Jackal
Registered
Having just completed the DSAT Blenders course, thinking about this and applying some aeronautical engineering knowledge, and seeing a PADI Master Instructor shaking a tank prior to analysing the O2 content of a tank, I have a short question regarding the stratification of Nitrox mixes. I thought the shaking of the tank was a pointless exercise prior to analysing it. For those non-technical people out there, stratification is the seperation of two gases or fluids, like water on oil.
Does stratification within a tank occur ? I argue that it doesn't with the reasons as follows.
Air is simply a nitrox mix of 21%. If it stratified over time, resulting in a layer of Nitrogen on top of Oxygen ( since Nitrogen has a molecular weight of 14.0067, and Oxygen 15.994), then if you attached a regulator on top of a tank, you wouldn't be able to breathe it until you shook it and stired it up (since all you'd be breating in would be Nitrogen ).
The homogenous nature of a gas means that it's constituent molecules are evenly distributed within the gas. Unlike a fluid, the gases have a continual motion, so are constantly mixing, but over time, this motion slows due to molecular friction with the tank walls. It would be interesting to leave a EANx 40 tank standing up for about a week, analyse it ( without moving the tank ), then shake the tank and re-analyse to note the different. Would it continue to be a 40 mix ? Thoughts ?
Fortunately in diving, the regular movement of a tank on and off the boat etc, would be sufficient to ensure a complete mix of the gases if they did stratify. So the only real reason for this question is when analysing, which is quite serious. If a EANx mix does stratify, and you analyse it, you might get majorly incorrect O2 contents, resulting in a significantly dangerous situation for the diver.
Now the really interesting question is Trimix. Since the molecular weight of Helium is only 4.0020 (ie: 1/4 the weight of oxygen ) does Helium Stratify from the rest of the air mix making up the Trimix. I've heard that if a tank has a slow leak around the valve, and it the tank is standing up, the Helium can bleed off first, since it is pooled around the upper end of the tank. This would result in a significant change in the tanks contents, and obvious danger to the diver. Anyone with any trimix knowledge, please comment on whether there is a requirement to re-analyse a tank within a certain time period of a dive.
Does stratification within a tank occur ? I argue that it doesn't with the reasons as follows.
Air is simply a nitrox mix of 21%. If it stratified over time, resulting in a layer of Nitrogen on top of Oxygen ( since Nitrogen has a molecular weight of 14.0067, and Oxygen 15.994), then if you attached a regulator on top of a tank, you wouldn't be able to breathe it until you shook it and stired it up (since all you'd be breating in would be Nitrogen ).
The homogenous nature of a gas means that it's constituent molecules are evenly distributed within the gas. Unlike a fluid, the gases have a continual motion, so are constantly mixing, but over time, this motion slows due to molecular friction with the tank walls. It would be interesting to leave a EANx 40 tank standing up for about a week, analyse it ( without moving the tank ), then shake the tank and re-analyse to note the different. Would it continue to be a 40 mix ? Thoughts ?
Fortunately in diving, the regular movement of a tank on and off the boat etc, would be sufficient to ensure a complete mix of the gases if they did stratify. So the only real reason for this question is when analysing, which is quite serious. If a EANx mix does stratify, and you analyse it, you might get majorly incorrect O2 contents, resulting in a significantly dangerous situation for the diver.
Now the really interesting question is Trimix. Since the molecular weight of Helium is only 4.0020 (ie: 1/4 the weight of oxygen ) does Helium Stratify from the rest of the air mix making up the Trimix. I've heard that if a tank has a slow leak around the valve, and it the tank is standing up, the Helium can bleed off first, since it is pooled around the upper end of the tank. This would result in a significant change in the tanks contents, and obvious danger to the diver. Anyone with any trimix knowledge, please comment on whether there is a requirement to re-analyse a tank within a certain time period of a dive.
