No Technical Training for Me.

[Mike Edmonston]

You shouldn't really need it. If you can do a dive or two on 80% at 3000 psi, that's not much different than using your 2100 psi of 100%, since you are using it shallower. I'll admit, it is a little less gas, but if you're only valid reason for using is 80% is "need a booster," then that's not very compelling.

just adding a little comedy is all

Cheers

Mike

 

[Rick Murchison]

So, your argument about buoyancy skills and 80% being safer is bunk.

Is someone here arguing that poor buoyancy skills is a reason to use 80%? I seem to have missed that. Could you kindly cite the post?
Or are you questioning the physics that it takes more deviation in depth on 80% (and more still on 50%) to get the same change in PO₂ that you get on 100%? That physical fact - that the lower the oxygen percentage of a deco mix the more forgiving it is of depth deviations w/r/t PO₂ changes - doesn't say anything about buoyancy skills requirements or desires. Indeed, as decompression offgassing is mostly (almost entirely) a function of PN₂ gradient rather than PO₂, and bubble growth threshhold a function of dissolved gas pressures (or micro-bubble interior pressures, depending on which theory you want to use) and ambient pressure, and as decompression algorithms are formulated with an expectation of tight depth control during both ascents and at deco stops, any supposed benefit in the reduced sensitiity of PO₂ to depth changes w/r/t the overall decompression problem is a red herring.
As a matter of fact, I have never, ever heard the "poor buoyancy skills" argument mentioned by anyone in support of using a lowered oxygen content in a deco gas, other than those who claim it is someone else's excuse for using 80% vice 100%. Never. If you can show me the "someone else" by name, or a credible source (agency document or scientific study) that makes such a claim I'd like to see it. The only proponents of "80% as a cover for poor buoyancy skills" that I can think of right off hand are you and GI3
Rick

 

[Soggy]

Is someone here arguing that poor buoyancy skills is a reason to use 80%? I seem to have missed that. Could you kindly cite the post?

Yes:

80% deco gas is VERY COMMON for a number of reasons. It increases your margin of safety for MOD for new divers with buoyancy issues (31 FSW).

Or are you questioning the physics that it takes more deviation in depth on 80% (and more still on 50%) to get the same change in PO₂ that you get on 100%?

No, I am not arguing that, but if you are diving at a real 1.6 on 80%, while there is a difference in how much PPO2 is affected by depth variation, it is miniscule. It takes 4 ft to change the PO2 by .1 with 80% and 3 ft with 100%. That's not very substantial and isn't very compelling (to me) of an argument about 80%'s benefits.

As a matter of fact, I have never, ever heard the "poor buoyancy skills" argument mentioned by anyone in support of using a lowered oxygen content in a deco gas, other than those who claim it is someone else's excuse for using 80% vice 100%.

I hadn't either, until I read Mike's post.

The only proponents of "80% as a cover for poor buoyancy skills" that I can think of right off hand are you and GI3
Rick

Not me, I'm the first to admit (and I believe I have in this thread) that many of GI3s baker's dozen points are pretty dumb and reaching, but many of them also have a good amount of validity and you won't find me using 80% as a gas.

 

[Rick Murchison]

Waves do not have an effect on your measured depth when you are drifting, even with a lift bag.

That is flatly false.
How great an effect depends on several factors - and basically, for a given wave height, the longer the period and the greater the depth the less the effect on a suspended object like a diver (if you're measuring at a stationary object on the bottom then the longer the period the greater the effect, the deeper the depth the less the effect) - but the effect is there nonetheless.
For an easy-to-visualize example, just think of a free-floating weighted buoy with just a little of it above the surface. Do the waves cause the water level along the side of the buoy to rise and fall? (If you answered "no" then I can't help you)
Now imagine increasing the weight until the buoy is neutral just beneath the surface... will it rise and fall the same amount as the waves passing over it? And if you measure the water pressure as the waves pass overhead will it be unchanged? (You can easily feel the wave generated pressure changes - pressure=depth - in your ears as you approach the surface on a rough day)
The math is complicated, because water pressure is exerted in all directions from the water's surface on down, which is why the deeper you are the less pressure change you get from the waves overhead - and the change is "buffered" by a suspended object's rise and fall - which will be something less than the height-to-trough associated depth change - with the water movement associated with wave passage, but the effect is there and it is measurable.
Rick

 

[Soggy]

That is flatly false.

No, sorry, it's not, or else I (and everyone else doing decompression in even mildly rough seas) would be bouncing up and down like a clown on a pogo stick every time they did a 10 ft stop. But, that doesn't happen. It just doesn't.

