What's the deal with these numbers?

[PatW]

Something that has not been mentioned are the restrictions of the buddy system. The idea is you have to have enough air for yourself to get down to 130' and get back up without decompression. You also need enough air to get you AND YOUR BUDDY back up should there be an O ring blow out or something else. For recreational diving, you do not want to get into a situation where a mishap is nearly 100% fatal.

Also, the 130 foot limit serves several purposes. It gives recreational divers a limit. Anyone who exceeds that limit should, at least, dimly perceive that going further demands extensive preparation.

Can people safely dive below 130. Sure they can, but I would think dives deeper than that would be just down, touch and go back up. That requires more discipline. Also the margin of error narrpws. When you get to having little or no margin of error, you are getting into technical diving.

 

[Hatul]

The so called "recreational depth" is just an arbitrary definition used by SOME agencies. It's not a law, its not a rule and its far from universal. The reasons they use ranging from narcosis, air consumption at depth but mainly as these agencies tend to train no-decompression diving and at 40m the bottom time is so short to go deeper as no stop would be a pointless bounce dive.

Other agencies just have different recommended max depth which increases as you go through the grades in a nice sensible progression and don't make arbitrary "rules" about a maximum depth.

BSAC for example use 20m for entry level, 35m for next level and 50m for the next level.

As for the term "recreational" i take that to mean i dive someone is doing for fun, as a hobby regardless of complexity or depth. "Commercial" is a dive done for money.

So i could have a 60m recreational "fun" dive and a 6m "commercial" teaching dive.

I think the term "recreational" as commonly used has a narrower meaning than that. It also means open water, single tank, and air or nitrox. Other types of diving like very deep or overhead barrier would fall into a technical dive.

Adam

 

[Bonairetrip]

Where physics meets physiology, inside the human body, is a very variable and analytically messy place.

The tables look so clear: if you are one this side of the number (your NDL time at a given depth), you are golden. If you are on the other side of the same number, you are screwed. The same in applying the MOD of a given gas mix. The same in calculating the amount of gas you will actually use. All of these are neat formulas or the result of interesting algorithms.

TS&M did a nice job describing how the models are validated. That said, they are not validated to you personally. So, in one sense, the OP is correct that the tables might not be a perfect fit for him. He might be an exception. The problem for the OP (and for all of us) is we generally have no way of knowing that [1, 2]. And the bigger problem: even if we did know, we would need to know for every dive. Yes, there can be much variation with an individual. [3]

Pehaps a lame attempt at an overly complicated analogy is in order. You are given a new car to drive (and you are a new driver). You are driving across the Baja (partly paved; partly not). As a new driver, you are told to go no faster than 60. Why? Going faster increases your risk of running out gas (mileage varies by conditions and you have limited refueling points). Going faster also gives you less time to recognize and respond to risks. You then work up to going 100. You have started to understand how your vehicle responds at speed. You learn about tunnel vision and how to manage that risk. You learn more about fuel planning and usage. You then work up to 130 where tunnel vision is an even bigger risk.

You also hear that the front wheels of your car might fly off at high speeds so people say you should never go faster than 218 mph. And then you learn that a guy dove that same car 509 mph and the wheels stayed on. (Of course you didn't hear of the drivers who had wheel issues at 219 mph, or 327 mph. Or for that matter at 195 MPH).

The bottom line: Treat limits as limits and not as targets. Diving is a sport with risks. The tables, rules and guidances are all there for a reason: to help keep you safe. Yes, there are those who exceed NDL times without symptoms. Yes, there are those who follow every rule and are symptomatic. But that doesn't mean you should ignore limits -- especially when your prognosis if certain events occur (i.e., CNS hit, O2 tox at depth).

As you gain more knowledge, the reasons for the guidance will become clearer. It is then up to you to decide if/when/how you will challenge them (hopefully safely). For example, I had hundreds of dives before I did my AOW. I don't think I was any safer after my AOW than before. But I did have a substitute for the card: lots of dives and lots of informal learning.

[1] An interesting exception to this is the issue of a PFO. We can surmise that the tables are overly liberal for persons with a PFO.
[2] I have never seen research that validates increased diving adapts the body in a meaningful way to accelerate N2 off gassing. Fitness, likely has an effect on risk.
[3] Too many reasons to list (including unknown). Dehydration is probably at the top of a long list.

 

[MaxBottomtime]

Another thought is your SAC. My dive buddy will use a steel 100 while I use a 119. Making the same dive and staying together, she will almost always surface with a few hundred psi more than I. Although the tables and computers say we have the same nitrogen levels, she has breathed a lot less nitrogen than I.

 

[dannobee]

IIRC, the guy who did the 509 ft dive on air, Dan Manion, recalls NOTHING of that dive and was almost completely incapacitated by narcosis.

 

[elmer fudd]

Another thought is your SAC. My dive buddy will use a steel 100 while I use a 119. Making the same dive and staying together, she will almost always surface with a few hundred psi more than I. Although the tables and computers say we have the same nitrogen levels, she has breathed a lot less nitrogen than I.

I'm guessing you're also physically larger than your buddy though. So presumably your body can handle more nitrogen.

 

[Rhone Man]

Another thought is your SAC. My dive buddy will use a steel 100 while I use a 119. Making the same dive and staying together, she will almost always surface with a few hundred psi more than I. Although the tables and computers say we have the same nitrogen levels, she has breathed a lot less nitrogen than I.

Just remember that gas breathed does not equal gas absorbed by your body. The amount of gas dissolved in your tissues is determined by pressure and time of exposure. In theory there is a microscopic difference because when you absorb part of the N2 in a lung full of air, the ppN2 drops by a tiny amount, so reduces the rate of absorbtion prior to exhaling - but to make any serious difference you would probably have to hold your breath for an impossible length of time. Otherwise if you continually breathe the same mix at the same depth, you are going to have pretty much the same time/dose exposure.

 

[vjanelle]

Another thought is your SAC. My dive buddy will use a steel 100 while I use a 119. Making the same dive and staying together, she will almost always surface with a few hundred psi more than I. Although the tables and computers say we have the same nitrogen levels, she has breathed a lot less nitrogen than I.

The laws of partial pressures would tend to disagree with you there. It's not a volume of nitrogen absorbed that's being estimated, it's the rate various tissues absorb and release stored nitrogen that'll may or may not have reached equilibrium with the surrounding environment.

The difference in gas amounts between you two is because of her lowered lung tidal volume(ie, smaller than you), and effort required to move mass through the water column... Usually at least when comparing men vs women(IMHOBPICO). You're still going to be bound by the same time limits, and the chances of DCS are still the same.

 

[Hammerhead]

The recreational limit for scuba diving is 130ft. But oxygen doesn't pose a substantive toxicity issue until 218ft. BUT the record for compressed-air scuba diving is 509ft! So WTF?!?!

I guess my point is I wonder how ambiguous these numbers are. And it's almost impossible to know just how ambiguous they are because virtually nobody pushes these so-called limits

I think you'll find that a couple of people who've posted so far have pushed 218ft on a single (air) tank. I'm not proud of it, it's just something I've done, but I'd fall into that category.

I believe Nemrod's mentioned some deep air stuff on previous threads as well.

 

[SailNaked]

you can dive past 130 you just need the Dive officer of the day to approve it or be a combat dive.

our limits and rules go all the way back to the navy. they invented the math and the "rules" You are free to do as you like. the numbers work mostly, and you can follow them at your own risk.

Enjoy everything you can, live while you can, there are no guarantees in life or death.