Recreationall TRIMIX

[FreediveWI]

Doug,

We have seen doubles come through with questionable tank-neck o-rings in them. If you fill them with air your would never know the diffrence, but if filled with trimix they leak out over time.

Jon

[dfreeman]

Jon,

Being up on my quantum gas partical theory, I understand why you see the change that you describe with tanks bleeding off. You have to have good seals when working with "fast" gases (hydrogen and helium). This phenomena is used in several commercial gas separation processes.

I am still confused about a helium binder.

Doug

[FreediveWI]

Straight helium would leak out faster than if we would add a shot of air to the tanks to mix with it it. This became more obvious as we added helium in one place and then drove the tanks across town to use a compressor. Now helium banks and compressor are in the same place.

BTW: we have never had a problem with stratification. We never need to roll tanks around or anything like that. Once you top it off they pretty much mix instantly

jon

[dfreeman]

Jon,

I understand what you are seeing in this case. I am just surprised it is as noticable as you indicate. I would recommend changing the seals and sealing surfaces before this leak becomes serious.

These gases will not stratify in the tank. It is against the Laws of Thermodynamics. It takes significant energy to separate these gases.

For an simple explanation of what is going on see:




For a good treatment on gas laws see:



Thanks for the feedback. I understand your observations much better now.

Doug

[FreediveWI]

THat's what we did to that set of doubles.

The reason I posted the other stuff was that it used to be a widely held myth that gases would stratify. People would tumble their mixes before they would analyze them. In reality just pressurizing them with air when you top them off causes enough turbulence to mix them right away.

Jon

[dfreeman]

Once these gases are mixed, they are mixed. It takes a lot to separate them. Another myth bashed.

Doug

[puddlejumper1]

Correct me if am wrong Doug. But isn't the most likely way that a seperation is going to occur is if one or more of the gases is changed into a solid? This would seem most unlikely at any dive mixing station I know. Isn't the melting point of He something like -460 -470Deg F. Way more equipment needed than what my LDS carries.

BTW what is your take on the necessity of a He analyzer?

[dfreeman]

There are 4 separation methods that I know of that are commonly used. Three of them (membrane, centrifuge, pressure swing adsorption) require a lot of additional equipment. Distillation is the commonly used method for high purity but must be done at very cold conditions, basically the oxygen and then nitrogen are turned into a liquid at several hundred degrees Rankine (I am too lazy to look up the exact numbers right now). This would present a serious freeflow problem if you tried to run the remaining gas through the regulator.

If you reheated the tank, the stratification would collaspe due to free convection and diffusion forces. I would expect the tank to be well mixed by the time you got it to room tempurature again. The Laws of Thermodynamics would keep it mixed.

The melting point of helium is very close to -460 DEG F, which is absolute 0, or 0 Rankine. Any lower temperature is meaningless in the real world. Again, too lazy to look up.

I worry about all gas analyzers. Oxygen analyzers are famous for drift and calibration problems. All important oxygen processes that I have experience with use three analyzers and a polling control logic to help keep the operation going when one of the analyzers craps out, which it will. If the problem is more severe, an emergency shutdown is initiated.

For the oxygen analyzers that I trust my life with in industry, they are frequently calibrated using air and a calibration gas at 16% O2. This give tha slope and intercept needed to accurately calibrate the instrument. We see sensor heads fail as early as 6 months. They are replaced every 2 years no matter what. Is this the case with your local dive shop? I will let you be the judge of that.

I am not as familiar with helium analyzers. Perhaps someone can relay the technology used in these sensor heads. However, my experience with analyzers is that for $800 you are getting a bear bones piece of equipment. Its is like buying a low end regulator, will it work in warm water? Sure fine. Bottom of Lake Wazee? Oh, boy. I do not have enough background with helium sensors to make a judgement.

As a Nitrox diver, this subject has concerned me quit a lot when considering oxygen analyzers. I do not have an easy answer. I actually trust the blending process over the analyzer. I would prefer to blend my own gases. Give me the nobs.

Doug

[MikeFerrara]

Hey every one. I'm new here but I recognize some names. Interesting thread so I'll throw my couple cents in.

I didn't see any mention of the fact that there are two IANTD rec trimix classes. The first is no deco. The second is Advanced rec trimix and has a max depth of 150, max deco of 15 minutes and a max Fo2 of 50% for deco unless the student has taken the decompression specialist class and then they can use 100%. The Advanced Nitrox is of course ïntro to tech and taught right lays down a solid skill foundation. With the addition of the rec trimix classes we can give students all the tools applicablr for those depths. It does seem like a lot of classes and I'm still thinking about the most efficient and economical way to combine them. If any one has any questions the IANTD standards are on their site. I haven't seen the materieals for the classes yet myself.

helium analyzers...

I just recently had the chance to use one for the first time. Over three days of mixing and analyzing there weren't any surprises. I feel better about not having one now. LOL

Another way to solve the problem of knowing what's in the tank of course is to top with nitrox instead of air. If you know what the nitrox was and that the rest is helium, all you need to do is analyze the O2 to know exactly what you have. I plan to set my fill station up for continuous blending and bank 32%. That makes it easy to mix the mixes that some refer to as standard because all are made by topping with 32%.

Which brings me to my next point. If gasses mix immediately, why are there so many products made to mix them?

I won't try to explain why this is but I can demonstrate it any day for any one who wants to see it. If I mix say 50% and add the O2 through my hyper filter veeerrryyy slowly. When I use my mix controller, which has a flow restrictor and my hyper filter which also has a flow restrictor I can't pump it anyway but slow. I can analyze it and read as low as thirty something percent. I can let the tank sit for a fairly long time and the reading won't have changed much. I can roll that puppy on the ground and it'll jump right up to 50%. The less air I'm topping with and the slower I add it the more pronounced it is.

I asked some one at Purdue about this and the answer was that the geometry of the container had an effect on how the gasses mixed.

I can't explain but it does hapen and I can demonstrate it repeatably.

I don't see it when mixing trimix or lower FO2 nitrox because I'm doing the final topping right off the banks and a slow fill from there is still way faster than off my hyper filter.

Oh, I read where some one mentioned Greg Such. I'd recommend him.

Mike

[Vtach]

Welcome aboard Mike! I have read some of your posts on the Deco Stop. Look forward to seeing more here.

Todd