You do get SURGE, moving you back and forth, but your depth is not affected. If it is, it is nearly imperceptible. This is one of those things that can be shown by just getting in the water. If you are drifting along in a 6 ft sea, your depth gauge will not reflect that depth change. Reason it out all you want, but my experience and understanding of physics (which isn't too shabby) tells me I am right.

I'm done arguing with you. You and I obviously have diametrically different views on diving in general and while I respect your much greater experience, your experiences do not jive with mine.

 

[MikeFerrara]

What kind of dives are these students doing that they need the tank full to the brim?

I don't boost O2 and I manage to get plenty of O2 into my tanks for any dive that I'm going to do. I keep a few O2 bottles around and bank them. I can usually get an al 80 up to around 2000psi. I can do a lot of decompression on 50 cu ft of O2.

So, you can get more 80% in a tank without a booster but you tend to need more of it (you start on it deeper and deco is a little longer). I'm having trouble seeing any real advantages.

 

[Rick Murchison]

No, sorry, it's not, or else I (and everyone else doing decompression in even mildly rough seas) would be bouncing up and down like a clown on a pogo stick every time they did a 10 ft stop. But, that doesn't happen. It just doesn't.

No, you wouldn't go bouncing up and down, and that's the point. As the waves pass overhead your inertia keeps you fairly level in space as the water pressure - depth - around you changes. This is just physics.
Think about it... I know you've felt it in your ears as you get close to the surface and waves pass overhead.
Rick

 

[H2Andy]

Rick, you're talking a change in depth as the "bottom" of the wave goes over, then the "top" and so on?

total change in depth would be what? the "height" of the wave?

 

[Soggy]

No, you wouldn't go bouncing up and down, and that's the point. As the waves pass overhead your inertia keeps you fairly level in space as the water pressure - depth - around you changes. This is just physics.
Rick

Even if that were the case (it's not quite, since you are often moving with the waves due to wind action), the depth gauge doesn't jump around showing dramatic (or even minute) changes in depth, even with gauges at a high sampling rate. Maybe my ears just aren't sensitive enough, but I certainly do not notice any change in my ears when nearing the surface due to wave action, nor do I feel my drysuit squeezing and relaxing. A foot pressure difference is quite instantly perceptible to me in my drysuit at such a shallow depth.

I'm talking about practical, real life situations. If you want to talk in circles and nitpick about minute theoretical pressure changes, go ahead. In real life, what you describe is not a problem and certainly is not a justification for using 80%. Use it because you like it or because you want to get out of the water 30 seconds faster on certain dives. We both know a group of divers who have done drifting deco in seas 12 ft or more with O2 after a 390 ft dive, and they had no problems, so I'm not sure why we are even talking about it.

 

[Thalassamania]

That is flatly false.
How great an effect depends on several factors - and basically, for a given wave height, the longer the period and the greater the depth the less the effect on a suspended object like a diver (if you're measuring at a stationary object on the bottom then the longer the period the greater the effect, the deeper the depth the less the effect) - but the effect is there nonetheless.
For an easy-to-visualize example, just think of a free-floating weighted buoy with just a little of it above the surface. Do the waves cause the water level along the side of the buoy to rise and fall? (If you answered "no" then I can't help you)

Actually it depends on the mass of the ball. If the ball is theoretical (e.g., without mass) it will remain in perfect reference to the air water interface. As you add mass to the ball it diverges from the perfect behavior of a water particle slightly because it must be accelerated up and down in space to maintain it’s reference to the air water interface.

Now imagine increasing the weight until the buoy is neutral just beneath the surface... will it rise and fall the same amount as the waves passing over it?

Sorry, it’s the same.

And if you measure the water pressure as the waves pass overhead will it be unchanged?

It will be unchanged (well … much less that the amplitude), for the reasons I indicated above.

The math is complicated, because water pressure is exerted in all directions from the water's surface on down, which is why the deeper you are the less pressure change you get from the waves overhead - and the change is "buffered" by a suspended object's rise and fall - which will be something less than the height-to-trough associated depth change - with the water movement associated with wave passage, but the effect is there and it is measurable.
Rick

I’m sorry Rick, trust the Oceanographer on this one. If you track a water particle beneath a wave it makes a circle and the water pressure remains the same. As you go deeper the circle diminishes in radius. There are several films, animated and shot in wave tanks that are always shown in basic Physical Oceanography classes that demonstrate this.