nitrox MOD/TOD

[TSandM]

Terms are often adopted and used in common parlance with their meanings blurred and rendered imprecise. There is a danger in that, since, if one does not know the derivation of the term or its original definition, it may be misused, or the entire concept may be misunderstood.

A case in point: When I was in OW, I asked the question, "Why does the residual nitrogen time get shorter as the depth of the proposed second dive gets deeper?" Nobody could answer the question. It was bumped up the chain of command to the senior instructor, who told me, "It has to do with the fact that the compartments load at different speeds." That answer wasn't very satisfying, but it was what I got.

Turns out, if you know where the term "residual nitrogen time" came from and how it was originally defined, the answer to my question is trivial . . . and further, it opens the door to understanding why George Irvine says you should do the shallow dive first. Many things can become clear if you understand the definitions of terms.

 

[victor]

Given the trend to make everything safer. The ndc times for many of the modern computers appear to be much less than they were the profile used my my sherwood source.
I have read on a number of occasions that if you want to extend your bottom time just dial in a higher nitrox number when diving 21%.
For me that is just wrong. However it would be interesting to compare a modern computer against mine on a 33m dive with a gradual ascent up the reef from that depth.
The only advantage of NITROX seem to be that you can stay deep for longer. However thare is an increased risk in that exceeding your depth can trigger seizures with little or no warning.
To avoid this they are decreasing the maximum recommended depth for a mix and reducing the time you can be at these depths.

Ergo we are back where we started.

For me the dangers inherent with a mix that limits how deep you can go but allows you to stay close to that depth for longer may outweigh any advantage given.

After reading some of the posts that came in while I was away. I must look into this further before I spend some of my hard earned money on getting a nitrox certification, new computer etc.

 

[H2Andy]

I have read on a number of occasions that if you want to extend your bottom time just dial in a higher nitrox number when diving 21%.

i'd never do that.

i have actually felt Type I DCS (pain only) simply by speeding up my ascent
(impatience). once i went back to slow-and-easy ascents, pain went away.

if simply speeding up your ascent can do that, i'd hate to think what diving
with air but dialing in 32% will do to you.

 

[HowardE]

The only advantage of NITROX seem to be that you can stay deep for longer. However thare is an increased risk in that exceeding your depth can trigger seizures with little or no warning.
To avoid this they are decreasing the maximum recommended depth for a mix and reducing the time you can be at these depths.

Ergo we are back where we started.

Sorry - this argument holds little water...

There are places (where I dive frequently) where there is a hard bottom at 65'. Nitrox 36% or as high as 40% for rec divers - is a perfectly safe mix to use at that depth, and you CAN NOT exceed an MOD at 1.4 or 1.6 whatever you want to call the MOD, and Nitrox useage greatly increases bottom time before NDL.

 

[Thalassamania]

I am glad that after a night's sleep H2Andy understands that “term of art” links MOD 1.6 atm ppO2. The manual does not permit a lower MOD, but rather clumsily confuses things. Efforts will be bent to correct this oversight, which was not noted before because most everyone reading it knew what was meant.

No one is suggesting that NOAA does standards for recreational diving, but they defined the term. I am not arguing over what the maximum ppO2 exposure should be, I am just suggesting that it creates confusion to co-op an existing term that is quite precisely defined (and is not a variable for a given mix) and redefine it on the fly. This makes for sloppy usage in a critical, potentially fatal area.

I have no problem with anyone doing whatever they want in the privacy of their own mind (home, boat, drysuit, bedroom etc.). All I was trying to say is that unless we all accept clear definitions of terms we have trouble communicating clearly. That is why I applaud the idea of a TOD as opposed to an MOD. MOD is the “edge of the cliff,” TOD is how close to the edge you are willing to stand.

MOD is something that is on the bottle as a warning: “DANGER - GO NO DEEPER THAN THIS!” It has nothing directly to do with the planned dive, except as an absolute limit. A TOD (or whatever you decide eventually to call it) is part of your dive plan. These are very different concepts, based on the same math, which are best kept clear and distinct.

 

[jonnythan]

The only advantage of NITROX seem to be that you can stay deep for longer. However thare is an increased risk in that exceeding your depth can trigger seizures with little or no warning.
To avoid this they are decreasing the maximum recommended depth for a mix and reducing the time you can be at these depths.

Ergo we are back where we started.

This is not the case at all.

1.4 is the generally accepted working limit. 1.4 gets you *deep* with Nitrox. For instance, on EAN32 you have to break 110 feet to get to a ppO2 of 1.4.

Nitrox isn't a gas for deep diving. It's a gas for recreational depths. It gets you a ton more bottom time in the 60-100 foot range, as evidenced by my post. Two 40 minute dives versus a 30 minute and 25 minute. Nowhere close to any danger range.

Some people like to dive with "best mix" nitrox to absolutely maximize their bottom time. They will actually choose the mix based on maximum planned depth. That's OK, but it does eliminate some flexibility in dive planning after your tanks are filled. It's still a perfectly acceptable method of course, but by no means the only way to do things.

EAN32, for instance, is perfectly applicable to all diving above 110 feet. It provides dramatic increases in allowable bottom time in all recreational ranges, particularly the 60-100 foot range where most recreational dives occur.

Take a Nitrox class.. it might open your eyes.

 

[PerroneFord]

I do best mix sometimes but not to extend bottom times. Usually just because I want to decrease nitrogen loading. My surface intervals are about 1-1.5 hours. Bottom times usually in the 40 minute range. Since I am doing freshwater springs, I have a hard bottom to work with and it's always less than 100ft. So sometimes I'll take EAN32, and sometimes I'll take best mix.

Both work great. And I agree with Jonnythan, take the class. I have done only one dive on air after getting my nitrox card.

 

[lowwall]

In point of fact, if one presents a hypothesis and it can be shown to be correct 95% of the time then that's what modern science calls a fact.

I know this is off the main topic, but it is important enough that it needs to be addressed.

Hypotheses have to be right 100% of the time to be considered facts. It only takes one verified observation to disprove or force the modification of a hypothesis. Once a hypothesis has survived a number of challenges for a long enough time, it will be generally acknowledged as a theory.

I think you are confusing hypotheses and confidence intervals. The latter is a statistical calculation of the likelihood that the results of an experiment were due to something other than chance. 95% (i.e. 19 times out of 20 you are witnessing something real, rather than random) is a common requirement for a finding to be considered true, although many types of experiments require much higher confidence levels.

This is a simplification, of course.

Back to the original question. It makes real sense to me that the Maximum should actually be a maximum and another term (working, planned, target,???) for the deepest you expect to go. If the term MOD has become too devalued to use in this way, you could always adopt terminology from aviation that I find quite compelling. Call it DNE - for Depth, Never Exceed.

 

[pickens_46929]

Generally, I plan 1.4 and only use 1.6 for deco at rest.

This is what I was taught in my recent TDI Adv. Nitrox / Deco procedures class.

Still provides wiggle room if there is some trouble... Imaging planning for a max. PO2 of 1.6 during the working portion of the dive and something happens....

 

[Thalassamania]

I'm not confusing them, but I'll grant you I'm using the word in terms of null vs. alternative hypothesis which are, as you no doubt know, statistical constructs. And since I'm basically a biologist, I'll live with 90%<G>.

DNE, that works for me and I'll use it from here on out (got to make a quick global change in the soon to be released Do It Correctly [DIC] Diving Manual) .

I'll let you know how the NOAA folks react.