DIY Liquid Nitrogen

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Mobius1

Joined
Feb 15, 2016
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Where I live, I have to drive an hour and a half to fill my dewar, and the price has more than tripled from what I used to pay. I'm currently paying $3.30 a liter. At this point, I am seriously tossing around the idea of making my own LN2, and was wondering if anyone else here has tried their hand at it, and if so, what kind of experience have you had with it?

If I did it, I'd do the simplest possible setup - The nitrogen membrane + cryocooler method. The cost of this equipment is not cheap, but it's less than I've paid for a lot of my other equipment, so it's certainly in the realm of feasibility for me!
 
I am interested in your methods and progress, should you pursue this approach.
 
Veratassium on YouTube did this. For most things we do liquid air should suffice. Not sure what the energy costs would be.

 
You are right, liquid air is just fine! That will make it seriously less expensive.
 
If it makes you feel any better, I also drive 3 hours round trip and pay about the same as you do for Liquid nitrogen
But I only do it twice a year. I batch process all the blades over a several days period and inventory them for the 6 months or so
The cost comes out to $2-3 per blade roughly

Harbeer
 
Well, because of health reasons I haven't been able to make a single blade at all in the last year. But I am a hobbyist, so I never did make a very high volume of knives, anyways. I could easily make a single blade on one tank, depending on how busy/motivated I am.

If I could make my own liquid air, I could just heat treat knives when I felt like it, and not feel forced to do them in batches like I have been.

It would just take stress out of the process for me, save me time, and allow me to enjoy the hobby more because I can do it at my pace. And it would pay for itself. Plus my obsession always drives me to improve my setup/processes, and I there's not much else I can buy in the way of equipment at this point.
 
While I have to drive around 2 hours round trip to get LN, it's only 50¢/liter, or $5.50 (including tax) to fill my 10 liter dewar. That's about 45 to 50 minutes each way and 15 minutes to pay and get dewar filled. The welding supply that's 15 minutes from house charges $6/liter!
 
Praxair right by my office used to carry it but they stopped. Apparently I was the only person buying it. Now I have to drive about an hour away to get. Couple times a year isn't a big deal. I have a 25l dewar.
 
From what I am finding after looking into this, the Stirling type cryocoolers are not as easy to find anymore, and thus are more expensive than I was anticipating. But this method would be ideal because it's all self contained and one small unit. I think a lot of these come from old cell towers, because they use them to cool superconductors.

The other method is you can get a cold head and hook it up to a helium compressor. This is the kind of system MRI's use if I'm not mistaken. The compressors seem fairly cheap, though who knows if they still have helium left in them. But I'm not sure if you can just hook up any compressor to any cold head.

Looks like I will have to gather more information. Or be patient and hope I can find a decent price on a Stirling cryocooler! If I'm not mistaken, they are in the realm of > $10,000 new. And it looks like the days of finding them for $300 on ebay are over.
 
So I've decided I'll go ahead and go with the helium compressor plus cold head method, since the Stirling compressors are just too expensive now.

One thing I was also thinking, is with a cryocooler, I don't think I'd even need liquid air, let alone liquid nitrogen. I could just put the knives directly on the coldhead to quench.

My main question on this, is what effect, if any, does an even colder cryo have on tempering? The GM cryocoolers can get as cold as 3K! That's a degree colder than liquid helium.
 
In complex steels requiring cold treatment , there are points on the cooling curve where different things happen.
The first is the Mf .. which occurs around -100°F/-70°C/200°K
Next are changes in the microstructure and carbides at -300°F/-185°C/90°K
Going below that should have little to no additional effect.

Of more importance are the time it takes to get to these points, and the time necessary to hold them there. The Mf is an instantaneous point. Once there you are done.
Cryo at the much lower point needs time for the changes to occur. This is usually 4 to 6 hours.
In both cases, the cooling should be continuous from the time the blade reaches room temperature after the quench. Any delay or rest period will decrease the benefits of the cold treatment. All cold treatments must be done after the quench and before tempering.
 
