Dive Computer for Recreational Use

[Matthew]

It's not so much the "use" that is prohibited, as it is the "blind trust in a piece of technology". I often use the following simple analogy to illustrate the difference:

Suppose you are on a cross-country drive in your car, which consistently gets 500 miles from a tank of gas. You see a gas station with a sign out front that says "Next gas station - 100 miles". You notice that you have driven 450 miles since your last fill-up, but your gas gauge still shows a quarter of a tank. Do you blindly trust your gauge, which is telling you that you can make it to the next station with 25 miles to spare, or do you go more by what you know about your own car's gas mileage from driving it?

The "DIR approach" is not to trust the gauge completely, when you have better information available that has been built up through personal experience. The idea is that you should have developed a "feel" for what sorts of profiles you get with tables, before you start using a computer - kind of like learning to add by hand before starting to use a calculator.

A dive computer is just one source of information about decompression profiles, and if you have built up experience with tables beforehand, you will be better equipped to make intelligent decisions about what the computer is telling you (and whether you "believe it" or not).

So using a computer for most rec profiles is generally considered OK, and may in fact be necessary if you are planning to do multiple repetitive dives over the course of several days (and want to get some decent bottom time out of them). Multi-level profiles in particular are pretty hard to manage in any other way.

As several folks have mentioned, once you get into the longer decompression dives, you will want to start relying more on tables for your planning. This gives you the ability to determine a priori how much gas you need, what stops to do, etc. It also allows you to develop and discuss a cohesive dive plan with your team, rather than trying to "wing it" based on what the computer says (or worse, what several of them say).

Well written post, thanks. I think you make a good analogy and your explanation of the DIR approach is easy to understand. And your 5th paragraph describes exactly how we dive, it's somewhat a pleasant surprise reading it in this forum.

 

[RTodd]

It's not so much the "use" that is prohibited, as it is the "blind trust in a piece of technology". I often use the following simple analogy to illustrate the difference:

Suppose you are on a cross-country drive in your car, which consistently gets 500 miles from a tank of gas. You see a gas station with a sign out front that says "Next gas station - 100 miles". You notice that you have driven 450 miles since your last fill-up, but your gas gauge still shows a quarter of a tank. Do you blindly trust your gauge, which is telling you that you can make it to the next station with 25 miles to spare, or do you go more by what you know about your own car's gas mileage from driving it?

The "DIR approach" is not to trust the gauge completely, when you have better information available that has been built up through personal experience. The idea is that you should have developed a "feel" for what sorts of profiles you get with tables, before you start using a computer - kind of like learning to add by hand before starting to use a calculator.

A dive computer is just one source of information about decompression profiles, and if you have built up experience with tables beforehand, you will be better equipped to make intelligent decisions about what the computer is telling you (and whether you "believe it" or not).

So using a computer for most rec profiles is generally considered OK, and may in fact be necessary if you are planning to do multiple repetitive dives over the course of several days (and want to get some decent bottom time out of them). Multi-level profiles in particular are pretty hard to manage in any other way.

As several folks have mentioned, once you get into the longer decompression dives, you will want to start relying more on tables for your planning. This gives you the ability to determine a priori how much gas you need, what stops to do, etc. It also allows you to develop and discuss a cohesive dive plan with your team, rather than trying to "wing it" based on what the computer says (or worse, what several of them say).

This covers it well. One point to add is that the typical computer diver doesn't even know how far his car will go on a tank of gas and relies solely on the distance to empty gauge. In recreational diving, that is the real problem. On most dives this doesn't matter. But, computers do break. In the past two years I have killed both a Vytec and a Uwatec Tec. While neither was ever used outside of gauge mode, both died in water. Since I was diving with DIR buddies, it was simply an annoyance. If I had been relying ont he computer, the diving day would have been toast.

The bigger gripe you will see from more experienced divers are that computers are hopelessly too conservative and don't give you any credit for proper ascent techniques.

As far as liveaboards, while I am in the two good dives a day camp, when doing 4-5 a day, the last thing I want to deal with is fussing with a computer between dives.

 

[lamont]

You can actually reset some computers (such as the Aladin TEC2G) now. Otoh, I think there are still some time restrictions before one can switch from "computer" to "gauge" mode... There are also computers such as some made by Cochran that will still show relevant infos after having been "aggravated."

I used my vyper for the first time in ages last weekend and it got all bent out of shape because it was diving 21% while i was diving 32% with O2 deco (and it only had about 3 minutes of mandatory deco left -- given the type 2 DCS it gets when you blow off just a tiny bit of deco it must have a PFO...)

it did throw up "Er" in the big numbers in the display, but it still functioned as a gauge...

