different perspective and may not in fact be in this position in the first place which is infinitely more important, and speaks more so to the mindset of DIR, than actually being in a position to *calculate* these profiles. We encourage problem solving before the problem occurs rather than getting into questionable situations and then hoping we are skilled or practiced enough to get out of them.
For sure these dives place any diver, diving under just about any table on the edge of the Buhlman generated M-values. I was under the impression from Seahunt that his concerns revolved around the fact that he engages in *saw tooth* profiles meaning that he bounces up and down and is busy lobster hunting and than therefore doesn't concern himself with his surroundings (i.e.; depth and time) but relies upon his computer to tell him when he reaches the M-value. That's problematic for a variety of reasons because given the blind reliance should the computer fail, lock up or provide inaccurate information the diver would have no idea all he/she would know is that the computer says it's OK to continue my dive or it's time to get to 15'.
When Seahunt and I attempted to dissect this profile both JJ and I discussed the fact that we would average our depths inasmuch as it is a saw tooth profile. Seahunt then countered that that isn't the case, even though we spent all that wasted bandwithith the last couple of weeks discussing saw tooth profiles. That notwithstanding, we addressed the issue raised which was Seahunt saying that his profile meant that if he said 85' max depth that he stayed at 85' the entire dive. We suggested that was impossible and that every dive has some up and down and all the computer does is calculate the average. At Seahunt's insistence we addressed the issue of a his suggested profile being calculated at max depth for the entire dive, which actually makes it easier and then that's where I believe a fundamental lack of understanding, and need for additional training became clear.
What we said in response to this claim is that if you are at 85' for the entire dive than the computer and the tables will be the same and there isn't a need for calculations. The M-value via a table and the M-value via a computer would be the same because they are based on the same algorithm. The only exception would be if you modified the computer algorithm, which I'll address below. That point didn't seem to be understood. It's sorta like saying that if you divide 286 by 32 and if you did it by long division or by a calculator you'll get different answers. The answer is 8.9375 no matter if you used an adding machine or did it by hand. And we were unable to get that simple point understood.
The perceived value, as I understand computers, is that if you use a table you'll need to plan your dive ahead of time and if you look at the table for a 90' dive it will tell you that you can spend 24 minutes at depth (using air tables for simplification). But the argument is made that you may not spend the entire time at depth and as such you would be *penalized* with shorter BT's because you would be without a way to recalculate your dive underwater. A computer was invented to alleviate this *problem* and if you did a multi-level dive the computer would acknowledge the depth variation and not require you to surface at 24 minutes. That's sound in principle. So I was puzzled that after 2800 dives Seahunt did not understand that principle and of course made it much more difficult to discuss multi-level, multi-day dives without a understanding of the basics.
To me, that demonstrated much I what we are trying to change, which is Seahunt has suggested that he is the typical California diver and that he does up to 7 dives a day, is a avid bug hunter and bounces up and down roaming the reef, and after 2800 hundred dives this principle still is new. If that doesn't answer the blind reliance on computer question I'm not sure what will...
Decompression theory is by definition an inexact science and what strikes me as strange is that many diver's will blindly follow a computer generated algorithm when they have absolutely no idea as to what it is calculating. What gradient factor? What conservation level?? I'm in these *discussions* all the time and what I hear often when I say you can stay at 90" for 35 minutes is * Well the PADI tables say 24, so if I follow you I'll get bent* (or words to that effect) But then if you speak to someone else they'll quote the NAUI tables or the Navy tables all of which provide for a different allowable time at the same depth. Further most computers will give you different times for the same dive. Seahunt told me yesterday that he uses 2 computers and they often compute different NDL limits for the same dive. That is common. I say this because knowing this list the way I know it I want to pre-empt what I can easily see as us having to provide algorithms that have been signed off by the Navy, Dan or whatever other potential hurdle that can be raised. It's important to note that the agencies, the navies don't agree and there are so many other factors to consider (i.e.; hydration, cold -v- warm water, physical fitness level, anxiety, PFO's, obesity). If you start paying attention to your profiles, while using the computer as back up in the beginning, you'll understand your limits better than a computer. Your computer doesn't know how you feel after a dive.
So * How is it done* ???????
So how we do is to first off gain a good understanding of the *planned* NDL limits. So we know going in, for example, the tables recommend 19 minutes at 100'. But everyone should know that if I spend 10 minutes at 100' and then continue my dive to 60' that I can spend added time because I'm not at 100'. If I spend the entire time at 100' the computer is of no advantage. Accordingly, you have an average depth of 80' in this example. The NDL limit is 30 minutes at 80'. So in the beginning I would use a 30-minute profile. I would of course be conservative in the beginning and pay attention to how I feel after the dive. I would incorporate deep stops, which my computer would penalize me for. I have said often that computers tend to bend you deep and then clean you up shallow, whereas deep stops help you, not hurt you. It allows the fast tissue groups to clean up, it allows the bubbles, such as they are, to shrink. If you ever do Doppler testing and compare a diver incorporating deep stops and a diver not doing deep stops, all other factors being equal you'll see a lower bubble count with a diver doing deep stops.
This is why we say DIR is much more than a standardized gear configuration. It's an approach to a dive.
We would not approach a dive on air, planning a saw tooth profile. We would use the right gas for the dive, to the extent available, or we would modify the dive. In all fairness to Seahunt he and I were on the same page here. We would not rush to 15' and then spend added time there, as your computer would have you do.
Most diver's, not all, but most diver's have a plan going in (you remember plan your dive and dive your plan) so this concept isn't as foreign as some would have you believe.
The way a computer adjusts it *conservation* factor is one of two primary ways;
1) It adjusts the depth in its algorithm that you are at. In other words, it runs its calculations for you at 100' by using a depth of 110', so by definition it provides for shorter NDL's. Conversely, it would calculate your depth at 90' if you wanted to adjust to a * aggressive* computer. Most divers have no clue what their computer does...
2) The other thing it does is add/subtract time in its algorithm. In other words if you are at 18 minutes, it may use 16 or 20 minutes when it performs it's calculations to add or subtract from available BT.
These *fudge* factors are commonplace in every computer on the market so all we do is get to know the parameterís of our diving, begin by following and using the computer as a learning tool, and then after significant and sufficient time, you'll be in tune with your computer. Furthermore, as I showed everyone yesterday, if you get totally confused or just feel uncomfortable, I always keep a set of tables in my wetnotes and I'm free to refer to them if needed.
I'm sure they'll be an onslaught of questions so let me anticipate a few of them.
1) This doesn't happen overnight;
2) We encourage training to gain the requisite understanding of decompression theory;
3) We are always available to do a few dives with and we'll discuss/ demonstrate how it works;
4) I'll turn off the killfile for this thread and will respond to any serious questions about this issue, but the second it starts getting into flames I'll killfile. I'm not willing to have this important issue denigrate into a flame war.
5) If you are uncomfortable asking questions about this issue on the bbs, you can e-mail me at email@example.com and I promise to respond to all serious inquiries.
And my final thought is that if there really is a problem with running out of available BT before you run out of air then we, as a dive community, need to continue to push for Nitrox/triox, trimix. I have been reluctant to put out a post that *instructs* people how to compute on the fly. I'll disclaim up front and then I'll add my thoughts and you are free to do with them as you see fit.
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