PADI tables vs TDI/USN tables vs Dive Computer

[vinsanity]

My background is PADI, but I recently took the TDI Nitrox class and in the process noticed that the PADI tables and TDI/Navy tables are pretty different and wanted to understand why. I searched the forum and found some very interesting old discussions summarized as roughly:

• Navy Tables were designed for Navy work, which is typically 1 dive to do as much of a job as time permits, and then come up for the rest of the day. So the tables reflect a maximum first dive followed by very long surface intervals for repeat dives. There is probably also some limit pushing because everyone is fit and there is a medic on the boat.
• PADI wanted to support for multi-recreation dive charter boat trip, so they made their own tables with tweaks to reduce surface intervals, which also included reduced the NDL. With multiple dives, you'll recoup the lost NDL time from the first dive in the form of sooner or longer repeat dives.

Assuming my understanding is at least roughly correct for a very complicated topic. What can I expect my dive computer's position on this to be? I currently dive with an AquaLung computer, but am starting down the tech diving path, so I plan to upgrade to a Shearwater Perdix AI shortly.

 

[tursiops]

You'll be much closer to the PADI tables than to the Navy tables.
Plus, using a computer you'll not have to make the "square-profile" assumption so your NDLs will be longer than what the tables allow.
As you progress down the tech path, you'll discover that the Shearwater uses an adjustable Buhlmann algorithm; there is much effort, angst and argument about what the adjustments ought to be!
The AquaLung use a variant on the Buhlmann algorithm called PZ+, although the details have never been released.

 

[boulderjohn]

What you wrote is essentially correct, but there is a bit more to it all.

• PADI did not just tweak the navy tables. They did their own original research to create the tables. Their primary finding was that the navy's use of the 120 minute theoretical tissue was unnecessary for recreational diving and led to unnecessarily long surface intervals. For much recreational diving, they found that the 40 minute tissues were OK, but they made the 60 minute tissues the basis for the tables.
• The two other major tweaks were shortening the first dive limits and making more pressure groups to cut down on the amount of rounding off that was necessary with the navy tables.

Modern computers are all pretty much similar to the PADI tables. If not, they would be out of business because people would not be able to do the second dives of a standard 2-tank dive schedule.

You will occasionally find people who claim they are using the navy tables or any of the other variations linked to agencies (NAUI, SSI,SDI, etc.). Often, if they explain in greater detail, you will find out they are diving within the first dive limits of the navy tables, waiting through the PADI-compatible surface interval, and then using the navy first dive limits again for the second dive. That is very much NOT using those tables. That is pretty much winging it.

 

[shurite7]

I agree with tursiops and boulderjohn, computers are much closer to the PADI RDP, or more accurately the PADI Multi-level wheel. As for going forwards, learning and understanding the wheel or the eRDPml will really help to understand what the computers are doing.

 

[vinsanity]

Thanks all!

I'll do some digging into the wheel and eRDPml next and see where that knowledge rabbit hole goes.

 

[Wibble]

Pretty much all technical diving uses Bulhmann with Gradient Factors nowadays. The PADI tables are for recreational diving, by definition technical diving is beyond that.

Most technical diving computers use Bulhmann and there’s plenty of good diving planners out there (e.g. MultiDeco). Avoid the Suunto computers as they use proprietary algorithms which means no other non-Suunto planners are available and they are incompatible with other technical divers computers and planners. Look at Shearwater computers, probably the most common technical diving computers currently sold.

Prior to your move to technical diving it’s worth reading up on decompression theory. Most people read Deco for Divers (Powell). Even if you don’t move into technical diving a knowledge of decompression is interesting and useful.

 

[boulderjohn]

Pretty much all technical diving uses Bulhmann with Gradient Factors nowadays. The PADI tables are for recreational diving, by definition technical diving is beyond that.

It is true that PADI's research was aimed exclusively at recreational diving, and their research did not touch on decompression dives at all; however, at the recreational level, Buhlmann on a computer behaves very similarly to DSAT, the computer version of the PADI tables.

 

[dmaziuk]

Pretty much all technical diving uses Bulhmann with Gradient Factors nowadays. The PADI tables are for recreational diving, by definition technical diving is beyond that.

You just need to cut a different set of PADI tables.

They will be sub-optimal for decompression diving because DSAT (PADI RDP) algorithm is supposedly using the fixed sea-level value for maximum allowed overpressure whereas in other models (based on empirical data) that value goes up with depth. As a result Buhlmann will generate shallower/shorter/fewer decompression stops; DSAT will generate unnecessarily long decompression schedules.

 

[Doc]

For the casual reader, it should be noted, as in OP, the reference to “Navy Tables”, those were designed to a “square profile”, dropping to a specific hard bottom depth (performing a task) spending X minutes there, then ascending.

Mostly we don’t dive that way.

PADI’s early tables used a “maximum depth attained” as a constant in the algebra. The plastic card didn’t care if you went to 100’ for one minute and spent the other 55 minutes at 30 feet. It put you at 100’ for 56 minutes and thusly pronounced you dead.

PADI’s “Wheel” is a representation of our recreational reality, multi level diving, where it was hoped that we would plan and adhere to specific depth profiles, avoiding “sawtooth” and erratic changes in depth.

As the Wheel was introduced- Computers were just about to appear in our LDS. The PADI Wheel, for a number of social and technological reasons was instantly a hard sell...not only as a gotta have-it dive widget, but more unfortunately, as a graphic representation of how these dive computers “thought”.

[PADI and other agencies were presented with a challenge. Should they attempt to explain how to use one of several computers being sold? Should they just ignore the teaching of how multi-level diving is computed (by explaining it thru the Wheel) or fight teaching computers? A combination? For a while, there was a transitional phase between Wheel and Computer. All the while, fighting internally over what the any agency’s OW student will endure....Understanding vs.” just look at the numbers, kid”.]

@shurite7 , I’m just trying and hoping to flesh out your comment. It speaks to the history of the evolution of our understanding, and it also explains the hardware, from paper tables, plastic cards, wheels, and the dive computer.

Here’s one...

 

[tursiops]

As another take on this table-Wheel-eRDPml discussion...
** The table was nice. It was discrete....you were at (say) 79 or 80 ft, no ambiguity. Your surface interval was discrete...no ambiguity. People learned it as a set of rules (do this, flip it over, do that, flip it back...)...but only a few caught on that all the information was on side 1; all side 2 did was some subtraction for you, just in case you couldn't subtract.
** The Wheel was a circular slide rule: an analog device. It was never exactly clear exactly where the arrow was pointing, is was not discrete, and gave ambiguous results.
** The eRDPml turned the analog Wheel into a digital equivalent. No more ambiguity. Cool.