There are one or two papers showing a small reduction in RA with liquid helium vs liquid nitrogen, but it has not been studied much. The helium shortage made liquid helium even more expensive and difficult to obtain.
 
There's so little RA left anyways, I doubt it would even be noticeable. But I also assumed the atoms can't really move much at such cold temps.
 
Another thing to consider is the stirling cryocoolers wear out. Thy are used in mri machines and some cryo freezers. Thy have a life span and get replaced. The ones seem on eBay generally are the used ones pulled out so who knows how much life you would get out of it any ways. With a good dewer like our 20L it would hold liquid for over 200 days. We where laying over over $70 to fill it. So we where averaging around 3 fills a year. So 200-300 bucks a year to keep liquid nitrogen on hand 24/7. Once the heat treating business took off we where having to fill much more often and it then started getting quite expensive. We then invested into a lab grade cryo freezer that we have set at -86°C. It’s 18”x18”x16” inside and we can pack it chuck full of blades.

And this is where I make my pitch for sending blades out for heat treating. If your just doing a handful of blades a year then you save WAY more money sending them to a heat treater. Even if your doing batches it’s generally a money saver to just ship them out and tart on the next batch while the first batch is being processed.
 
Well, for me, it's more about being able to do as much of the process as possible myself. Hell, I'd even make the steel if I could reasonably do so!

Collecting and stabilizing my own wood, making my own micarta, etc.. That kind of stuff is half the satisfaction of it for me!

Also, learning and trying to understand the metallurgy is quite a big part of my joy as well. And honestly, being a hobbyist probably allows me to focus on things that others can't so much, because I am not so concerned with volume, consistency, or time.

Of course, I do get what you are saying though, and I know a lot of makers send their blades out. I'm all about doing what brings you the most satisfaction and the most pride in your product, so if that's someone's preference, more power to them!

JTknives said:
Another thing to consider is the stirling cryocoolers wear out.
Click to expand...

I have heard about the Stirling coolers wearing out, but I've also heard the brand I mentioned earlier has a pretty impressive lifespan. Honestly the helium leakage is my primary concern, though you can supposedly rig them up to be refilled. But since I am going the helium compressor route, it should be a lot simpler to refil. Though I am currently trying to determine the cost, since a lot of compressors for sale are empty or low on helium.

There is also a brand you can find for cheaper, called Cryotiger, but you need special refrigerant with them (PT-13 or some such), which is proprietary to the company, so I am reluctant to go that route, as well.
 
I am starting to wonder if the Achilles heel of this idea is the need for Ultra High Purity Helium. I've been looking at the costs, and this stuff is crazy expensive. So if this type of set up is going to be worthwhile, I think it is dependent on how much these things leak helium. If you have to refill them every year or two, that can cost hundreds (I think?).

The other question, is do they leak much when not in use, or is the leakage mostly from use? Most of the used compressors I see for sale, have thousands of hours on them, whereas my usage will be in the order of hundreds. You also have to replace the absorber every 10k hours, but with my kind of use, I doubt I'll ever come close to that amount.

Part of me wants to dive into this just for the novelty of it, haha. At this point, my plan is to be patient and find some Ebay deals that come around (Namely something close enough for me to pick up instead of paying shipping.). What I've determined are the bare minimum purchases required to make this work, is a helium compressor, cryopump, helium supply/return/refill hoses (these are NOT cheap because they are special pressurized hoses!), UHP Helium tank, and a helium tank regulator. A helium absorber may also need to be purchased depending on the compressor. My goal is to keep total costs under $1500 but that may be a pipe dream.

On a different subject, the dwindling amounts of helium in the world sure makes me wonder about the future. Maybe someday we'll be able to extract it from other planets! The need for helium 3 and 4 is only going to increase as technology progresses!
 
I think you are starting to see why everyone doesn't have their own liquid air machine. It may not be cost effective.
 
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