 

[TSandM]

(and it only had about 3 minutes of mandatory deco left -- given the type 2 DCS it gets when you blow off just a tiny bit of deco it must have a PFO...)

Good one!

The problem with the "Er" mode is that it puts the dive time number in the little tiny digits down in the right-hand corner of the screen again . . . I LOVE gauge mode on my Vytech, where the time numbers are right in the middle and about an inch tall.

 

[b1gcountry]

I've been biting my tongue just a little bit here, but why don't you just take the time to set your computer up correctly for the mix of gas you are using? I kinda thought that was the point of the entire DIR philosophy...taking the time to do things 'er, well, Right.

Tom

 

[TSandM]

Well, of course, you're right; but I dive the thing in gauge mode 90 percent of the time, so I forget that I have to tell it every day which mix I'm diving, because I just don't use it as a computer very often.

DIR divers are human, not Borg, no matter what my avatar suggests . . .

 

[b1gcountry]

....

 

[DIR-Atlanta]

your 5th paragraph [about multi-level diving] describes exactly how we dive, it's somewhat a pleasant surprise reading it in this forum.

I think most divers probably do the same thing. The one down-side to doing dives this way is that it's very difficult to do any sort of "team planning" beforehand (at least with respect to depth and time). It's fine to say "max depth == XX", but that's about the extent of what you can do. You can't really break the dive down into smaller increments, because in most cases you can't really predict ahead of time what any individual person's computer is going to say about their gas loading at any particular point in the dive.

So the way most people approach this is to swim around at their max depth XX until their computer says "time to go up". If they have enough air left to extend their bottom time, they ascend to a shallower depth where there's enough no-deco time left to let them look around a bit more. That scenario then gets repeated until they eventually reach the surface. And of course, there is no way to accurately predict at what point during the dive that a person's computer is going to tell them that it's to "time to go up", so there's no way to plan for that ahead of time.

That effectively forces people into doing a "trust me" dive with their own computer, which is where they can run into trouble if they don't have a good feel for what some of the standard decompression models will allow. That's why I think it's really important for folks to have a good bit of table diving experience before they make the jump to a computer. That puts them in more of a "trust but verify" mode, which I think is safer in the long run.

 

[DIR-Atlanta]

Just a couple more random follow-up thoughts ...

One other problem with dive computers is that they frequently have undocumented "features" that penalize divers for violating some hidden "diving rule". For example, some divecomps have been reported to have a "feature" that virtually prevents any repetitive dives below 80 feet, if made within 12 hours of some previous dive (any dive - even something as benign as 10 feet for 10 minutes). This "rule" is not based on any sort of scientific research or known decompression theory - just someone's decision that "divers probably shouldn't be doing deep repetitive dives, so we are going to make sure they can't use a computer to do them".

I've seen other computers that will severely limit repetitive dive times, just for exceeding a 30 FPM ascent rate during any part of a previous dive. I had a friend once who was using one of those wrist-mounted Suunto dive watches. He would sometimes get much shorter dive times on his repetitive dives, and could never understand why. He finally realized that he was getting an ascent rate "violation" that was caused by occasionally raising his arm too fast to vent his BC. After he figured out what was happening, he switched the computer to his right wrist, and had no more problems with shortened repetitive dives.

Lots of folks will make the claim that "this forces divers to be more conservative, and that is a Good Thing". I agree that up to a point, being conservative is a safer way to dive, but I also realize that it can be carried too far. I prefer to make my own decisions about just how much conservatism I need to incorporate into my diving, and I am reluctant to abdicate that responsibility to an electronic device. This reduces my diving to the lowest common denominator, and I like to think that I am capable of a little more independent thought and decision-making than that.

Diving is all about risk assessment and risk management, and I believe that to be the purview of the divers themselves, not the gear manufacturers (who have their own agenda, which often runs counter to mine). I find that as I have gotten more and more knowledge and experience, I have had less and less need for the electronic "brain".

 

[lamont]

I've been biting my tongue just a little bit here, but why don't you just take the time to set your computer up correctly for the mix of gas you are using? I kinda thought that was the point of the entire DIR philosophy...taking the time to do things 'er, well, Right.

Tom

The problem comes when you start doing deco and want to shape the curve because you don't necessarily agree with the VPM models and don't want to risk your computer becoming useless if you have a gas loss issue and don't inform your computer about the change in deco plans... The UI problems occuring on the surface by not setting the computer to the correct nitrox mix are symptomatic of larger problems that can occur underwater in an emergency. The way to eliminate those UI problems completely is to not use computers...