Forgot to set computer nitrox mix, what a PIA

[TSandM]

Many of us learn a method of figuring deco that doesn't involve a computer . . . but often dive a computer for a while after learning it, before we feel confident enough to work without a backup. The inciting event that makes most of us put the computer in gauge mode is precisely what happened to the OP -- a computer inadvertently set on 21% when one is diving 32. Yes, if you do the time the computer asks for, you can clear it -- but depending on where you are and what you are doing, the deco obligation can grow to be quite ridiculous, especially if you are using a computer that gives you no credit for time spent below the ceiling.

To the OP: I have read the whole thread, but don't remember if your computer would still give you depth and time . . . our Suuntos will, although they will not give deco information. If you have depth and time, you have tools for figuring your allowable no-deco limits.

 

[KeithG]

Plan: Immediately (upon determining that something has gone wonky) ascend slowly to 15-20ft and suck down all but the last couple of hundred psi.

Saw this suggestion recently from a Suunto manual. Not sure it passes the sniff test.

This "rule" is just a plain bad idea. Why would I spend 2 hours hanging at 15 feet?

Assuming you are doing a recreational dive, then you have a buddy. They have not gone into deco (unless both of your computers failed on the same dive...). Just calmly end your dive NOW and surface normally.

If you are not doing a recreational dive, then you will have proper backups. A dive computer failure is no big deal. Follow your plan.

 

[kr2y5]

Why am I not able to fix the nitrox mix setting?

The demand likely does not justify the cost of implementing such a feature, in terms of additional hardware, complexity, risk of bugs, etc., the computer would have to re-compute all the tissues based on a saved dive profile in memory after retroactively modifying dive parameters, and most contemporary dive computers probably do not even have anywhere close to the computational power to do so...

A more interesting question would be, whether one could see a marriage of things like V-Planner and Subsurface, whereby you could download your actual dive profile to a mobile device, have your smart phone or your laptop crunch all the numbers by treating it just as if it would treat a multi-level dive entered by hand, and to have it plan a repetitive dive for you based on the actual exposure on the first dive. A surface interval could be a great time to analyze the first dive, and to make small adjustments to the repetitive dive plan, available gas permitting. The dive planning software even on modern tech dive computers is pretty unusable, but modern cell phones and tablets have more than enough horse power to do all of this, and then some... I would be very much interested in purchasing such software, if anyone were to invest the time to implement it.

 

[boulderjohn]

I am not sure I understand precisely what the OP did during his ascent and safety stop. It is possible that he did something similar to something I did a couple of years ago quite intentionally.

I arrived in Florida and set out for the first dive of my vacation, a fairly routine recreatioal dive that should have presented no issues whatsoever for NDLs. I was planning to use my Shearwater Predator, and I set the mix appropriately. For some reason that I don't remember, I was not using my normal spg but was instead using a Suunto Cobra, an air integrated computer, instead for that purpose. I dutifully set that for the correct mix I was breathing. About half way through the dive I checked my air pressure and happened to notice that the Suunto was giving me a ridiculously short amount of remaining NDL time. I looked at the Shearwater and saw I was never going to get near the NDL. Then I realized what the problem was. The last time I had used that Cobra was quite some time before, in the Denver area. The Suunto does not correct for altitude automatically, so it was still set at its highest altitude setting. I realized that if I continued the planned dive, the computer was going to be well into deco. I continued the dive, knowing that I would really not be anywhere close to deco. As I was doing a safety stop, the Shearwater was telling me that I didn't have a glimmer of a problem--I could ascend any time. The Suunto was telling me I had to stay 10 minutes. I was at the time that the boat crew wanted us to ascend. Well, I knew that if I ascended, the Suunto would go into error mode, but what did I care? In error mode I still got what I wanted from it--PSI. I was following the Shearwater anyway, so up I went.

I wonder if much the same thing happened. The OP looked at the computer and thought, "Oh, crap, I forgot to set it for nitrox." He may have then thought that because he had done that identical dive so many times before, he knew he was not in deco and could safely ascend. He just didn't know he would lose computer functions when he did so.

Now, that still leaves a lot of room for improvement, but if that is what happened, I feel a little better about the decision to ascend when the computer was telling him not to.

 

[MrChen]

Many of us learn a method of figuring deco that doesn't involve a computer . . . but often dive a computer for a while after learning it, before we feel confident enough to work without a backup. The inciting event that makes most of us put the computer in gauge mode is precisely what happened to the OP -- a computer inadvertently set on 21% when one is diving 32. Yes, if you do the time the computer asks for, you can clear it -- but depending on where you are and what you are doing, the deco obligation can grow to be quite ridiculous, especially if you are using a computer that gives you no credit for time spent below the ceiling.

