A question about the Partial Pressure of Oxygen

[Doppler]

Not sure there is an agreed upon definition for the boundary between rec and tech diving....

Actually there is, for the US (except Louisiana) at any rate. But it may confuse the issue for you since rec and tech are the same thing. A definition was used in a court trial several years ago and from an agency perspective (certainly the one I worked at) that definition sticks.

Recreational diving is what you and I do... we dive for fun. Recreational diving includes technical and sport diving... watching pretty fishes and diving in cave to 100 metres on a rebreather are both forms of recreational diving.

Other forms of diving include: Military Diving (for example Royal Navy Clearance Diver), Commercial Diver (underwater welder; someone who works on oil rigs and whose working conditions are subject to the scrutiny of a quasi government organization such as OSHA) and Scientific or Research diving (underwater archaeologist associated with a museum or university for example).

I believe you mean the definition between sport and technical diving... there are many definitions trying to tweeze those two apart. I've taught technical programs for almost 20 years and the boundary is more clouded now than it used to be... Nitrox used to be tech!

Doing open water decompression dives on air/nitrox is probably not considered tech diving (I certainly wouldn't consider it so).

I would, but then that's me and what the heck do I know.

 

[Doppler]

1. How is the metabolic use of the oxygen a factor? I metabolize the oxygen I breathe all day long, but the 0.21 atmospheres of oxygen available is quite adequate.

I was talking about a sealed system... a CCR... which is the only technology available that comes close to achieving what you want.

With a workable loop volume, the quantity of oxygen available to you will be quickly diminished. No CCR or SCR is able to operate successfully at depth to maintain 0.21 bar of oxygen pressure since the diver will metabolize enough rapidly enough (even at an average rate of 1.5 litres a minute) to send any automatic solenoid into spasms.

2. In the hypothetical apparatus in my OP, the 0.21 atmospheres of oxygen would be maintained during ascent.

How? I am unaware of any available technology which can maintain this setpoint and adequately support oxygen use, the drop in ambient pressure AND control partial pressure within 0.1 bar... If your machine is set to 0.21, that gives an operational slop at best case of 0.11 and 0.31... I would not recommend the lowest at anytime, especially underwater.

You asked why is this not done... the answer is that it is neither possible nor practical and would be far from best practice since I suspect the first couple of test divers would likely pass out from hypoxia... "there goes our funding!"

 

[FPDocMatt]

How? I am unaware of any available technology which can maintain this setpoint and adequately support oxygen use, the drop in ambient pressure AND control partial pressure within 0.1 bar... If your machine is set to 0.21, that gives an operational slop at best case of 0.11 and 0.31... I would not recommend the lowest at anytime, especially underwater.

Right, I understand now. Others have clarified all this for me, but I appreciate the refresher.

I was wondering why anyone would need more than 0.21 bar of O2, but it isn't the need for a higher bar of O2 but rather other considerations--such as decreasing the ppN2, or the fact that you consume O2 in the rebreather necessarily dropping your ppO2 as you breathe, etc.

Anyway, I just received my new copy of Deco For Divers, and am enjoying working my way through it. Whoever suggested it, thanks, it's a fabulous book!

 

[feitr]

I would, but then that's me and what the heck do I know.

Yea probably a grey area - I don't really think clocking up a few minutes at 3m because you went somewhat marginally over ndl is tech but if you were diving doubles and had 30 minutes of deco stops that is probably getting closer. As you said, when it came to the difficulties of definitions i was more referring to the boundaries of sport and recreational tech diving so there will always be smudged boundaries in what slots in where.

 

[DevonDiver]

I don't really think clocking up a few minutes at 3m because you went somewhat marginally over ndl is tech but if you were diving doubles and had 30 minutes of deco stops that is probably getting closer.

Most of the major agencies define recreational diving in the context of retaining an ability to immediately ascend to the surface. Hence, diving "within recreational no-deco limits". That's good enough for me.

Exceeding an NDL is a game changer. It's a very easy threshold to understand. Beyond the NDL, the diver no longer has the capacity to immediately ascend to the surface, should a problem arise. The consequence of this is that their planning and conduct of the dive must now mitigate any foreseeable risks to ensure that decompression can be completed. Recreational divers don't face those risks and don't have to mitigate them through training, preparation and equipment. Mitigating those risks adds a much higher degree of technicality to the diving planning and management.

When it comes to definitions, it helps if you focus on the appropriate factor. A dive isn't technical or recreational.... it's a dive. The approach to the dive is what varies. The diver, and the skills, drills and procedures they employ is what differs.

Yes,... a recreational diver can exceed an NDL, using a single cylinder, on air, with no redundancies, no gas management, no pre-practiced procedures, following their computer without a precise dive plan or forewarning of the deco plan. That doesn't make them a technical diver. Neither does it make the dive 'technical'. There is no technicality in their approach - no mitigation of increased risk. If a failure happens, they will bust their deco obligation and rely on pure luck to safeguard them from DCS.

In contrast, a technical diver will only exceed an NDL if they have calculated every factor with precision, understanding their gas management in relation to the dive and pre-calculated deco schedule, having intelligently equipped themselves to ensure that any foreseeable contingency can be resolved underwater without recourse to premature surfacing. This approach has a high degree of technicality and mitigates the additional risks that occur when NDLs (or mixed gasses, or overhead environments) are encountered. If a failure happens they are absolutely confident that they can honor their deco obligation and rely on effective training, equipment and procedures to safeguard them from DCS.

It's much easier to define the diver, their mindset...or the approach to the dive, rather than define the dive itself.