How are helium tanks filled?
June 10, 2016 1:13 PM   Subscribe

Just something I was wondering about. Are helium tanks vacuum-pumped before filling, or are they just full of air? If the latter, how does the air escape? Are they suspended valve-side downward so that the helium goes up past the air and pushes it out or something like that?
posted by baf to Grab Bag (3 answers total) 3 users marked this as a favorite
I am going to admit up front that I don't really know. However!

I do know that the kind of tanks that helium is stored in can have gas stored at very high pressure - I don't know to what pressure they are usually filled, but they *can* be filled to something like 200 atmospheres.

So if you opened the tank valve completely and let it equalize, it would contain 1ATM of regular air. Then, if you forced helium into it up to 200 ATM, you'd have 1ATM worth of air, and 199 ATM worth of helium - it would be 99.5% helium.

That would probably be just fine for fillin up balloons. So it's possible they don't really bother to form a vacuum first.
posted by RustyBrooks at 1:25 PM on June 10, 2016 [1 favorite]

Best answer: Evacuating the air is actually only a problem the very first time you fill it up, as long as the valve that lets the helium out is one way — it doesn't ever let air in to replace or mix with the helium. This means when the tank is "empty", it is actually filled with helium at one atmosphere pressure (ie., the same pressure as the outside air). Then, when it is hooked up to a compressor that pumps in more helium to fill the tank again, there's no need to evacuate any air, you're just adding helium to what's already in there.

For helium that's just going to be used for party balloons, they probably do it Rusty Brooks's way the first time. After that, when that tank is empty, it is still actually 99.5% helium, which means that after the the second filling with pure helium, that 0.5% remaining air is further diluted, again by 1/200th, so you are now down to .0025% air. And this keeps getting smaller as the tank is re-used.

For applications requiring extremely pure hydrogen, you would indeed have a device that first de-pressurized the tank, which can probably be done down to 1/200th of atmospheric pressure or more. When you then fill that tank up to 200 ATM, it would be the same as the above example, .0025% air and 99.9975% hydrogen. And the next time around, that purity would become even greater.

Here's a rundown on the actual standard purity levels of helium that are available. "Balloon grade" ranges from 99.99% down into the 80 percent range. Most helium is supplied at Grade 5 purity which is 99.999% pure.
posted by beagle at 1:58 PM on June 10, 2016 [3 favorites]

I don't know about helium, but the similar argon bottles we use for welding (where the gasses have the be high purity to avoid Issues) have a vacuum pulled on them before filling. Airgas, etc., have the equipment to do it because you they have to. Whether helium gets the same treatment depends, I'd wager, on your purity requirements.
posted by introp at 11:22 PM on June 11, 2016

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