Diving with gradient factors for a new recreational diver

[tursiops]

On my first dive with a Perdix on the default setting (tech mode) my reaction was "wait . . . I had way more no deco time last week." (Compared to my Aqualung i300.) Algorithm comparisons are always interesting. For a first dive, I think the Navy Dive Tables give you a 25 minute NDL at 100 feet and Buhlmann 99/99 gives you something like an NDL of 17 (?) and setting the GF high lower takes it down from there. In theory, all of the algorithms provide some concept of an acceptable level of risk, but there are always gray areas. I think the default on the Perdix is 30/70.

I've published this table before, but it adds to your point about first-dive NDLs. The US Navy tables are way out of line with everything else, so perhaps not the best comparison to make.

 

[boulderjohn]

Remove is probably wrong word selection on my part. Not having, separating, excluding probably better.

Sorry, that still doesn't help.

Are you suggesting that teaching how to use tables is the same thing as teaching decompression theory, and are you suggesting you can't do one without the other?

 

[HomeBarista]

The idea that it is obvious is contradicted by a once popular thought that the key to avoiding DCS was protecting the fast tissues, and if you did that, you did not have to worry about slower tissues.

I understand little about the onset factors of DCS, though I hypothesis I formed from my misc reading is that there may be some merit to being extra careful with the fast tissues first, which are more vital organs too, in DC algo design, after which to medium tissued based on current performance diving. Especially so for recreational divers.

Looking at his computer, I saw that it was in error mode. We stayed 15 minutes. I took him up to the surface and had him breathe oxygen for a while to be safe. He was OK.

So glad to hear you were able to catch this and help him.

Years ago we used to think the proper thing to do in OW training was to teach students how to use tables and then send them out into the diving world knowing they will not use those tables and will instead buy a computer, with no instruction on how computers work.

One of the problems with computers is the RTFM problem. If you read the manual, the whole thing, there is a good chance you will come out of the experience thoroughly baffled. After page after page after page after page after page of stuff you will never need to know, you will realize that the stuff you needed to know was scattered throughout those pages, but you somehow missed them or can't remember them. That was indeed my experience with my first computer, The stuff I really needed to know, which could have been concentrated in two pages, was scattered throughout the manual, without clear headings to help you find them.

Ditto. Computers should be taught too. My experience after learning tables was that a computer would be so much more enjoyable to use. But then I met the Suunto! As someone who handles some UX design elements at work, the Zoop Novo's manual and user interface controls baffled me beyond imagination... As someone mentioned, it's sad (many?) manufacturers don't create a summary of the critical pieces of info and training videos how to handle the situation you need to be able to handle. So sad to think this is for liability reasons, but as someone who resides in the US i can easily see this being the reason.

 

[scubaozy]

Sory, that still doesn't help.

Are you suggesting that teaching how to use tables is the same thing as teaching decompression theory, and are you suggesting you can't do one without the other?

A cmas 2* diver will know how to use decompression tables in contrast to an aowd. Aowd will have to get a dedicated tec training for instance decompression procedures until he is exposed to anything comparable. The problems you described people not understanding what computer is telling them is less likely to occur with a 2* cmas diver because their training goes beyond ndl.

 

[boulderjohn]

I understand little about the onset factors of DCS, though I hypothesis I formed from my misc reading is that there may be some merit to being extra careful with the fast tissues first, which are more vital organs too, in DC algo design, after which to medium tissued based on current performance diving. Especially so for recreational divers.

This idea originated in decompression diving, and it is one of the reasons for deep stops in decompression diving. Some people actually said you can shorten total decompression time by making deeper, longer first stops. "Take care of the fast tissues, the the slower tissues will take care of themselves."

Well, that turned out not to be the case.

But you can still find those old articles, and you can still find people who hold to that belief as if it were chiseled on stone and brought down from a mountain top by a prophet.

