Question on small dewars?

Kentucky

Kentucky

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Dec 13, 2008
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Ive been looking for several years for a small dewar..I see small dewars for sale all the time on the auction sites but everyone I see looks like it has no lid or threads/place to insert a lid..Is this the norm or is there something Im missing..Ive looked locally for years but never ran across one..Dont want to pay $250+ shipping for a large used one.About the going rate. Ive never seen one of those mysterious $100 dewars people talk about..Any thoughts on the smaller dewar/flasks or are they just like "containers" for lab settings? thanks
 
Do most of the ones that we would want to use have threads? The ones that I looked at had the thick foam insulating plug. and as som are meant to ship stuff, i assume they have some way of securing the lid. It seems to me that the smaller dewars are either containers for the lab or small ones to store LN. I know some guys use the 10 liter model. but the ideal ones that I see are the "big mouth" 34 or 35 liter models where you don't have to worry about the mouth being 2 inches or less across. a bit mouth reduces the time that your LN will stay liquid, but with those big boys, we are talking a month or more. if you can get a large one for $250 that isn't beat up or missing parts, I would jump on it. I thin that some of the Taylor-Wharton 10 and maybe 5 liter dewars can be had with the 2.5 inch mouth.
 
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Honestly Im just wanting something that is smaller because we don't do that much stainless and usually no more than 3-4 at a batch if that..Dont need a large dewar that lasts a month because in all honesty the vast majority would go to waste..We can get dry ice 10 minutes from the house for .99 cents a pound. I know the dry ice/acetone bath dosnt have the carbide effects of Ln but at -106° or so it takes care of the RA just fine(or at least we get 1-2 points rc from it)..Most of its Aeb-l..None of the super steels like elmax.
I can also get Ln close to the house but have a hard time justifying so large of an expense for such a small number of knives..
 
Unless doing HT in good size batches weekly, LN is a luxury, not a necessity. Use dry ice and alcohol and you will be fine.

If you go the LN route, get a large dewer. They are made to keep the LN as long as possible. The tops are just foam plugs ( loose plugs at that), and do not "screw down" and seal. All the large dewers are is just a vacuum insulating container that prevents the LN from boiling away too fast. The small containers are for pouring the LN into for use in labs and such and then pouring back into the large tank when done. Many smaller dewers are just a open cylinder with vacuum walls and have no top.
 
A screw-down lid would be a safety nightmare, or at least could be, and I suspect this is the reason you don't find them.

As heat gets into the Dewar, it is absorbed as latent heat by the LN boiling off into gaseous Nitrogen. The boiling temperature is a function of the liquid/gas concerned and the pressure it is under; -195.8 degC/-320.44 degF at normal atmospheric pressure. The rate at which the LN boils away is directly related to the rate at which heat flows into the LN.

It is basically the same process as we see with water in the kitchen kettle. If we put the kettle on the stove, its temperature will rise to 100 degC/212 degF, where at standard atmospheric pressure it will start to boil. It will continue boiling and the temperature will remain at 100 degC/212 degF as long as there is still liquid water present. When the last water turns to steam (gas), the temperature of the kettle will rise beyond 100 degC/212 degF.

If we use a pressure cooker instead of a kettle, the steam produced above the boiling water develops pressure, which increases the boiling point of the water. At 15 PSI(gage) the boiling point of water is around 120 degC/248 degF and in the kitchen our pressure cooker relief valve lifts to maintain this temperature and pressure. If we did not have the pressure relief valve, the temperature and pressure would simply keep rising until something failed, probably catastrophically.

Pressure vessel failures of all kinds have killed and maimed many people over the years. As a result, there are many rules and regulations to minimize the likelihood of it happening in the future, and to establish who will be held liable if it does.

Because the boiling point of Nitrogen is so low, ambient temperature is sufficient to develop enormous pressures in a closed vessel.

Pressurized industrial Nitrogen is supplied at 3000-4500 PSI (200-300 Bar) in the same sort of heavy cylinders as Oxygen and Argon. There is no liquid Nitrogen present in these cylinders at normal ambient temperatures.

Dewars are very lightweight and this is a large part of the reason they work so well. They cannot be redesigned to work as pressure vessels and still work well as insulating vessels, so Dewars intended for LN are usually designed such that they simply cannot be pressurized. An easy way of doing this is to have a plain neck.
 
i had a 10l and it was handy till it lost vac. neck was right about 2 inch and for my kitchen knives that was becoming a limiter. when i upgraded i went with a big mouth 20l (3.5inch mouth) because of the mouth size i get the same amount of use time as i did on the 10l (just over a month) so i plan on HT and LN so i can get 2 large batches out of each tank to keep the cost per blade as low as i can last tank was jsut over a buck a blade but i had cut a whole sheet of XHP up to do that
 
Thanks guys..I knew dewars had to have a vented lid. I suppose I was just hoping to find a smaller one with a vented lid for transport.
Timgunn1962 that is a lot of good info, thanks for posting that..
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Stacy We do have excellent results with the dry ice slurry we use and its cheap at that.I think your right.I just don't think we do enough stainless to justify such a large expense and such a large filling expense to have it all go to waste..
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If I run across one at the right price we'll buy it, just not at the prices Im seeing right now..
 
