NDL

[GearHead]

UGA once bubbled...
Jeff,

If you're diving a VyTec in gauge mode, can I trade you my Vyper?

I understand paying attention to dive limits independently of a computer, and I can understand some people no wanting to use a computer, but why would you use a Vytec in gauge mode when a unit probably $800 cheaper would do the same thing (as a gauge)?

Check out the Suunto site, UGA, if you read a little about the differences between the Vyper and Vytec, I'll bet you'd figure it out.
But to save you the time, I'll tell you. The Vytec has integrated hoseless gas data. So Jeff, myself, and many other Vytec users have tank pressure displayed in the bottom left hand corner of the device.

 

[MikeFerrara]

IMO, the problem with thinking in terms of NDL is that it tends to make divers think that they can stay down for X amount of time and ascend. Many give no consideration to the actual prifile inbetween. I suppose this is true for tables also but I think computers make it really easy.

I own two computers. Both were purchased before I ever gave any of this much thought. I got lucky though and both computers are models that just won't lock up on me no matter what I do to them. One is a nitrox single gas computer the other is a dual gas computer. The problem is that I often dive with more gasses than that and the computer doesn't know it so I use them for bottom timmers. I still enjoy using them because depending on the gasses I'm using I can see the difference between what the computer says and what I say.

I'll try to get to the point. Many divers do what I call ride the computer. If the computer says they have bottom time left and they have gas all is well. Often these divers will ascend when they see their NDL getting very small. They later move up again when the see it getting small again. When they run low on gas they ascend maybe doing a short safety stop on the way out of the water. Some of those divers get bent on the plan ride home. Either way they hit the surface with way more bubbles in their system than what I think I have even when I leave the water with my computer saying that I still owe 40 minutes of decompression. I don't do that to myself. I have seen divers surface from 80 ft to see where they were and go back down to finish the dive because their computer still showed no- deco time. Unfortunately many of us we need to learn something about technical diving before we feel we learn how to do a recreational dive.

Avoiding the NDL isn't the objective. The objective is to surface with minimum (bad) bubbling. In this sense there is no such thing as an NDL and if there is we really don't care. Limiting time at depth is only one small part of what we're after.

BTW, I sent a get well e-mail today to a guy who, last I heard, was still walking with a cane a month after a dive where he stayed within the limits of his computer.

 

[ERP]

Here's a question I have for the tech trained divers on the board.

How many of you would do the 6 or 7 dives a day that many people using dive computers on live aboards do?

I ask because to me this seems to be the epitomy of mindless computer use.

Personally I'm still in two minds about computers, I dive my Vyper in computer mode, but I usually just pay attention to the depth/time readings, using the irritatingly large NDL display just for reference.

 

[O-ring]

ERP once bubbled...
Here's a question I have for the tech trained divers on the board.

How many of you would do the 6 or 7 dives a day that many people using dive computers on live aboards do?

I ask because to me this seems to be the epitomy of mindless computer use.

Personally I'm still in two minds about computers, I dive my Vyper in computer mode, but I usually just pay attention to the depth/time readings, using the irritatingly large NDL display just for reference.

I have done 3-4 a day, but I don't think I would do much more than that unless there was something VERY cool to see. I think it's a personal decision of risk vs. reward. For me, the risk probably isn't worth it. I would prefer to do a couple really long dives than to keep doing repetitive dives, but that's just me.

 

[MikeFerrara]

ERP once bubbled...
Here's a question I have for the tech trained divers on the board.

How many of you would do the 6 or 7 dives a day that many people using dive computers on live aboards do?

I ask because to me this seems to be the epitomy of mindless computer use.

Personally I'm still in two minds about computers, I dive my Vyper in computer mode, but I usually just pay attention to the depth/time readings, using the irritatingly large NDL display just for reference.

Don't you love that! They make the NDL display big enough to see at a hundred yards but I need bifocals to see the depth and time. Proof of who the stupid boxes are designed by and for.

