USN Tables vs computers

[Nemrod]

If you are male, age 20-28, able to run 2 miles in 12 minutes and do 15+ pullups, and always have a deco chamber on your dive boat, then the Navy tables are probably a good match for you. If these are not true than I'd suggest you use significant caution in using the Navy tables.

What if you are 61 and can do that?

I am going with the computer considering human nature to occasionally make an entry to the table error. But, you guys realize that most technical divers use a bottom timer and tables? The advantage to a computer is that it can handle continuously changing depth and credit the diver (or penalize) on the fly so while the computer appears more conservative in practice it might not be.

On a square profile dive to 120 feet I have 15 minutes. But, using a computer, I have stretched the dive to over an hour, of course I was only at 120 for a minute or two. Which is more conservative? I think the real issue here is that most of the divers trained today or even in the last decade really do not know or understand the application of the tables or how to plan a dive or do multilevel dives and repetitive multilevel dives using tables.

N

 

[Akimbo]

As mentioned, US Navy reped tables never were much of a priority since dive teams are usually large enough that your rotation into the water rarely comes up more than twice per day.

One factor a recreational diver needs to consider is the dive profile. A computer will optimize your bottom time because it gives you “credit” for portions of the dive spend above your deepest depth. A military or commercial diver spends the vast majority of most dives planted at one depth so that “credit” is of marginal value.

As a result, tables have an inherent conservation factor for most recreational dives that have a meandering depth profile. The downside is the Navy doesn’t collect much real world data past one reped/day. Toss Nitrox into the equations and using your computer and setting it to the most conservative setting probably makes more sense. For added margin, you can tell your computer you are using a leaner Nitrox mix than you really are — but you have to track your own O2 clock if you are pushing bottom time that hard.

 

[boulderjohn]

PADI had their tables written with repetitive diving in mind, so they have generally shorter NDL's on the first dive, but generally longer NDL's on the subsequent dives.

For anyone planning repetitive dives, as you are, this is the key issue. The US Navy tables were the standard for the early era of recreational diving, but they led to extremely long surface intervals that made repetitive diving a real problem. The research that led to the PADI tables was inspired by the need to to get in more than one or two recreational dives in a day. If you are gong to follow the Navy tables for a 4 dive day, it will be a very long day.

 

[KevinNM]

What if you are 61 and can do that?

Age is considered a risk factor for DCS. So no, those tables were not developed via tests on 60 some year-olds. They were based on tests on extremely fit young men in essentially perfect health. The scuba police won't stop you if you want to dive Navy tables to the limits, but it seems of questionable wisdom.

 

[tbone1004]

Kevin, age is not considered a risk factor by DAN at this point. They have observed increased dead space in the lungs, but there is no evidence that age is directly related to DCS risk factor. Fitness however is very much related and that usually goes hand in hand with age. There was a recent podcast on Pod Diver Radio where Dr. Pollock addressed age and DCS. It's a good listen if you have 15 minutes for it.

 

[Nemrod]

Age is considered a risk factor for DCS. So no, those tables were not developed via tests on 60 some year-olds. They were based on tests on extremely fit young men in essentially perfect health. The scuba police won't stop you if you want to dive Navy tables to the limits, but it seems of questionable wisdom.

I think you misunderstood. Some 61yo including possibly me can run two miles in 12 minutes and do 15 pull ups. Okay, what with my titanium leg and busted hip grant me 15.

My point is that while age might be a factor (unproven) it is not the primary factor and from what I see of many 21 yo today their youth will not offset their other contradictions and that goes for anyone.

And do not misstate what I said which no where implied an intent or recomendation to dive to or beyond either the tables or computer limits.

N

 

[PeterNBiddle]

Way too many factors to consider in order to provide a good answer.

However, I will add that you shouldn't really "let the computer do it all for you."

As you can see on the table above, the USN tables are quite a bit more liberal at shallow depths, and they more or less even out at greater depths, but there are lots of different algorithms, so it really depends.

The benefit of using a computer is for real-time tracking, and for non-square profiles. You can apply conservative factors on the computer rather easily, so you could make the computer dive more conservative rather easily. Bottom line, though, the computer is primarily a convenience tool, that offers additional real-time tracking benefits.

This table is AWESOME. It can pretty much be the punctuation on the end of any debate about tables vs. comps. Thank you so much for posting it.

 

[boulderjohn]

This table is AWESOME. It can pretty much be the punctuation on the end of any debate about tables vs. comps. Thank you so much for posting it.

Really?

What does it tell you about multi-level dives and repetitive dives? Aren't those the two areas about around which the debate revolves?

 

[tbone1004]

Where'd the table go?

Either way, it doesn't actually show any relevant information to this discussion. There are however graphs showing comparisons of some algorithms for limited repetitive diving, but it also doesn't really matter.

Here's the only actual answer to this question.

Tables will almost always be more conservative than dive computers, they assume square profiles at maximum depth. Very few dives are conducted like this, they also have ranges for their SIT's, not calculating real time. More conservative doesn't mean better.

If you want the most accurate information and actual control over your diving, you need to invest in a computer that has user adjustable gradient factors. At that point you can control how much nitrogen loading you are allowed to surface with and adjust accordingly. I.e. set the top number to 70 and you surface with 70% of your nitrogen loading, set to 80% and you get longer NDL's but you surface with more nitrogen loading. It tracks accurately, and your NDL's are always set to allow you to surface with a theoretical % of nitrogen loading in your tissues. You come up and don't feel great, set it one notch down, you feel really good, try it one notch up for the next dive.

Shearwater has defined the following for its recreational nitrox mode
Low 45/95
Med 40/85
High 35/75

Many tech divers use 30/70, but that is for planned decompression, and generally only two dives/day. Set them where you want and go from there. For reference the Low setting 45/95 is fairly similar to the PADI tables for square profile diving.

 

[Akimbo]

Now that we have buried Bruce in minutia, one point is clear. Keep your DAN insurance current.