Dry ice in a dewar - I wonder

JTknives

JTknives

Blade Heat Treating www.jarodtodd.com
Knifemaker / Craftsman / Service Provider
Joined
Jun 11, 2006
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Now befor we get into this let me put this out there. I’m not an expert, just crazy.

Ok so this came about as a matter of necessity not an “I wonder”. So Tuesday I picked up 10lbs if dryice for some AEBL heat treating. I got home and realised I had jumped the gun. The blades where not ready and needed some more work and another 12hrs of tumbling. So being that I did not want to waste it I dumped it into my empty dewer, Why not right? Well today I peaked inside and it’s all still there sitting peacefully snd minding it’s own business. So I had another thought. I wonder what the temp is down inside the dewar. I have allready showen that the temp above liquid nitrogen up to the foam plug is -300° to -320°. And that it’s perfecty fine to suspend blades in this cold atmosphere. So why not with dry ice. I know this topic has been beaten to death here. To slurry or not to slurry. Normaly I would agree with using a slurry if the top was open and exposed to the atmosphere temp. But if it’s contained and allowed to cool it’s enviroment then why not.

So I did a coupon if AEBL. Same material as my last testing series on AEBL. So I know exzactly what the hardness should be at every point when everything is done by the book. So coupon got 8min at 2000°. Then into quench plates. I then cleaned up the surfaces and hardness tested it. Weird thing is is it will skate a file but only measured 56rc. I then suspended it in the dewar above the dryice. And set the timer for 10min. When it came out it was smoking and building up frost. I warmed it up and went to the hardness tester. It tested at 61-62 which is identical to the previous test I did that got a slurry soak. I then tempered it at 300°. I then went back to the hardness tester and got damn spot on 64.5rc. So according to these numbers the results are identical. I would actualy put money on the dewer creates less of a warping problem the the slurry. It’s genlty cools down where the slurry is quite the shock to the steel. I know I have heard that you need to cool it down fast but this is an air hardening steel and does not require a fast anything.

So I leave it up to you to decide weather or not my results are bogus or truthful. All I can say is that I’m going to be doing it this way from now on unless I can be shown hard proof that’s its wrong. Nothing wrong with questioning tradition. If we didn’t then we would still be edge packing and quenching blades facing north.
 
Don’t ask me I’m not a metallurgist. But it happens, and happens a lot with AEBL. I did a post on here where I showed all my test results from testing 5 coupons from start to finish through the full range of tempering.
 
"IF" the temperature above the dry ice is at the -95F range, it would work just fine. Did you happen to check the temperature?

A question JT - since you've got LN and dewar, why were you using dry ice? Just happened to be out of LN? It's my understanding that once you get the dewar LN is less expensive than dry ice because it keeps so long - is that true?
 
Yeah with a good dewar LN keeps for a long time. But the thing with LN is you can’t just top it off when you got some extra cash to do so. I pay right at or just over $3 per liter and it’s 20 liters. That’s allmost $70 to fill, it does last for a LONG TIME. But with me having closed the heat treat side of things till after blade I don’t need LN. The funny thing with LN is I feal I burn through it quicker the fuller it is. Has somthing to do with the blades being submerged into the LN and causes it to boil off rapidly.

Now I’m just assuming but it holds water with traditional insulation’s. The colder it is outside the less efficient the insulation in your walls is and the more you have to heat the inside. So I’m wondering if a dewar is more effencent when used with somthing that’s 3 times warmer. If so then the dryice could be cheaper by a large margin especially considering I would only have to keep a small layer in the bottom. Seams like I test is in order.
 
JTknives said:
Yeah with a good dewar LN keeps for a long time. But the thing with LN is you can’t just top it off when you got some extra cash to do so. I pay right at or just over $3 per liter and it’s 20 liters. That’s allmost $70 to fill, it does last for a LONG TIME. But with me having closed the heat treat side of things till after blade I don’t need LN. The funny thing with LN is I feal I burn through it quicker the fuller it is. Has somthing to do with the blades being submerged into the LN and causes it to boil off rapidly.