I think my last post pretty much suggested that I think the profiles many do on their Caribbean vacation is pure insanity. Talk about blind trust in some one else. Don't get me wrong I have often done six dives in a day (I teach) and my profiles are not always what I really want when mixing classes but I don't ride the computer.

 

[MikeFerrara]

O-ring once bubbled...

I have done 3-4 a day, but I don't think I would do much more than that unless there was something VERY cool to see. I think it's a personal decision of risk vs. reward. For me, the risk probably isn't worth it. I would prefer to do a couple really long dives than to keep doing repetitive dives, but that's just me.

Me too. When I get to Florida I do two dives a day max and sometimes only one. They are really cool dives though.

In Missouri I do one "dive" then after a SI take my son for a spin around the caver. Usually I'm on 50% for his cavern dive cause I'm a chicken. LOL

 

[Charlie99]

MikeFerrara once bubbled...
Avoiding the NDL isn't the objective. The objective is to surface with minimum (bad) bubbling.

I just wanted to repeat the above; and add that you need the time, and gas supply, to do a good ascent and deco.

In SeaJay's recent thread, he commented that his mistake was to go beyond his NDL. Another way of looking at it, his mistake (besides not understanding his computer, and not folllowing his plan) was to go past NDL WITHOUT THE GAS supply he needed to do a good decompression. Looking at his ascent profiles, he also appears to be lacking the needed knowledge, but the most important thing is the amount of GAS.

This is particularly important in the case where the computer diver "rides 0 NDL" back up. That sort of profile starts by loading up the faster tissues, but as you ascend, while keeping close to NDL, you load up the medium speed tissues up close to their surfacing limits, as modeled by the 40 and 80 minute halftime compartments, and it takes a long time to deco those back out to more conservative levels while using air or 32%. This is particularly true for repetitive dives, where the medium and slow tissues start off with significant N2 loading.

My target is to not get out of the water with any tissue above 90% of M0, and much prefer to be below 80%. Luckily, there are computers that have a bargraph display of M value of leading tissue -- but you can also have a pretty good idea by just playing around with any of the deco programs and plugging in a variety of dive plans.

Charlie

 

[MikeFerrara]

Good points Charlie. One of the things I do with my students is to show them how I plan a 100+ dive including equipment and gas. I wouldn't have done SeaJay's dive without my doubles and probably a bottle of O2. If I go to 100 ft on a wreck (everything of interest at about the same depth) I have no intention of being in a hurry either at depth or up shallow. A dive like that on a single 80 is exactly what I avoid. You just don't have time to play and if you push the "NDL" you don't have time to spend shallow to finish it right.

But Charlie, these ideas aren't going to sell Caribbean trips now are they?

 

[O-ring]

Don't get me wrong I have often done six dives in a day (I teach) and my profiles are not always what I really want when mixing classes but I don't ride the computer.

The only time I have ever had problems was when DMing an AOW class. I had to keep going down to hide things for the search and recovery dives. I think I probably did 10 descents that day. My left ear wouldn't clear after the last one and I couldn't hear out of it until I was almost back to the dive shop.

I noticed the 2003 DAN report included a separate category for dive professionals this time...makes sense given the profiles you guys have to do.

 

[MikeFerrara]

I try not to be stupid when doing training dives. I don't worry too much if it's just OW. The ups and downs can be tough on the ears but it's all shallow. Once in a while I just don't have any choice but to mix different classes though. Even then I try to set it up so I have a chance. If I have to do an Advanced nitrox dive and teach AOW or OW in the same day my profile for the AN dive is good. I give myself a decent SI and give my OW students a great deal of hovering practice whyile we deco ah, ah, I mean do a good long safety stop. Sometimes the ESA stuff still makes me drop bullets in my pants though. I worry more about the bouncy bouncy stuff than anything else.

Even when it comes to instructors getting in trouble it's mostly just stupid stuff. Even (especially) instructors get thinking that they can do anything without having trouble and as a result they do the real dumb stuff.