setting to nitrox to reduce over conservatism on dive computers

[Akimbo]

Well, this is an interesting thought. Keep your DAN insurance current.

 

[Dr. Lecter]

Trust the computer you bought, or get another computer that you like better. If you start messing with the way this one works, it is no longer valid, and anything that happens to you as a result is on your head.

What, exactly, made the computer "valid" in the first place? This is surely not the brightest idea I've ever heard, but running predive simulations with computer A at 21% and seeing what %O2 computer B needs to match computer A's NDL for the same dive is not all that different from RD with a sanity check against a known model. Indeed, this being NDL diving, I'd suggest the margin for error is wider.

I'm surprised to see any GUE diver complain about the safety of this approach given their own seat of the pants techniques.

 

[MXGratefulDiver]

I'm surprised to see any GUE diver complain about the safety of this approach given their own seat of the pants techniques.

.. the GUE approach is actually pretty conservative when you take all factors into consideration ... but it only works well if you do that.

If I'm understanding Lynne's post correctly (and I think I am), she's saying pretty much the same thing about the computer ... you can't just change one variable and assume you'll get the results you think you will.

... Bob (Grateful Diver)

 

[Dr. Lecter]

.. the GUE approach is actually pretty conservative when you take all factors into consideration ... but it only works well if you do that.

If I'm understanding Lynne's post correctly (and I think I am), she's saying pretty much the same thing about the computer ... you can't just change one variable and assume you'll get the results you think you will.

... Bob (Grateful Diver)

Is it assuming if you verify your change against an existing, unaltered model before diving? I have no doubt that if instead of doing X minutes on air with Computer A at 21% and Computer B at 25% (which setting you'd previously found to give the same NDL as Computer A on air) you decided to stay in the water for X+60 minutes, there'd be some divergence. But as silly as what this guy's doing is, it's not that silly. Similarly, if he's considering this approach for a lot of repetitive diving, I'd be much more inclined to agree with Lynne's concerns.

You don't need to know every detail about how each computer's algo works to get a reasonable approximation of safety--certainly RD users don't know all the factors that affect their deco process, nor is RD's "algorithm" sufficiently well-thought out to be anything more than a general band within which most divers are safe (assuming appropriate gasses and other factors). All of this stuff is ballparking it: having a dive computer run a known model against an actual plot of your time/depth with the gas you're truly breathing is probably the least ballparky way of doing it. This approximation of one computer against that method is surely no less valid than RD (which you yourself have said is best checked against a DC for sanity's sake).

 

[NAM001]

I was just curious whether there was a processing difference between nothing more than the mode air/nx-- that used different degrees of conservitism.

If you're asking me I meant diving mix such as 32% but treating it as air.

 

[tech_diver]

"A common complaint I hear is that newer dive computers are overly conservative. The reason given is that computer manufacturers are afraid of litigation."


I think whoever told you that isn't connected to manufacturing dive gear. The manufacturers of gear are completely different organisms than people who develop deco algorithms. People who build dive gear don't adjust deco models for liability any more than training agencies adjust tables for liability.

 

[MXGratefulDiver]

Is it assuming if you verify your change against an existing, unaltered model before diving? I have no doubt that if instead of doing X minutes on air with Computer A at 21% and Computer B at 25% (which setting you'd previously found to give the same NDL as Computer A on air) you decided to stay in the water for X+60 minutes, there'd be some divergence. But as silly as what this guy's doing is, it's not that silly. Similarly, if he's considering this approach for a lot of repetitive diving, I'd be much more inclined to agree with Lynne's concerns.

... I believe you ARE agreeing with her concerns. As she said, if you're doing this to make one computer resemble another, without knowing how the algorithms work you might be able to make them reasonably match for a specific dive profile, but then changing profiles or doing repetitive dives may result in more divergence than you expect. She even gave fairly accurate reasons why that might be true.

... Bob (Grateful Diver)

 

[padudle]

Many people dive nitrox with their computer set to air to get more conservatism.

This guy set his computer to nitrox and dove air to get less conservatism.

Pay your money and take your chances.

Actually, in this case he was diving with two computers. His old computer was set to air like always. His new computer was set to nitrox to match the old computer.

 

[Akimbo]

This might help you to appreciate all the warnings that have been posted. Decompression algorithms continuously calculate based on time, depth, altitude, PPO2, and residual diluent gas variables (I think I remembered them all). In addition they may also factor in conservatism settings, exertion level settings, water temperature, and age parameters. These parameters are then used to continuously calculate all (or at least most) of different tissue permeability rates and figure out what it takes to prevent bubbles from developing that will cause you to get bent.

You can lie to your computer by telling it that you are using a lower oxygen mix than you really are to increase conservatism or safety margin, except that you have to manually track OTUs (Oxygen Tolerance Unit). Not a bad idea if you are diving days away from the nearest chamber.

There’s more to different algorithms than apparent conservatism. Some use more short deep stops than others. Some “weight” tissue calculations differently. Many are more “conservative” on one profile and not another. You don’t need a PhD in math to appreciate that trying this trick in reverse can generate a recipe for a Table 5 chamber ride.

… He could test the setting by running the computer in dive planning mode and comparing bottom times with what his old computer allowed…

And some use exactly the same algorithm, but newer revisions have been adjusted because the older revs produced unacceptably high DCS hits rates on certain profiles. Also, because they are computers, we can’t discount small bugs in the code that have been fixed.

 

[DepthCharge]

I've seen this term "NDL" several times in this thread. Can someone explain to me how you can dive and have NO deco
Later,
John