I just watched this (https://youtu.be/jLX1-tNnvEo?t=897)(14:57) and the guy in the video used vacuum and argon gas to remove any oxygen from the melter but I don’t understand why is the argon used if you are already vacuuming all the air out anyway?
In: Chemistry
Same way sand could be used to displace the water in a pool.
Argon is heavier than regular air. So you pump in argon and to folks up the space, pushing the air out.
It’s difficult to get a perfect vacuum. If you absolutely need to remove all the oxygen, you’d displace it with something inert (like argon) then suck out as much of the argon as you can.
I didn’t watch the video, so hopefully I’m not talking out of my ass.
Think of it like if you had a glass of bleach
You can pour the bleach out, but there is going to be drops clinging to the glass. So you fill it back up with water and pour it out again. Now there are drops but they probably aren’t bleach any more
You can’t vacuum all the air out. So if you NEED there to be no oxygen/water vapor in melter, then you are going to this style of pump/purge. Pump out the air, fill with argon, pump out again, repeat as needed
>if you are already vacuuming all the air out anyway?
That’s the thing, it’s not taking all the air out.
A vacuum pump does not pull absolutely all of the molecules from an environment. Let alone a “regular” lab one. A perfect vacuum is actually nearly impossible. High grade vacuum chambers specifically meant for it can’t even replicate deep space levels of vacuum on earth.
So pulling 90%(made up number, don’t know the actual specs of his pump and chamber), and replacing it with inert gas, and repeating the process, will get effectively close to removing all of the oxygen. It’s like rinsing something soapy once. One pass might not get it all, but after a few times of getting most of it, it’s functionally good enough.
Its nearly impossible to create a perfect vacuum.
Lets imagine that the vacuum pump leaves 1% air in the tank. Now when he fills it again with argon it will mix with the air and become a mixture of 99% argon and 1% air. Now he turns on the pump again it will leave 1% of the mixture in the tank which means there is only 0.01% air and 0.99% argon in the tank. Now he just needs to repeat this until there is as little air as he wants.
Eli5 version:
When youre brushing your teeth and you spit out the toothpaste it will leave a taste in your mouth but its hard to spit out the last remains. Thats why you rinse your mouth with water and the water takes out some of that toothpaste each time and you have to repeat it couple times to get the taste out fully.
ELI5: you have a glass full of mud (oxygen/moisture) and you want to use the glass to make untainted lemonade (whatever the science experiment is). so you pour out the mud, fill it with water (argon gas), dump, rinse and repeat until theres no mud left. Now you have a glass with water in it and hopefully no mud that you can use to make your lemonade.
Seems like the final goal was have a chamber with no oxygen or moisture. He doesnt leave the chamber in a vacuum state (he closes the vacuum and argon valves but the pressure meter is nominal not negative) but instead filled with as high concentration of argon as he can that wont interfere with what hes doing. He’s essentially rinsing the chamber of oxygen and water using the argon
Breathe in. It’s pretty easy for your lungs to get air in, right? Now put a cup around your lips to make a seal and try to breathe in. It’s a lot harder, isn’t it? There’s still air in the cup, but you can’t suck it all out with your lungs.
When there’s less air around, it’s harder to “suck” the air out of an area. So, the more you suck out, the harder and harder it becomes to suck. You eventually need to get different machines in order to keep “sucking” the air up, and that’s expensive.
So, instead of trying to suck harder, in the video they just make it easier to suck! The argon gas they pump in is a gas, but it doesn’t do anything, unlike oxygen which can cause things to burn. So now it’s easier to suck, and they can keep doing it. Eventually, the argon washes away all the oxygen, like how when your hands are muddy you can wash away the mud; at first there’s a lot of mud in the water, but eventually it’s just water in and water out, and you know all the mud’s off.
Ok, so there’s two things to talk about here.
If you don’t want oxygen, you can try to just pull a vacuum. But pulling a really good vacuum is difficult and impractical for anything but really expensive systems, so you still end up with some oxygen. And in a lot of cases, some oxygen is still bad. You want nearly no oxygen.
Flushing with argon is a fairly good way to do that. You pull a vacuum, refill with argon. Now the leftover oxygen is mixed with the argon. So you pull the vacuum again. You’re left with less oxygen. Repeat a few times. I personally do it three times in my work, it serves to bring it down to an acceptable level for my needs. So I could just now pull a final vacuum, BUT!
The second thing. You might NOT want vacuum. Say, when you’re melting things. Especially, as it were, using a plasma which actually requires gas in-between.
The one property of vacuum is that it’s an absence of stuff. And nature… Well, abhors a vacuum. So if you have something in vacuum and and can turn to gas, it will. Especially hot liquids.
You might be familiar with a fact that the boiling point of water drops as pressure drops, say on top of a mountain. Well, it’s the same for everything else. Even metals. You can start getting a metal vapour coming off liquid hot metal. And unless you literally aim to do that, it’s usually undesirable. You usually want something close to atmospheric pressure.
On the other hand, that’s exactly the point of vacuum drying.
Plus, another issue is sealing. If you have vacuum, stuff from the outside REALLY wants to come in. Well, what you can do is very slightly overpressurise the inside instead. That way, instead of air coming in, some argon leaks out. But you can just top it up. And that way, you can get away with a system that isn’t perfectly sealed, which is cheaper and more tolerant of errors.
A couple of reasons. First it’s virtually impossible to make a perfect vacuum. To have an oxygen free environment it’s far easier to pull a 99% then fill with a pure gas, at which point you have about 0.2% oxygen. Do that a few times and you’ll have an oxygen free environment.
Second he’s dealing with molten metal. Under a vacuum molten metals have a much lower boiling point, as he’s trying to make a very precise alloy having some evaporate at different rates wouldn’t be good.
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