Go talk to people that actually use LN on a regular basis. A friend of mine works in the cattle breeding program at a large university and he just found me a 20 Liter Dewar with a 3.25" neck for $160. Talk to your local LN vendor and find out who some of their regular customers are and you may just find a good used one locally. Good luck.
 
From what I have been by folks who use it, the cost of LN in the quantities that we use is almost insignificant. its just another relatively low price consumable. Availability might be an issue for you, Phillip.I don't know if you would have to drive a substantial distance toward Lexington to get the stuff. I can get it right across the street from my shot. The cost of a dewar and my limited use of high alloy steels so far has been what has kept me away.
 
Another thing that hasn't been mentioned is that with a small dewar, even if you can find one with say a 2.5 inch neck or larger, you may still be DEPTH limited. The thing will just be too short for big knives. From what I remember from looking for them, you can find ones in the 10 liter range where that is not a huge issue.
 
I have and use a 10 liter dewar regularly. As has been said about the top, just a foam type plug. I paid $275 for it used.
I get 2-3 weeks out of it depending how often I open it. I usually do blades and springs for folders in batches and try to stay
ahead of myself with them. I often have 20-30 sets of blades and springs heat treated and ready for fit up at any given time.
So I don't heat treat every week. Right around $25 to have mine filled locally. Good luck, I could'nt be happier with mine- total
control, and Rc check every blade and spring.
Ken.
 
Ken, what is the neck size and internal height on your dewar? For some of us, the "gateway drug" for cold treatment was stainless steel kitchen knives and they are often just too big for a lot of smaller dewars and even some bigger ones because of blade width vs. dewar neck width.
kc custom said:
I have and use a 10 liter dewar regularly. As has been said about the top, just a foam type plug. I paid $275 for it used.
I get 2-3 weeks out of it depending how often I open it. I usually do blades and springs for folders in batches and try to stay
ahead of myself with them. I often have 20-30 sets of blades and springs heat treated and ready for fit up at any given time.
So I don't heat treat every week. Right around $25 to have mine filled locally. Good luck, I could'nt be happier with mine- total
control, and Rc check every blade and spring.
Ken.
Click to expand...
 
jdm61 said:
Ken, what is the neck size and internal height on your dewar? For some of us, the "gateway drug" for cold treatment was stainless steel kitchen knives and they are often just too big for a lot of smaller dewars and even some bigger ones because of blade width vs. dewar neck width.
Click to expand...

The inside of the neck measures 2 1/8", the total depth below the stopper would be 14", so agreed bowies and large kitchen knives are out.
I think I've actually done fillet knives maybe as big as 15" length total.

Ken.
 
That probably big enough for a lot of what we do. How long do you think it takes for a blade to stabilize at the temperature you require to quickly deal with the RA without considering long soaks to create those funny carbides?
kc custom said:
The inside of the neck measures 2 1/8", the total depth below the stopper would be 14", so agreed bowies and large kitchen knives are out.
I think I've actually done fillet knives maybe as big as 15" length total.

Ken.
Click to expand...
 
I usually suspend them on a wire for a bit instead of an absolute "dunk". Once immersed sometimes I take them out
in an hour, sometimes overnite. I can't say that I can see any difference with a long soak, but I can tell a difference
between LN, and dry ice alcohol-- enough to make any difference? I doubt it.
Ken.
 
Look to see if your university nearby has a "dollarsaver" publication. Not widely known but universities offload general equipment this way pretty often.
 
I have no problem getting Ln(plenty of places sell it around here) but I live in a very mountainous rural area..No universities and no cattle farms so generally no cheap source for used dewars.
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Im just curious..Im in it for the RA reduction..A friend of mine does have access to Ln for his knives and has a RC tester..The added hardness has been the same(tested) for both Ln and dry ice slurry..About 1-2 rc..I know the dry ice slurry dosnt get cold enough for carbide formation but it seems like it gets plenty cold enough for just as much RA reduction..At least according to the hardness readings..
 
One more quick question? I know some hang blades above the Ln and some dunk them..What if say you dewar isn't tall enough to completely submerge them..What about part of the handle sticking out of the Ln? would that be ok or would it cause a problem with maybe warp?
 
According to my friend who works with LN every day, the vapor above the liquid is ~-160. So the vapor alone is enough to convert RA. Formation of ETA carbides requires complete submersion and a long soak.
 
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