Re: Dive Computers-Really Long

[ Follow Ups ] [ Post Followup ] [ California Scuba Diving BBS ] [ FAQ ]

Posted by Kendall Raine on September 25, 2001 at 10:17:05:

In Reply to: Dive Computers?? posted by BadFish on September 24, 2001 at 21:24:29:

As Marta said, at least two schools of thought here. Some people wouldn't dive without them and some people believe they'll lull you into a false sense of security on top of not being very good predictors of decompression stress to start with. Lots of people in between, too.

You asked short questions, but thorough answers are anything but. I tried to stay under control here and I could have gone on... and on...

Some thoughts:

1. Computers are great at computing. They'll recompute your depth every 30 seconds or less which results in a very precise recordation of your dive profile. This information is then run through an algorithm, typically some varient of the one development by Buhlmann, and tell you how close you are to what it says is your NDL. There are some which use different algorithms and I'll discuss these later.

2. Some computers will carry this even further and compute your decompression stops for you if you exceed that hypothetical NDL.

3. Lastly, some computers will do this for gas mixes other than air and a few will allow you to change gas mixes during the dive thus computing some theoretical benefit from lowering the inert gas portion of the mix.

4. Several points to make here: How good is the algorithm? What do you do if the things stop working? How intellectually lazy do you get over time? What's the cost/benefit of not using a computer?

How good is the algorithm?

This is an issue whether you're using a computer or a table. Without getting into the details too much, the alogorithms break down into two broad types: dissolved gas based models and free phase (or "bubble") based models. The Buhlmann family of models, as all neo Haldanean models, are dissolved gas models that seek to define a time/pressure curve beyond which inert gas will change form from liquid phase to gas (bubble) phase. You'll see people on here refer to "M" values and such. These are statistical coefficients developed theoretically and modified empirically to define pressure ratios on various theoretical compartments serving as proxies for varying types of body tissue. Lots of theory and math used to guess how inert gas behaves in the body. The other family of models is the free phase type which includes the Reduced Gradient Bubble Model (RGBM) and Varying Permeability Model (VPM) with the former being a multi-dive expansion of the latter. These assume that bubble growth is a function of both micro-bubble seed excitation as well as dissolved phase gas kinetics. People who spend their time really thinking about this stuff are focussing on the RGBM/VPM models as a quantum improvement in the sophistication and accuracy/efficacy over the neo-Haldanean models. These free phase models promote the use of slower ascent rates and much deeper stops as a means of "cleaning-up" inert gas early in the ascent. Both models are available in table form, as software for your PC/MAC and in submersable computer form. In both cases, the alogorithms used in submersible gauge form are abbreviated compromises of what their original designers had in mind.

So, are the bubble models better than the neo-Haldanean models? That's unclear. They should be. There is growing evidence to suggest they're more effective than the neo-Haldanean modles, but they're new enough that little research is really available, particularly as applies to sport divers, to say definitively one way or the other.
As far as I know, only the Vyper and the Explorer dive computers use RGBM. As a software/PC based system, Abyss uses RGBM and Decoplanner uses a modified Buhlmann designed to approximate VPM. Both are works in process.

What do you do if the things stop working?

That's the rub. If the computer quits after five dives into a multi-day trip, there is no way to switch back to tables unless you've been depth averaging carefully during the previous five dives. Since the purpose of the computer is to relieve you of the work of doing this, that is unlikely and you're faced with a theoretically long surface interval before you can assume you're "clean" and start in on the tables. You could use a Doppler to check if you're clean, but that itself involves a whole other level of complexity and uncertainty. Bascically, you're screwed unless you wore two computers the whole time.

How intellectually lazy do you get over time?

Based on the deco knowledge of a lot of computer users, very. The computer does lots of math for you. That's different than doing the thinking. Computers compute, they don't think. Without knowing much about the assumptions that go into the various models and the theory, as it evolves, of how inert gas acts in the body, you're relying on a "black box" trying to solve the mysteries of a chaotic system. That, to me, is the biggest negative of using a computer.

What's the cost/benefit of not using a computer?

- Without a computer, pre-dive planning becomes more important.
- You need to have greater precision, others might say restrictions, on the execution of the dive.
- You need to have greater awareness, others would say distraction, on your depth and time.
- Without all the above, your indicated NDL's will be much less over time than with a computer.


- You're compelled to think about what you're going to do and actually doing on the dive.
- You'll probably dive more conservatively, at least early on in your career. Your ascents will be slower and you'll learn to listen to your body because you won't have the crutch of a machine telling you you're OK.
- Over time, you may well end up with more bottom time than with a computer as you gain confidence in your technique and use of PC based advanced models. Many computers use algorithms padded to prevent Joe Bubblebutt from getting bent.
- You'll save money for other pieces of gear.

Others will no doubt jump in here as the whole computer question has historically been a hot button. My advice is evaluate the responses critically and see through the pat answers and polemics. One thing I do, as well, is consider the range and magnitude of experience of the person giving advice.

Sorry to be long winded.

Follow Ups:

Post a Followup




[ Follow Ups ] [ Post Followup ] [ California Scuba Diving BBS ] [ FAQ ]