Gas blending, stratification and very long dip tube. Any experiences?

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Agro

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When blending into 50 lt cylinders I often find the mix does a stratification. Even after 7 days I do not measure what I expect, the gas is closer to the last topping gas, it does not perfectly mix. When laying down the cylinder for 24 hrs I get the perfect mix, no stratification anymore. But the 50 lt cylinder is hard to handle.

Now I was told I shall use a very long dip tube on the valve. 50 lt cylinders normaly have no dip tube at all, they do not need so because they are always stored upside. So I shall fix a dip tube of about 20cm/8 inch on the valve, going into cylinder. After blending I shall open the valve for a short time. This is said to create a turbulence inside the cylinder, IF there is a long dip tube. Without dip tube I get no turbulence. That's the theory. I dit fix such a dip tube and started blending. Result:

Test No. 1:
Cylinder A, no dip tube, perfect mix, no stratification. This happens sometimes,
Cylinder B, long dip tube, perfect mix, no stratification. Was it because of dip tube or not?
Test No. 1 is worthless.

Test No. 2:
Cylinder A, no dip tube, heavy stratification.
Cylinder B, long dip tube, no stratification.
Cylinder C, no dip tube, heavy stratification.
I layed down cylinder A and C for 24 hours, stratification was gone, perfect mix.
So it might have been the effect of long dip tube or just luck.

Before I take off all the valves on my 50 lt cylinder and add long dip tubes I ask you about your experiences. Does anyone use longer dip tubes and does the procedure of a short valve opening?
 
I have no experiece to relate ...

But I do have a question regarding what type of dip-tube you were using. I would think that the type -could- influence the turbulance. Did you use a side-ported or end ported style?

Additional factors influencing the mixing could also be:
- the fill rate of secondary/tertiary components.
- the order of filling. I would imagine filling lightest to densist would be the best for promoting mixing although that may be impractical.
 
I produced the dip-tube by myself. Length 20cm you can not buy. It is end ported. I was told this is important, it causes turbulences down in the cylinder's center and up on the wall.

I tried very fast filling from other cylinders and slow filling from compressor, both created stratifaction.

Until now I only mixed oxygen, air and nitrox. No helium into 50 lt cylinders so far.
 
assuming the last gas is air or EAN32, I've found that if you blend slowly so the O2 levels are down to something reasonable, then "pulse" the air in. Easier with quarter turn valves, but if you hammer it in while pulsing, I've found it blends a lot better
 
I have found long dip tubes to help, laying down works too as the surface area between the gases is quite high.

But I think the bigger question is why aren't you continuous blending with a nitrox stick?
 
I will not start working with nitrox stick for several reasons. Let's not start this discussion, thank you.
 
I came across this trick in the Navy that worked within an hour or two on industrial K bottles. Stand the cylinder up in a cool and shaded area. Expose a heat source like warm water or a 100 watt incandescent light bulb near the bottom. The differential temperature accelerates mixing.

Rolling cylinders works faster, typically around 20 RPM for as little as 10 minutes. The process depends on the course finish inside the cylinder. We would literally resort to rolling cylinders up and down the pier to get cal gas to read correctly until some anonymous sailor suggested using heat.
 
I will not start working with nitrox stick for several reasons. Let's not start this discussion, thank you.

wrong place to ask that question. What is your aversion to a nitrox stick? We'll still answer the original question, but most of us are very curious when people say they won't use a stick since it is VASTLY superior to any other blending method for nitrox.

@Akimbo that's friggen brilliant! I wonder if a hair dryer/heatgun on low would be too much.
 
I wonder if a hair dryer/heatgun on low would be too much.

Just monitor the temperature differential of the metal with your hand. A few degrees works, more differential works faster until the there is enough heat to warm the top of the cylinder. A 2"/50mm deep bath 110° F/43° C water (used for hot water suits) worked great in 50° F/10°C weather on K bottles.

I imagine a 60 watt incandescent light bulb near a set of doubles would be fine unless you are in a big hurry. Of course that is after the cylinders are cooled down from the filling process.
 
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