PADI tables finally going away?

[Quero]

I already give a lot of practical emphasis to watching the NDL on the computer and telling my divers (both students and fun divers) not to let the computer count down to less than 7 minutes NDL on the deep portions of our dives (>18 meters).

I don't know that I have quantified this with my students, but I like it! What made you settle on 7 minutes?

Because some readers may not have access to PADI tables, I'm going to take a risk here and attach an image of one side of the PADI air table so that you can see how I came to the 7 minute number. If there are copyright issues, please feel free to remove the image. (It's only one side, and it really can't be used properly if I don't post the other side, so I'm hoping it passes moderation for purposes of illustrating the discussion.)

When you look at the recreational depth deep dives (which PADI defines as those from 18 meters/60 feet and beyond), you see that the shaded boxes at the extreme time limits for the dive are all omitted if we subtract 7 minutes from the black box NDL time limit. That is, at 60 feet, the NDL is 55 minutes; if we subtract 7 minutes from that, we get 48 minutes, which will keep the diver out of the gray area if s/he dives a square profile and sits at 60 feet for the entire dive. For the next depth column, 70 feet, by subtracting 7 minutes from the black box 40 minute NDL listed, we get 33 minutes, again outside the gray area. The next deeper depths, 80 and 90 feet get us well out of the gray area. Anything over 100 feet is a contingency depth and we are meant to teach students to stay shallower than 100 feet.

In other words, there's nothing magic about 7 minutes. It's just convenient to have a single number to recommend to people, and you know that if they lose track while shooting a picture or something and go down to 6 or even 5 minutes, they're still well within limits.

I make the recommendation for shallower dives, too, especially if we're doing three or more dives and have been deep on the early dives. To be honest, I don't often come very close to bumping limits on those since the NDL times are so long (over an hour), unless we've had a realatively short surface interval after a series of deep dives. This can certainly happen on the liveaboards, but not so much on the day boats.

 

[The Chairman]

Interesting. I like it. Do you teach a deep stop, and how long are your safety stops?

 

[Quero]

Interesting. I like it. Do you teach a deep stop, and how long are your safety stops?

If we're doing something approaching a square profile deep dive, such as on our wreck (which sits in sand at 30 meters/100 feet with the top at 18 meters/60 feet), yes, I do teach deep stops. Half of our maximum depth for one minute. On reef dives, we're always doing multi-level dives, which gives the same effect as a deep stop--since we're not doing direct ascents but just working our way shallower from about the middle of the dive onward. Regardless of what the NDL time might be on the computer, I always do 3 to 5 minute safety stops, though sometimes divers are sometimes surprised that their computers show the stop as being completed--it's always because we spent the last 5 minutes of the dive puttering around in the shallows (if there's no surge) instead of doing a static blue water "stop." For blue water stops, I generally stick closer to the 3 minute time since we're invariably drifting.

 

[The Chairman]

Thanks. Air permitting, I teach a 2 minute deep stop and a full five minute shallow stop. That fights the drowsies. I don't have the luxury of the ascending reefs here in Florida. We have mostly sand close to shore where I dive.

 

[Blackwood]

IF you understand what you are looking at, and also understand how the variables interact with each other. Do most new divers have any grasp of that? Most that I see don't.

Maybe not (I don't dive with many new divers), but that's neither an argument against computers or for tables. It's an argument for quality, comprehensive instruction.

I think we're agreeing that it's possible to "fly" a PDC to the NDL. We're just reaching different conclusions whether that's a good thing or whether it's just encouraging people to completely turn off their own brains when it comes to dive planning.

Hmm, I wasn't meaning to take a stance on flying a PDC (or table) for that matter to NDL. I was merely using "the limit" as an easy starting point.

But given that my statement began with the phrase "If an aware computer diver looks down..." (which was meant to say that he pays close attention to his computer and thus is able to predict how it acts), I think we're on the same page with respect to people turning off their brains. In retrospect, it came out wrong (does seem like I was meaning he accidentally hit "the limit").

 

[Kingpatzer]

The summation point seems to be that proper instruction of decompression theory focuses on planning a safe dive that includes not using the NDL as a target but a number to avoid. Which is an idea with which I agree. A focus on what conservative dive planning should look like, as well as continually stressing (and modeling) such planning in all phases of instruction is essential to developing students who will continue the practice post certification.

I see two follow-up questions to this point.

The first question is how deep an understanding does a beginning diver need to have about decompression theory, how NDLs are derived, what they imply, and so on. Personally, I'm of the view that ingrained functional behaviors are more important than deep understanding, but that of course some level of comprehension of the underlying theory is helpful. Others have a different view, of course, but I'm wondering if it matters much in light of my second question.

The second question is, in some ways, the real issue in all of this. Does it matter given the culture of DM led trust me dives being the normal introduction to diving outside of the class experience? When the majority of OW students leave my class at least, they are shortly thereafter headed to some dive destination where they are going to be told the dive plan, told to gear up and drop in, and not be given the option of creating their own plan. Moreover, the majority of the time those plans are going to be very near the NDLs because divers want their "money's worth" of time underwater. So the dives are structured to at least push available gas (usually violating everything the divers were taught as students about gas planning) and usually pushing NDLs as well.

When the 'professional' examples are generally so poor, why do we expect a different long-term outcome than what we're seeing regardless of if we're talking about tables or PDCs?

 

[Blackwood]

The first question is how deep an understanding does a beginning diver need to have about decompression theory, how NDLs are derived, what they imply, and so on. Personally, I'm of the view that ingrained functional behaviors are more important than deep understanding, but that of course some level of comprehension of the underlying theory is helpful. Others have a different view, of course, but I'm wondering if it matters much in light of my second question.

Regarding the part I bolded, I agree 100%. That's a great way of stating when I mean when I say "functional decompression theory."

In addition, I think that it would be valuable if students were taught that ascent rates are an explicit part of NDL calculations (and that if you break the max ascent rate, you negate the specific NDL computed for it). Many times I've read or heard people say "I ascended too fast, but I was within NDL so it doesn't matter." That shows a fundamental lack of understanding about what NDLs are, and depending on the actual ascent rate and computed NDL, they may in fact violated critical m-values.

As to the minutia of various tissue models, bubble models, hybrid models, etc., I think that can be left for those who want to learn about them.

 

[boulderjohn]

OK, we don't want "trust me" dives. How do we avoid them?

In most resort areas, DMs lead dives, and in some cases (Cozumel, for example), it is a legal requirement. These DMs will lead multi-level dives based on computers. A square-profile trained diver without a computer has no choice but to use a "trust me" approach to diving if he or she wants to do those dives. How will they know when the dive is unsafe?

I have had three occasions when a DM led an unsafe dive in terms of Deco. In all cases, it was the second dive of a two tank dive, and we had a different DM for the second dive. In all cases, the DM treated the second dive as if it were the first. In all cases, several divers rebelled during the dive and let the DM know things were not right. In one case the divers just completed the dive well above the rest of the group, at a safer depth, but in two cases the DM was confronted and the entire group ascended. They overrode the DM's decisions, and they took control of the dive.

How were they able to do it? They looked at what their computers were telling them.

 

[The Chairman]

How were they able to do it? They looked at what their computers were telling them.

Holy Grass Roots, Batray!

 

[Blackwood]

In most resort areas, DMs lead dives, and in some cases (Cozumel, for example), it is a legal requirement.

I think the law requires a guide for conservation purposes. Maybe my experience was abnormal, but none of the DMs ever tried to control my profile, and looking around, people were all over the place in terms of depth.