To the OP: I have read the whole thread, but don't remember if your computer would still give you depth and time . . . our Suuntos will, although they will not give deco information. If you have depth and time, you have tools for figuring your allowable no-deco limits.

Yes, it still provided bottom time and depth.

---------- Post added July 19th, 2014 at 05:15 PM ----------

There are some assumptions that I was breathing 32 which is inaccurate. Whether I need to "prove" something or not is arbitrary, I'm a grown adult and I'm putting my experience out there for judgement and feedback. I also didn't realize I had a deco obligation on the surface, I ended my first dive shortly after realizing what happened at depth. I cycled through the menus and that's when I realized it was on air. I made conscience decisions, whether the best ones or not, they made sense to me at the time in a situation I had never come across before.

What I've learned from this is if it happens again, I'll do my deco "obligation" even though it is a false positive, I'll pull out my handy dandy cell phone during the SI to run my table dive planner, and dive the tables on the 2nd dive. The computer won't be accurate anyways and will probably go into deco again even with the correct mix on the 2nd dive since it has over calculated my nitrogen loading from the first dive.

The above makes sense, I get it.

One suggestion given to me is to program one of my computers for air and another for my proper mix and dive the air profile. Once the air computer goes into deco, practice a deco ascent and stop. I'll probably do this on the 2nd dive of a 2 tank trip. I like lowviz's recommendation on practicing an oral inflation. I'll throw this in every once in awhile.

 

[lowviz]

Saw this suggestion recently from a Suunto manual. Not sure it passes the sniff test.

This "rule" is just a plain bad idea. Why would I spend 2 hours hanging at 15 feet? ....

It isn't a "bad" idea when you have no idea how far you are into deco. It is also easy to remember and works at any time and depth.

I purposely chose it because it is one of the worst (crude and simple) possible "beneficial" plans for handling deco. Now that the OP has this, he may see the real advantage of obtaining more training with respect to unintentional deco in a recreational setting.

 

[MrChen]

I wonder if much the same thing happened. The OP looked at the computer and thought, "Oh, crap, I forgot to set it for nitrox." He may have then thought that because he had done that identical dive so many times before, he knew he was not in deco and could safely ascend. He just didn't know he would lose computer functions when he did so.

Now, that still leaves a lot of room for improvement, but if that is what happened, I feel a little better about the decision to ascend when the computer was telling him not to.

It's exactly what happened.

Now the kicker is I jumped in for a 2nd dive without planning it.

---------- Post added July 19th, 2014 at 05:37 PM ----------

Now that the OP has this, he may see the real advantage of obtaining more training with respect to unintentional deco in a recreational setting.

I have many plans to continue training. SSI Rescue and SDI Solo are on my todo next list. I'll knock out Rescue soon and plan on taking the Solo class this winter.

 

[BRT]

A computer is only as good as the inputs. Garbage in, garbage out. It's not that the computer hates you and wants to get even for not following the rules. You have put it in a situation that it cannot calculate a safe way out of. So at that point it must leave you on your own with just depth and time information. What else could it do? Guess? It just doesn't have a workable solution.

 

[lowviz]

...//... I'll knock out Rescue soon and plan on taking the Solo class this winter.

Very good. Solo will be the first formal course where they really stress that you can get hurt diving. After Solo, consider TDI's Decompression Procedures, but first find an instructor that is all about the kind of diving that you do. No dives are required for this course. Purely informational, but you do walk away with your own deco tables and a lot of truly useful information that you can incorporate into your recreational diving. And I don't mean adding deco to your rec diving, it's more like always knowing where you are and how to get home.

After that, Advanced Nitrox.

-all suggestions, nothing more than that.

 

[KeithG]

It isn't a "bad" idea when you have no idea how far you are into deco. It is also easy to remember and works at any time and depth.

sorry, i disagree. if you have no deco idea AND this is your backup plan, then you failed before you hit the water. you have not followed your training.

as a rec diver, you will not go into deco. so no need for this silly procedure. rec & deco = fail. rec & computer fail = end dive calmly now. nothing special required.

if you are in fact a deco diver, then you will already have an agreed upon valid backup plan. it will NOT involve a "random length" safety/deco stop at 15 feet.

if you get into a situation where you have no idea how far you are in deco...well that is just stupid.