In dive, manage having EAN programed into computer while diving air?

[flymolo]

It's all a matter of how close you get to the "M" value and how well your body handles DCS. Technically we all get DCS, its a matter of what we observe. I dive a very conservative gradient factor (35/70) and use that as my guide to keep me away from M. I watch my surf GF delta5 and other factors on my computer as a decision factor in where to be (depth) and when to end my dives in many situations because I have that liberty. Most of my dives are just "fun" dives. We may have a very high level plan but not done to a tech level of planning as things so easily change on our local weekly shore dives. We may play with a harbor seal @30ft, observe a 6 Gill shark at @120ft or spend extra time trying to get that perfect photograph of a GPO or wolf eel. We spend more time in events like this than you can really plan for. It's easy to say plan, but we all know the truth in dives like that. Yes you can average but that does not take into effect tissue loading the way a computer can. A computer gives a diver a better estimate of how close to M they are using all tissue compartments in the Bühlmann model. A table can't do that as pointed out above. A table can give you an idea but those table do push us toward M more than I like. I would love to have to do less guessing and have more real data to work from and where adjusting an air mix on the fly and having it recalculate the model would be great in "emergencies" like that. Computers already have the capability to recalculate TTS based on available gas or gas that is switched while in dive. This calculates TTS based on your GF/conservatism settings and what it takes to get to the surface. Those are fixed numbers. There is no reason a computer can not easily do that inverse over your profile. Even if its just a adjusted tts or ceiling icon/number vs an overall change of the data. I dove for years on tables without a computer. There's a reason we all use computers know and that flexibility is amazing. I just see this as a bit of a hole in computer design for the modern rec diver.

And that being said, knowing the rules of thumb listed above are great to know and will be used by me in situations like that. Any estimate is better than a none!

I don't mean to beat a dead horse here, but i thought your original question was roughly, "how do I devise a reasonably safe ascent plan on a dive where halfway through I realized the wrong gas was programmed into my computer". I think having the practical skill to come up with a "close enough" approximation for that, using tables, is ultimately more useful in the long run, than the new dive computer functionality you are suggesting.

 

[EFX]

Still think it would be a great feature to be able to recalculate your dive profile using an updated gas in dive. All the telemetry data is there so its just a matter of recalculating. Yes the option would need be fool proof to implement but I can see it being useful in situations like I had. We (most of us) dive a computer for a reason, we have the data so its already there.

[---- table jockeys should leave the room]
The dive data is there in the dive log. There may not be enough data points to give a finely tuned profile but I think it is feasible regardless of the legal (are there any?) aspects of doing this. You could simply select the new (proper) gas and tell the DC to recalculate. Once the calculation finishes the DC continues to calculate using the correct gas. Going from say, air to nitrox or mix is not a problem as you would end up with more NDL time or less deco time. The problem will be going from nitrox/mix to air where if you're late in the dive you find out you missed a deco stop or that deco is required and you've run down your gas. But still, it's better than nothing.
[---- table jockeys may come back to the room now]

 

[johndiver999]

the benefits of retroactively changing your mix while underwater and during a dive would have to be balanced by the increased complexity and the significant possibility that a diver might make another mistake and (while diving air) change his mix to nitrox on accident. It is not like it is a common situation and nobody has the user's manual underwater, so why the heck would a manufacturer want to deliberately allow all this hocus-pocus?

The dangers of punching in nitrox (when diving air) are minimized by the fact that the diver should probably just use all his air on a safety stop - assuming he is not confident that is not necessary. So the reasonable solution to this mistake ain't that complicated.

Allowing people to deliberately go the other way (change air to nitrox underwater) and then have the computer give them directions to skip a deco stop (when they have the gas for it) seems really risky from a liability standpoint for a recreational computer manufacturer.

 

[dmaziuk]

I thought you could not. Isn't the nitrogen loading in the fast tissues a diffusive process where the concentration is proportional to the pressure? So the rate is proportional to average depth but the nitrogen load is the integral of the rate and therefore not linear. All the models are systems of differential equations, there is no linearity in them that I am aware of.

The rate is log2 and integrating logarithmic functions is unpleasant enough already, but the load is proportional to time. You can go by average depth alone, but you could just as easily and likely just as accurately go by the phase of the moon.

 

[Blackfrogfeet]

Couple of options. Although a bit surprised you were that close to deco? What GFs were you running? You can memorize any air table but really all you need to remember is the 60ft for 50mins and 100ft for 20mins and you can extrapolate from those setpoints. 15-20years ago the Navy tables had 60ft for 60mins and that's what we all planned around (with analog depth gauges)

30ft = indefinite
40 = 120
50 = 75
60 = 50 (another setpoint but in days of yore this was an hour)
70 = 35 (NAUI says 40 but its easier to just add 5mins for each 10ft shallower than 100ft)
80 = 30
90 = 25
100 = 20 (setpoint)
110 = 15mins (5mins less than at 100ft)

Deeper maybe skip the air NDLs.

If you look at GUE Fundamentals materials from 5+ years ago it actually includes the 30m/:20min set point for air diving with average depth/ on the fly planning for when 32% is not an available gas. Pretty sure it uses 40min at 18m though instead of 50, just because it's easier to work with uniform 5min increments between depths.

 

[Blackcrusader]

the benefits of retroactively changing your mix while underwater and during a dive would have to be balanced by the increased complexity and the significant possibility that a diver might make another mistake and (while diving air) change his mix to nitrox on accident. It is not like it is a common situation and nobody has the user's manual underwater, so why the heck would a manufacturer want to deliberately allow all this hocus-pocus?

The dangers of punching in nitrox (when diving air) are minimized by the fact that the diver should probably just use all his air on a safety stop - assuming he is not confident that is not necessary. So the reasonable solution to this mistake ain't that complicated.

Allowing people to deliberately go the other way (change air to nitrox underwater) and then have the computer give them directions to skip a deco stop (when they have the gas for it) seems really risky from a liability standpoint for a recreational computer manufacturer.

Diver was on 21% air but did not change his computer from 30% nitrox. So he would be changing from 30% to 21% not the other way round. Also the diver could see his dive buddies dive computer and see it's reading as they would have been together at depth and time for the dive. I had a diver do this when he did the same as I was on 21% air and was not too close to NDL.
He did an extended safety stop and didn't dive again till the next day.

 

[HKGuns]

Would that mean scanning/reading the gas analysis tape attached to your cylinder?

The point of diving mixed gases is you must do the correct analysis and that goes onto the cylinder. As part of the pre-dive checking (like does it work) you read the analysis tape and check your computer.

Alas, it's not being pedantic, the penalty for getting that wrong could be death!

Clearly you didn't read the entire thread or you'd realize your comment is totally out of context with the subject.

 

[Wibble]

Clearly you didn't read the entire thread or you'd realize your comment is totally out of context with the subject.

I know. My point is that you *must* analyse your gas and mark up the cylinders.

Lovely to have tech to tell you this, but why not just do best/safe/mandated practices?

 

[Wibble]

Also, never miss the opportunity to kick Suunto, they won't allow you to change the gas whilst diving (e.g. if you've got it programmed for 32%, you can't change it to 21%) unless you get the computer out of the water for a few minutes.

 

[J-Vo]

I would probably just use ratios. 21% vs 30% is ~13% change in nitrogen. Round to 15% for safety and easy math. Subtract that form your preferred GF High. 70 -15 = 55% GF High. Either program that into your Shearwater or just ride the GF99 data to the surface.