 

[boulderjohn]

A cmas 2* diver will know how to use decompression tables in contrast to an aowd. Aowd will have to get a dedicated tec training for instance decompression procedures until he is exposed to anything comparable. The problems you described people not understanding what computer is telling them is less likely to occur with a 2* cmas diver because their training goes beyond ndl.

It is also less likely to occur if someone is taught how to use a computer.

Decompression theory tells us how nitrogen enters and leaves our tissues while diving, and it tells us why that can be dangerous and why we must use a safe diving protocol. Once you have learned that, you can learn how to manage your dives to avoid DCS. That management can be done via tables or a computer.

Because many people learned decompression theory and how to use tables at about the same time, they often do not realize they are not the same thing. Mark Powell wrote the book Deco for Divers, and if you read it all, you will learn a heck of a lot about decompression theory. You will not, however, learn how to read tables, because knowing how to read tables is not part of understanding decompression theory.

I have not taught any OW classes for several years, but when I did, the computer version of the course, included an online simulator that allowed you to move a generic computer through a variety of circumstances and watch how it reacted and how it provided guidance. That was a much better illustration of tissue loading and unloading than a table.

 

[Eric802]

Years ago we used to think the proper thing to do in OW training was to teach students how to use tables and then send them out into the diving world knowing they will not use those tables and will instead buy a computer, with no instruction on how computers work.

When I got my OW and AOW, computers were barely a thing (1988 & 1990). PADI just taught the tables, IIRC, and I think the most I heard about computers was something like "they have computers now that do this stuff for you". But there certainly wasn't training on it. Which is why, after reading all about how ubiquitous computers are now, I made sure that my refresher course included a bit on using dive computers.

Probably hard to instruct students on how computers work when there are so many different manufacturers and algorithms and such.

 

[boulderjohn]

Probably hard to instruct students on how computers work when there are so many different manufacturers and algorithms and such.

OW courses teaching computers teach the key generic concepts that all computers can do. There are really only 4-5 concepts that are critical and not obvious on the face of the computer. The student is told to learn the ways their computer handles those 4-5 items when they purchase it.

 

[Wibble]

Suunto D9tx vs Shearwater Perdix. Opinions...

The Suunto requires you to read the manual -- RTFM -- and re-read it periodically to remind you how to do things with it.

The Shearwater Perdix is so simple to use in comparison. With the Suunto you have to memorise all the different key presses; there's four buttons and there's a short press and long press. On top of that there's the built in conservatism of a Suunto which prevents you from doing certain things. For example switching to 100% at 6m/20ft. Perdix is fine; it'll let you change to a higher PPO2 gas although it will complain at you in very simple terms. The Suunto won't let you change until you're 2% below the max PPO2, so you have to be above 6m/20ft to "unlock" the 100% switch, i.e. break your ceiling by going above 6m/20ft just to switch gas on the damn computer.

Suunto has this crazy conservative thing they do which is where all its PPO2 calculations are out by around 2-ish percent. For example 28% nitrox at 40m is a PPO2 of 1.4 (dose over mix = MOD = 1.4/0.28 = 5ATA = 40m/130ft) Do this on the Suunto and the MOD is calculated as 38.3 metres.


Basically the Suunto's nigh on impossible to use underwater, the screen's confusing and the user interface is diabolical.

Then there's the Shearwater Perdix. Two buttons, simple interface, easy to use with a wonderful screen. The only time I've needed to refer to the manual was to work out what GF99 means.

 

[scubadada]

I made myself a spreadsheet that is similar to the one @tursiops posted, but it concentrates only on Buhlmann and the PPS algorithms, with and without conservatism. The range for GF high extends lower than most would ever need. The colors show roughly how the PPS algorithms track to Buhlmann.

This comparison only applies to a clean, 1st dive, as Buhlmann and the PPS algorithms behave differently on repetitive dives. I have previously posted some my experience with DSAT (2089 dives) and Buhlmann (937 dives). I would say that this statement applies to all or most different decompression algorithms.