Dive Computer No Deco Computations Question

[boulderjohn]

Sheesh. Well, at least you tried to help. Some people just can't be helped.

Really makes me wonder, though. Why did they buy the computers if they aren't actually going to use them? While I'm not a fan of lying to your computer by setting to Air when diving NX32, I understand why some do. However, if you then just ignore the warnings and guidance of the computer, what good is it. I just can't understand the reasoning there.

It was once a very common practice. People with the necessary credentials to be published in scuba magazines recommended diving with nitrox but using air tables/computer settings for many, many years. It never made sense to me, but they were the respected scuba authorities. Go back through enough ScubaBoard discussions and you will see it argued many times.

 

[dmaziuk]

but then you're outside your operating envelope, not anymore in a normal dive.

how should the algorithm continue?

There is nothing stopping it from tracking gas loading above the ceiling. What it can't do is recalculate you a different ceiling -- not until you off-gas enough to have one above your current depth.

Edit: speaking straight-Haldanean model like ZH-L. Bubble voodoo can be somewhat more complicated.

 

[David Carron]

It is the best you can do under the circumstances. Far better than just being locked out and told to go spend the rest of your gas at a safety stop. The Suunto approach is outdated. It is the same advice given if you overstay your NDL while using tables.

You certainly might think so, but consider this case:

GF 50/80 you descend to 50m and stay there for 20 minutes resulting in 45 minutes of decompression starting at 18 metres.

Same GFs, your buddy executes the same dive but at 15 minutes, he ascends rapidly and directly to the surface in 1 minute where he remains for 2 minutes before redescending to 50 metres in 2 minutes. At the 20th minute of the dive, your buddy will be told by his shearwater that despite the suicidally dangerous profile the recommended ascent is only 35 minutes of decompression starting at 9m.

Whilst this is clearly an extreme case, actively reducing the decompression obligation doesn't really seem to be "the best you can do under the circumstances".

 

[rongoodman]

but then you're outside your operating envelope, not anymore in a normal dive.

how should the algorithm continue?

On the tissue loading graph on my Shearwaters I would expect to see one or more compartments registering above the M-value. with the calculations continuing as before and appropriate warnings. What to do about it would be up to me.

 

[JMarc]

It is the best you can do under the circumstances

It is the simplest you can do. But the best? Stopping to compute offgazing when above ceiling would be better (probably not significantly better).

When I learned, and dove with, tables we had exceptional procedures for infractions... they all implied longer (and sometimes deeper) stops than the initial plan. Not the reduction you get if you simply continue to compute an haldanian gaz load.

There are limits to these procedures (for one they are NOT validated and the limits are not well established) and there are situations for which the best is "get out of the water now, call for help, get on pure O2 and hope they come quickly enough to save you".

 

[tursiops]

You certainly might think so, but consider this case:

GF 50/80 you descend to 50m and stay there for 20 minutes resulting in 45 minutes of decompression starting at 18 metres.

Same GFs, your buddy executes the same dive but at 15 minutes, he ascends rapidly and directly to the surface in 1 minute where he remains for 2 minutes before redescending to 50 metres in 2 minutes. At the 20th minute of the dive, your buddy will be told by his shearwater that despite the suicidally dangerous profile the recommended ascent is only 35 minutes of decompression starting at 9m.

Whilst this is clearly an extreme case, actively reducing the decompression obligation doesn't really seem to be "the best you can do under the circumstances".

It is the simplest you can do. But the best? Stopping to compute offgazing when above ceiling would be better (probably not significantly better).

When I learned, and dove with, tables we had exceptional procedures for infractions... they all implied longer (and sometimes deeper) stops than the initial plan. Not the reduction you get if you simply continue to compute an haldanian gaz load.

There are limits to these procedures (for one they are NOT validated and the limits are not well established) and there are situations for which the best is "get out of the water now, call for help, get on pure O2 and hope they come quickly enough to save you".

We are in Basic Scuba, guys. Perhaps you should not be inventing extreme scenarios to try and prove a point. The underlying issue here is whether a Suunto locking you out during a dive is a good idea or not. Do you think it is?

 

[David Carron]

We are in Basic Scuba, guys. Perhaps you should not be inventing extreme scenarios to try and prove a point. The underlying issue here is whether a Suunto locking you out during a dive is a good idea or not. Do you think it is?

Well, between "no violation with a small amount of deco" where the result of the model is assumed good and "huge violation with a middling amount of deco" where the result of the model is known bad, there's a continuum so it seems fairly intuitive that somewhere along that continuum the correct behaviour for the computer should be to stop displaying the result of the model. As far as I am aware there is unfortunately no decompression theory to guide us on this one though.

Suunto's decision to not display what they believe to be an unacceptably erroneous ascent profile probably has its roots in their in-house decompression expertise and legal teams neither of which I feel particularly competent to comment on.

 

[johndiver999]

It is the simplest you can do. But the best? Stopping to compute offgazing when above ceiling would be better (probably not significantly better).

When I learned, and dove with, tables we had exceptional procedures for infractions... they all implied longer (and sometimes deeper) stops than the initial plan. Not the reduction you get if you simply continue to compute an haldanian gaz load.

There are limits to these procedures (for one they are NOT validated and the limits are not well established) and there are situations for which the best is "get out of the water now, call for help, get on pure O2 and hope they come quickly enough to save you".

This is really amazing advice - to me anyway.

If I violated a required decompression (stops) and I was able to arrest my ascent (and felt little or no deleterious effects) and I had sufficient gas reserves, you can be damn sure I am not going to the surface and instead would put together a reasonable ascent strategy. This would probably involve a descent to a depth well below (maybe 20 feet) below the deepest decompression stop, hang for a minute and if I still feel OK, then slowly ascend to the first planned decompression stop. And then follow some kind of deco schedule that is practical for the situation.

To just go to the surface (when there is no compelling reason to) is not something I would do.

I have seen several instances where a diver makes it all the way to the surface and gets on the boat and is a little bent and just goes back down on nitrox and completely resolves all symptoms in 15 minutes or so. These anecdotal experiences lead me to believe that you probably have a little time if you screw up to attempt to remedy things. If a diver is paralyzed or incoherent, then the situation is different.

 

[dmaziuk]

Suunto's decision to not display what they believe to be an unacceptably erroneous ascent profile probably has its roots in their in-house decompression expertise and legal teams neither of which I feel particularly competent to comment on.

You are assuming that low-power Suuntos and Seikos can continue to work after you push them out of bounds. As far as anyone knows "folded RGBM" may be interpolating between a bunch of pre-computed tables and once you're off the table's edge, all it can do is show GAMOVER on its screen.

 

[David Carron]

You are assuming that low-power Suuntos and Seikos can continue to work after you push them out of bounds. As far as anyone knows "folded RGBM" may be interpolating between a bunch of pre-computed tables and once you're off the table's edge, all it can do is show GAMOVER on its screen.

It's not that much of an assumption. Weinke describes "folded RGBM" in quite considerable detail in his paper "RGBM in depth" from page 33 onwards and the calculations don't get any more complicated if the diver exceeds the ascent ceiling. It's quite possible, of course, that Suunto's implementation deviates from the original but there's no real reason to assume that it does.