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  • Tri-mix shelf life?

    I might be opening up a can of worms but here goes. Does anyone know if there is a shelf life on Tri-mix's with Helium (like for SS)? This is a debated subject in the diving wold, though not the same mix as it is mixed with Oxygen instead of Argon, but it is a Tri-mix. Some say that by the gas laws the Helium being so much lighter than the other gases, over time the gases inside the tank stratify, or seperate into layers. Others say once the gas is mixed, it stays mixed. Now this is from people that have analysers to test the mix's they are using. Anyway, if the gases layered, you would get a nearly pure gas output on inital use after sitting for a period of time. Anyone out there have any real world experience on this? If getting a tank of Tri-mix to have around for those SS jobs, would it be better to have a small tank so it gets used and then fresh fills as needed, or just the normal size and its no big deal?
    "The only source of knowledge is experience." Albert Einstein

  • #2
    It would depend on how the gas was mixed originally, and if the cylinder has an enduction tube installed.

    Most companies wont fill industrial blends in small containers. The exceptions are with the very popular gases, like 75/25 or spec gas mixes.

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    • #3
      Lay the tank on the ground and roll it around before using, works for me.

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      • #4
        the short answer is no, the gasses will never unmix.

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        • #5
          Yes, they can unmix - or stratify. The determining factors are how the gases were originally mixed, the order that they were filled and if the cylinder has an enductor tube to create a venturi during usage.

          Exotic gas mixtures that contain dioxides and hydrocarbons are much more prone to stratify. These cylinders are provided with an expiration date.


          Gas quality matters, especially in mixes. Make sure that your supplier does more than just fill the cylinder with two different gasses. Fill order, temperature, pressure, mixing type, valve / enductor tube type, are all variables that will affect your mix quality and shelf life.

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          • #6
            Does the air we breathe separate on its own? NO it does not. Welding gasses are recovered by distillation; there is no place on earth where all the argon resides or where you can pump pure nitrogen. Once mixed it should always be mixed.

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            • #7
              engnerdan-
              I believe that it would be illogical to compare gas molecules in a pressurize container to the gas molecules in the open atmosphere that are affected by numerous physical forces.

              To provide the information that I still stand by, I consulted my friend who happens to be the regional Spec Gas Specialist for Airgas. I think that he has a handle on the physics of the subject.

              Gases of similar masses will have more stability in the mixture overtime, such as 75Ar/ 25/ CO2. The blends that contain He and H2 are going to be the most suscepable to stratification.

              However, you will never be able to just remove a pure gas from a stratified mix. As soon as the cylinder is opened, currents in the cylinder will be created. This means that you will be removing a mix of some sort, just not as homogenized as it was originally. This is where the benefit of the enductor tube is realized - during gas withdrawal.

              If the inductor tube is installed and the gas is stratified - it may not be noticable to the welder. The venturi effect will remix the gas upon usage.

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              • #8
                The methods you describe in mixing gasses are accurate to some extent, in the speed of which the gas is homogenized. But, once in the cylinder, the gas mix with homogenize on its own, the more poorly it was initially mixed, the longer it will take, but after some days, the gas will be mixed and will stay mixed, So any long term storage will only help.

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                • #9
                  No, thats not accurate. Time will not help homogenize the mixture. Gravity aided with time and zero input from other physical changes such as temperature and vibration or motion will only aid in the mixture stratifcation. The lighter components will rise, the heavier components will sink - just like oil and water left to sit.

                  Helium and Hydrogen rise as compared to argon or co2. You cannot state that just because its in a mixture, the helium will ignore the law of physics.

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                  • #10
                    I would imagine it depends on the density of the gases and the % in the mix. I don't think you could say the same for all mixes. If argon will displace air then it should displace or be displaced by other gases. Interestingly you don't seem to run into this in the atmosphere, perhaps because air is always moving or maybe Nitrogen and O2 are similar in density and mix readily. I don't really know.
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                    • #11
                      I'm glad to see this brings as much debate as it does in the diving world. I have serviced many dive tanks and they mostly all have a tube in them, do welding gas cylinders have them normally or is it an add in? Is Tri-mix for SS the best gas for corrosion resistance compared to the other gases that can be used for welding SS, (salt water use)?
                      "The only source of knowledge is experience." Albert Einstein

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                      • #12
                        You do run into gas stratification in an open atmosphere. Ever hear of a confined space. Breathable air can be displaced by no breathable air. and in so doing if the gas is heavy enough it will displace all other gasses and create a very dangerous situation.

                        I realize that in most confined spaces It's the level of breathable oxygen that we are concerned with but if you dump enough Co2 or Argon into a open topped vessel it will push out all the other gases
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                        • #13
                          Its quite accurate.

                          1500psi is plenty of energy to overcome gravity. Each gas in the tank will attempt to fill the volume of the container, and they will, regardless of the atomic weight of the gasses in question.

                          You cant compare liquids to gasses because they don't behave the same.

                          Originally posted by GTA/SPEC View Post
                          No, thats not accurate. Time will not help homogenize the mixture. Gravity aided with time and zero input from other physical changes such as temperature and vibration or motion will only aid in the mixture stratifcation. The lighter components will rise, the heavier components will sink - just like oil and water left to sit.

                          Helium and Hydrogen rise as compared to argon or co2. You cannot state that just because its in a mixture, the helium will ignore the law of physics.

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                          • #14
                            Originally posted by Laiky View Post
                            I would imagine it depends on the density of the gases and the % in the mix. I don't think you could say the same for all mixes. If argon will displace air then it should displace or be displaced by other gases. Interestingly you don't seem to run into this in the atmosphere, perhaps because air is always moving or maybe Nitrogen and O2 are similar in density and mix readily. I don't really know.
                            You say it doen't happen in the open atmosphere, check out the biggest killer listed here. http://www.toolboxtopics.com/Gen%20I...d%20Spaces.htm Most people are not aware of confined space, and many think it is only inside a small tank or closed vesele, but it can actualy be aopen top pit or open top tank were there is no vertical ventalation. So that shows that stratifacation can happen in the open atmosphere. So the real focus of the question being has anyone every had a problem with a stored helium mix not welding properly when first used after sitting for a while?
                            "The only source of knowledge is experience." Albert Einstein

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                            • #15
                              Gas stratification is not caused by one component of a gas mixture "coming out of solution."

                              It is caused by one gas being introduced into a container with a laminar flow (such that turbulent mixing does not occur), and the stratification due to different densities is temporary. Eventually, the gasses will mix through diffusion.

                              But we are not talking about the transient state when a new gas is introduced into a container of another gas or mixture of gasses.

                              We are talking about a gaseous solution.

                              When any mixture of gasses are put into a container, whether it is 2, 10 or 100 gasses, once mixed, they will remain mixed forever.

                              And even if they are introduced individually, they will mix on their own eventually. This is called entropy. Order moves to disorder spontaneously.

                              The very definition of a gas is a state of matter where the molecules expand to completely fill their container. When all these molecules expand to evenly fill their container, they are going to get all mixed up an jumbled up on their own.

                              Now if a pressurized container leaks, this is another matter as the gasses with lower molecular mass will leak through at a faster rate than those with higher molecular mass. And the loss of pressure is spending energy to gain order. This is how gaseous diffusion works.

                              I would recommend an introductory Chemistry course to anyone who thinks gas mixtures spontaneously separate.

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