Random trimixes

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lermontov

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A buddy has some random Al80 tanks with various trimix/ heliox ratios and was wondering if we could decant them all together somehow to get one mix - anyone got any suggestions - we could use a whip to transfer back and forward to an empty G but how well will they all mix - ranges from 11/89 to17/38

hopefully we can get it all mixed and see what comes out so we can use it for a group dive
 
are they all full at 3000 psi? are you ok with needing topoffs afterwards? any decanting will leave you with less than full tanks. everything will mix just fine, just you will need to plan it out ahead. you're probably going to need to draw up a spreadsheet.
 
If you connect 2 tanks with a decanting hose, they will equalize but after that they won't mix if you'd leave them connected and opened (I wonder how many weeks it would take to have an equal mix this way).

Best way for this approach is probably to decant each tank into an empty buffer tank, which requires a booster (and lots of drive gas!), preferrably a 2-step booster if you want to empty each tank into the buffer. The end mix that this would produce can be calculated when you take the pressure and analysis of each AL80 - calculate the volume of each gas, add it all up and that's the theoretical end mix. Filling the divetanks would require the booster again. Temperature differences caused by decanting and boosting will influence the analysis.

I guess this time consuming and labor intensive process doesn't sound appealing. Another approach is determine the mix you want to dive with, decant from the AL80 straight into the dive tanks and top up the required gasses to achieve the desired end mix. Also time consuming and might require a booster as well.

So...not easy. Mixing from a helium and oxygen buffer is much easier.
 
If you have a booster and aren't worried about drive gas, I would just boost everything into a bank, let it all cool down and mix for a couple of days and then analyse whats in there. If the resultant mix makes sense, go do a dive at an appropriate depth and enjoy, else it might make sense to treat it as a bank for future fills/top-offs, makes for fiddly maths but at least you aren't wasting too much helium.
 
You can run them through a compressor. You just need access to a compressor which you can rig with a feeding bag.
 
thanks for the suggestions - It looks like whatever we do will be time consuming but that's ok at least it won't waste the He. Boosting it into one large store tank and then adding/topping up will work. it might be that we have to use two large tanks and send it back and forth a couple of times to mix it
 
what @RainPilot said. if you boost it all into one tank it should mix itself up. The heating/cooling/pumping action does a lot more than just transfilling, and getting it all in one tank will at least let you figure out what you've got and adjust it after the fact. Good luck!
 
I regularly pump leftover trimix into a bank cylinder (250cf). It can take quite awhile (I suggest leaving it a week) for the different layers to mix since my tank is vertical and the surface area between layers isn't large, they also are usually at different temperatures from running through the compressor which is also slow (3cf/100L minute) so its not blasting it in with lots of turbulence either. If you can lay the storage bottle on its side they'll mix faster.

Then I analyze it for O2 and He and either use it as-is for CCR diluent or as a base for making a useful mix.

edit: in my experience the differential compression and different volumes of all the different mixes will make calculating what they "should" combine into impossible. You will definitely need a helium analyzer. Start by pumping the highest helium gas into the storage bottle and then sequentially add the lower percentage mixes so they push the higher fractions of helium through the filter towers (wasting the least). This has the side benefit of pumping the higher helium fractions at lower pressures for less blow-by. And the last and least helium %age will be put on the top of all that lighter molecular weight gas. So as that cools and sinks it will mix better. The gases will layer in the storage bottle based on temperature moreso than molecular weight.

Oxyhacker has various ideas on how to make a trash bag booster. The easiest is to just put a wine cork in your mixing stick inlet with a trash bag on it. Then use a welding reg to keep that bag mostly inflated while running the compressor.
 
Too much work for me. What is your dive going to look like?

My O2 sense

Blend the highest He to the Lowest He and have your blender match O2 levels (hyperoxic) across the group, He levels won't be critical but need to be calculated.

PS: If someone came in my shop and asked me to circle jerk and bunch of mixes around my time would be more than dumping the mixes and going anew.
 
https://www.shearwater.com/products/perdix-ai/
http://cavediveflorida.com/Rum_House.htm

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