setting to nitrox to reduce over conservatism on dive computers

[NAM001]

Is that not what Rick was saying. When you put in a fo2 you are switching to nitrox mode. I asked earlier what the differences were when a fo2 was put in as opposed to air. All my computers allow me to go to air after a nitrox dive as far as i know, I will have to recheck to verify this.

Mine dont have any "nitrox and air modes", you set the o2 percentage and off you go and it can be set higher or lower at any point you want. It also allow gas switching during theh dive...

---------- Post added June 28th, 2014 at 01:20 PM ----------

Perhaps it is again timne to reask the question, What is the difference, as far as the computer goes, in diving with air or diving with 21%? Is it only a display difference or are there other things that change???

 

[DepthCharge]

Ah, I see... It was a rhetorical question. And I thought you were inquiring in earnest. May I suggest wearing a PFD at all times as you can also drown in a mud puddle.

Do you use V planner for a 10 foot, 10 minute dive or do you accept that the risk does not equal the effort? If the answer is the latter you understand NDL's. These are the limits in which the effort of doing decompression stops does not equal the risk of incurring DCS.

I'm not sure we are understanding each other. There is no such thing as a NO DECO DIVE is the point that I'm making. Your deco obligation is equal to your ascent rate. You do a dive to the NDL (unlike your example, which is not to the limit), come up to fast and you stand a good chance of taking a hit.
Later,
John

 

[kelemvor]

Perhaps it is again timne to reask the question, What is the difference, as far as the computer goes, in diving with air or diving with 21%? Is it only a display difference or are there other things that change???

Why does everyone seem to assume there is a difference at all?

 

[NAM001]

Its a good point Kel. I myself am curious about it now. My thought was that if you were using air, the puter took the worst case situation (new diver) and ran more conservative than if you were doing nitrox assuming that you are more experienced and need less safety factor. I always assumed that most of the differences were display related. like no OBG now I noticed that in my petrel the default conservatism is different than in the predator. 35/85 vs 30/80 and then when you go into rec mode on the petrel you get 3 generic settings like low med high.

In more direct answer to your comment. The assumption, if there is one, lies in 2 computers giving different results, and that being the product of level of conservatism as opposed to using different algorithms.

Why does everyone seem to assume there is a difference at all?

 

[DaleC]

I'm not sure we are understanding each other. There is no such thing as a NO DECO DIVE is the point that I'm making. Your deco obligation is equal to your ascent rate. You do a dive to the NDL (unlike your example, which is not to the limit), come up to fast and you stand a good chance of taking a hit.
Later,
John

You didn't make a point, you asked a question.. but I do understand.

However, saying there is no such thing as a "no deco" dive doesn't really tell us anything at all. In the application of theory to practical application we have to make some differentiation, or come to some conclusions, so we can move forward to action. That's the reason behind the creation of tables. Research was done to determine the relatively safe limits one can dive (time/depth) without incurring a "decompression obligation" and the amount of decompression time needed at specific depths, if those limits are surpassed. They did this so people could move out of the laboratory and begin working underwater.

According to the PADI tables (for example), people can surface directly if they do not approach the black mandatory time limits, and will need to do formal decompression stops if they surpass them. The ascent rates are clearly defined. These numbers represent PADI's (DSAT's) practical application of decompression theory as applied to the recreational dive setting.

This does not mean one shouldn't do safety stops for non decompression dives, they are a good strategy for avoiding rapid ascent rates, over expansion injuries and sub clinical DCS. But it's important to know that a direct ascent is available if the situation requires it. the safety of recreational diving is predicated on the concept of direct access to the surface and is why basic divers have many general rules of thumb:
Any diver may call any dive at any time.
No real or virtual overhead conditions.
Avoiding deep depths or violating NDL's.
Quick release weights.

A couple of agencies have created something called "minimum deco", which is the inclusion technical decompression procedures to the recreational setting, but this is really an artificial construct developed to address their over all philosophical approach to diving and not any real need for extraordinary offgassing in that realm.

 

[Tigerman]

Is that not what Rick was saying. When you put in a fo2 you are switching to nitrox mode. I asked earlier what the differences were when a fo2 was put in as opposed to air. All my computers allow me to go to air after a nitrox dive as far as i know, I will have to recheck to verify this.




---------- Post added June 28th, 2014 at 01:20 PM ----------

Perhaps it is again timne to reask the question, What is the difference, as far as the computer goes, in diving with air or diving with 21%? Is it only a display difference or are there other things that change???

Theres no difference what so ever. Not even on the display. The computer shows the exact same information regardless of my nitrox mix.. (po2, depth, time, ndl being the most notable ones).
Let me rephrase that, the only difference is how much time I have before I get a deco obligation...

 

[Searcaigh]

Perhaps it is again timne to reask the question, What is the difference, as far as the computer goes, in diving with air or diving with 21%? Is it only a display difference or are there other things that change???

I never use my Suuntos on AIR mode, always NITROX and if diving air will change it to 21%, there should not be any difference

 

[Jaydubya]

In my IDC I noticed the new PADI material used the term no stop limit instead of no decompression limit. This is a step in the right direction. Of course decompression happens on every dive whether we have a mandatory stop or not.

If you are starting to get to the point of exceeding the no stop limit on your chosen computer or set of tables, it's time to get some decompression training.

Most of the conservatism differences in dive computers seems to surface on repetitive diving- and repetitive diving is precisely the time when you need more conservatism. Not less. I would not recommend lying to the computer about your gas mixture.


Sent from my iPhone using Tapatalk

 

[kr2y5]

I put in a few profiles into V-Planner, here is an example that may help. Suppose you dive with a computer that uses VPM/B +2, you make 45 min. dives to 60 feet on air. The algorithm suggests less than 1 minute deco at 20 feet. If you ascend slower or do safety stops, perhaps your computer may qualify these as dives within NDL since it may never suggest a deco stop. Now, you switch to a computer that uses Buhlmann ZHL-16C with 30/70 gradient factor. If you did the same profile on that computer, it would suggest 10 min. total deco, so in the (misguided) attempt to "fix" its excessive conservatism, you will change the mix to 29% Nitrox to "restore" the profiles generated by the previous computer. So far, so good... Now, suppose you happen to stay just 5 feet deeper (and you can simulate the same for staying 5 minutes longer, etc.). Assuming that I did not make any typos anywhere and punched the right buttons, the new "conservative" computer on ZHL-16C with GF 30/70 will suggest you less than 5 minutes of deco at 20 feet on your 29% mix, and again, depending on the way you ascend, or if you do extended 5-minute safety stops, perhaps it may not even dawn on you that you are doing deco dives. In comparison, the old "good" non-conservative computer on VPM/B +2 on air would have suggested almost 9 min. of deco. So, by following your "fixed" conservative computer, you will be getting out of the water in half the deco time of what your old "good" computer was suggesting. Clearly, in attemping to "fix" one profile, you ended up making your "what if" scenario unsafe. Hopefuly, this example helps to see why fiddling with a single parameter, such as % oxygen in the mix, produces badly distorted results.

Also, I would not use the word "novel" to describe these diving practices... this word carries a positive sentiment, whereas the majority of people responding to this thread clearly implied that this is an accident waiting to happen...

 

[dmoore19]

I am not sure why a person would buy a computer that doesn't match up or doesn't give you the results you want. Why would you want to trick it and use it in a way it wasn't designed to work. Simple solution, buy a computer that uses the algorithm you prefer. Otherwise what good is it? I can use a big screwdriver as a pry bar but it isn't the right tool for the job.