Recreationall TRIMIX

[dfreeman]

Mike,

This is a great observation. What is happening is that there is less turbulence from the gas entering the cylinder the slower you add the gas, reduciong the mixing. This observation answers a question that I have had for a while. With no mixer in the scuba tank, what is the controlling form of mixing when blending gases? The choices are: free convection from temperature difference, diffusion, free convection from density difference, or flow turbulence. All of these have an effect, which is the big one, especially working with the supercritical conditions at scuba tank pressures where gases act more like liquids.

In your experiment, you are adding the heavy gas first and then the light gas, this will minimize free convection from density difference. You are adding the gases slowly, therefore it is more difficult to set up a thermal convection cell since there is not a lot of temperature difference from the inside to the tank surface. What is left is diffusion and turbulence. Gases usually diffuse rapidly, but at these pressures they act more like a liquid, slowing diffusion. In your experiemnt, the flow is low and there is not much turbulence for mixing.

The results is that the gases layer on you and only mix after filling by diffusion and this diffusion is slowed by density difference. The geometry of the cylinder enhances this effect.

There are bar drinks where you can layer different density liquors in a glass if you add the liquors in the right density order and very carefully so that you do not intorduce turbulence. Give the glass a shake, the drink rapidly mixes due to the turbulence. The shape of the glasses used are tall and thin, like a scuba cylinder.

Tip you tank over and rolling it does the same thing. Now there is enough turbulence to mix the gases and away you go.

Your observations answer several scuba gas mixing questions that I have been curious about but have not bothered to calculate (the physical data is hard to find at these pressures and I know that I will probably have to deal with Bessel functions in the end based on past experience).

Keep the information coming.

Doug

[WIdiver_Paul]

"Acceptable Risk" is a paradigm that changes with time and awareness, as people begin to realise the dangers associated with things they may not be aware of. When I was young, my grandpa's buick had no seatbelts. It also had no crumple zones built into the frame and engine compartment. It had no airbags. It had no antilock brake system to stop accidents from happening in the first place. It had no side-impact zones. Auto makers had to find ways to make autos safer, regardless if the user was aware of the dangers, and they had to find ways to make them easy to use, to make driving safer for everyone. They had to make auto safety easy for everyone to get used to. This is the same idea behind recreational triox, to make safe semi-deep recreational dives as safe as possible, and accessible to those who are interested in becoming safer divers at that depth.


Anyone taking recreational trimix needs to have a nitrox certification, which means they already know what PO2 they are diving at and their O2 exposure. The oxygen is not the issue.


Here again this statement reinforces the validity of what you are arguing against:
Recreational triox was conceived to make diving safer with added helium at recreational depths without the half dozen classes, months of instruction and thousands of dollars of money wasted on instruction to get a student to a level possibly beyond the level at which they may dive. If they made having a commercial driver's license a prerequisite for driving a car, we'd all be riding our bicycles to work! You don't need a degree in medicine to put on a band-aid. Diving is founded on the principle that people assume personal risk to dive. Not everyone wants or needs to have a full cave card and training at 300' to dive 130' on helium.

[Omicron]

Paul - you summed that up just about perfect

[WIdiver_Paul]

If you want to learn some insight why this "whole" concept is an important one, and maybe just
to learn a little bit more about why recreational trimix is a REALLY REALLY GOOD IDEA, check out
this link:

[puddlejumper1]

Wow that was really grand! What a great revelation the prophet opened his mouth again. If you want to try and convince me why it is a good idea than I would not quote Irvine at me. BTW already read it!

[WIdiver_Paul]

How about Jablonski? Don't like him either???

[puddlejumper1]

How about this one?




Before you go spouting off about stupid childish responses look to thy own self. Wow Irvine stated it it must be true. Jeeze this guy is so great all that he quotes can not possibly be wrong. Just reading his presentation must make it all ok. Justify the great hose routing issues. Show me were mass deaths have occured from doing it another way! Use something other than twenty year old cases (incorrectly) and you will start making sense. Till than look out for the cliffs, what flavor you need cherry or raspberry. BTW that post you quoted was my polite way of ending a discussion that would obviously degenerate into a flame war. So let it die and stop trying to justify why you are a follower. You should not care and I certainly do not.

[Deep Blue]

OK, Now this is starting to sound like a bunch of other boards that I have left. It is quickly degenerating into a flame war. Lets get back to arguing the merits, of the arguments, not the personalities.

So far, the discussion has been pretty good, lets not let in degenerate at this point.

To me the entire issue of DIR is about acceptable risk. Puddlejumper, you may be willing to accept the risks inherent with a non dir setup. I am not. I am not saying that a non DIR setup or non DIR diving or non DIR buddies will automatically get you killed, neither is anyone else that I can tell. We are simply saying that we believe DIR is safer and more optimal.

Unless you can come up with a reason that the configuration that you are using is better, at least be honest enough to say that the reason you don't want to dive DIR is simply because you don't want to, not because it isn't better.

The whole "don't you tell me what to do" and "prove my setup is dangerous" arguements are a smoke screen when people don't have a basis to defend their setup or diving practices. If you argue based on the merits and strive to dive in the most efficient manner, I believe DIR is the way to go.

Actually, I think that the links to George's articles are excellent, and even though he has a very abrasive personality, there isn't anyone out there further out on the cutting edge of deco theory and extended range diving.

Now PLEASE lets argue the specifics and not let emotion and egos get into it.

[jasondbaker]

Dirk, thanks for helping us get back on track.

I don't consider myself truly DIR since I have not taken DIR-F (waiting for next June). But, I have adopted many of the principles in terms of gear configuration and buddy team philosophy.

I chose DIR because it fixed a couple gear configuration problems I was having and it philosophically made sense to me. I have no problem with people who try different configurations or diving styles. Most of the people I dive with are not DIR.

I did not investigate DIR to minimize the risks associated with diving.
I didn't have time to try every gear configuration or style of diving. I wanted a methodology that was proven, streamlined, and "scalable" -- i.e., I can use it for OW, tech, wreck, etc. DIR presents one of the clearest set of guidelines for diving today. I guess I'm a follower in that sense. I'll let other people do the tinkering and exploring.

I don't know if DIR is "the best" way to dive. I do believe it is better than most.

[WIdiver_Paul]

Well said, Jason.
I'll be joining you in that class.