Useable internal height in dewar?

S

Scaniaman

Joined
Jun 15, 2012
Messages
1,423
Hello people,

When using a dewar for cryo, is the neck part any good temperature wise? I've understood that the whole chamber gets cold enough, even with only a little LN2 at the bottom. But the neck, does it get cold enough too? It kind of dictates what model/size to get.

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Drop your knives blade down, you'll be fine
 
On all the dewars I've had and used the neck is not as long as depicted in the photos you posted. The blades do not need to be submerged.
 
Thanks.
Does anyone use a dewar so small, they have to keep part of the blade in the neck if the dewar?
 
A small 10L Dewar is about 13" tall. That should work for knives up to 8-9" OAL.
A 15L and 20L Dewars are about 16" tall. That should easily do a 12-13" OAL blade. Longer blades will work, as long as the edge is in the cold zone. The handle area isn't as important, and as long as it is in the neck thermal conduction should take care of dropping the temperature low enough to reach the Mf.

There is little economy in getting a small Dewar. 15L is really the minimum, and 20L is better. The larger units will usually hold the LN longer.
 
I have my eyes on a 15 L with 80 mm opening. I think the internal chamber height is appr 300 mm, so that should cover most things. But my furnace can take almost 600 mm, but I would rarely make a something that long.
I only HT maybe 4 times a year. So plan is to go and get maybe 5 liters LN2 the day before or so, should be enough. LN2 place is 15 min drive away, so pretty good.
 
Scaniaman said:
So plan is to go and get maybe 5 liters LN2 the day before or so, should be enough. LN2 place is 15 min drive away, so pretty good.
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That's nice having LN so close. How much per liter do ya'll have to pay in Sweden? OR, in your area? Here in my area the price runs from 50 cents/liter to $6/liter depending where I get it.
 
Ken H> said:
That's nice having LN so close. How much per liter do ya'll have to pay in Sweden? OR, in your area? Here in my area the price runs from 50 cents/liter to $6/liter depending where I get it.
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Last time I checked (two years ago I think) it was around 11-12 USD per litre incl taxes
 
So I am ending up ordering a 20L with 80 mm mouth/neck.
It's 607 mm high.
Internal chamber height 380 mm.
Seems like good numbers all over, for my needs.
 
I like that 80mm neck - better than my 50mm neck. While it will loose bit more LN thru the larger neck, with 20L it's still gonna last longer than my 10 L which is around 6 weeks (if I remember correctly).
 
Yes the larger neck gives a higher evaporation rate (0.13 vs 0.088 L per day compared to a 50 mm neck). But it won't matter to me, as I only plan to go and get a few liters of LN2 the day before heat treat or so (which only happens like 4 times a year). I went with 20 L as the internal height was 380 mm, compared to 350 mm in a 15 L. It will give enough room even for long chef knives.

Yes I think 80 mm will be good, I can't see myself needing mote than that.
 
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I like your plan, especially since your source is so close.

My 10 liter dewar is empty so I just now measured it. It's got a 9" neck, and 20" full depth giving 11" (280mm) of height inside the dewar without getting into the neck. When I do long chef knives I let the tang stick up into the neck area as needed.
 
Ken H> said:
When I do long chef knives I let the tang stick up into the neck area as needed.
Click to expand...

I was thinking about getting a smaller one, and doing this too. But I figured most of the time I'll leave the blades in the dewar over night and temper in the morning, so it is nice to be able to put the plug in the neck.
I did find a very good deal on a 10 L though, the 20 L is going to be almost 5x the cash 😱
 
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