Now I’m just assuming but it holds water with traditional insulation’s. The colder it is outside the less efficient the insulation in your walls is and the more you have to heat the inside. So I’m wondering if a dewar is more effencent when used with somthing that’s 3 times warmer. If so then the dryice could be cheaper by a large margin especially considering I would only have to keep a small layer in the bottom. Seams like I test is in order.
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It is most definitely more efficient with a smaller temperature delta between its contents and its surroundings. Conductive heat transfer rate (heat flux) is directly proportional to the temperature delta and inversely proportional to the thickness of the insulation. In the case of a dewar flask, this math gets way harder because (presuming a perfect vacuum) the only conductive heat transfer is through the flask walls. Fortunately, there is so little conductive transfer, it doesnt matter much. The rest of the transfer is radiant (where the math is much easier).

In all cases, the rate of transfer is proportional to the temperature difference.
 
Where did you hear that colder outside weather made the house insulation less efficient? You have to heat more because there is more heat transferred to the colder outside. I hardly ever heat my house in July, but that isn't because the insulation is better when it is 90F outside :)

The R factor is the same at zero as it is at 90. The difference is that the differential is greater.

In a dewar, there is a vacuum between the inside and outside. A vacuum transfers almost no heat.
 
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Stacy E. Apelt - Bladesmith said:
Where did you hear that colder outside weather made the house insulation less efficient? You have to heat more because there is more heat transferred to the colder outside. I hardly even heat my house in July, but that isn't because the insulation is better when it is 90F outside :)
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Well, to be totally pedantic, the coefficient of heat transfer (expressed generally as energy / area * degrees) is listed at a specific temperature. Insulating properties change with temperature. Gasses increase conductivity with temperature, liquids decrease, metals decrease, nonmetallic solids increase (there may be exceptions to this). That being said, the conductivity changes are pretty small over a few hundred degrees of temperature change, the bigger issue is of course the difference in temp between the inside and outside as per the thermal conductivity equation:

q = - λ * ΔT / Δx

where:

  • λ is the thermal conductivity of the material,
  • ΔT is the temperature difference across the object,
  • Δx is the distance of heat transfer (the thickness of the object), and
  • q is the heat flux, measured in W/m^2.
That all beign said, in a vacuum flask, conductivity would need to be measured over path along the surface of the flask (in other words, Δx is not the thickness of the flask but rather the path from the top of the liquid to the point where the heat transfer is taking place). And for a vacuum flask, this is relitively tough calculus because you have to basically integrate over a large and comple surface shape over a range of ΔT and a range of λ. When you do do all this math (which I haven't in many years, I just remember it sucking), you find that conductive heat loss is tiny and the majority of your heat loss (or gain in this case) is from radiation. Which the flask is also designed to minimise, and whose rate is also proportional to ΔT.
 
I know when you look at fire bricks there is a btu conductivity rating for different temps.

Just makes me wonder how much longer dry ice wi keep in a dewar.
 
JTknives said:
I know when you look at fire bricks there is a btu conductivity rating for different temps.

Just makes me wonder how much longer dry ice wi keep in a dewar.
Click to expand...

Ostensibly much longer if you keep the air flow down. The latent heat of sublimation for dry ice is almost 3x the heat of vaporization of nitrogen. Couple that with the much smaller temperature delta, it should last a very long time.
 
I thought about doing a simple test by weighing the dewar and calculating the lose in weight over time.
 
JTknives said:
Yeah with a good dewar LN keeps for a long time. But the thing with LN is you can’t just top it off when you got some extra cash to do so. I pay right at or just over $3 per liter and it’s 20 liters. That’s allmost $70 to fill, it does last for a LONG TIME. But with me having closed the heat treat side of things till after blade I don’t need LN. The funny thing with LN is I feal I burn through it quicker the fuller it is. Has somthing to do with the blades being submerged into the LN and causes it to boil off rapidly.

Now I’m just assuming but it holds water with traditional insulation’s. The colder it is outside the less efficient the insulation in your walls is and the more you have to heat the inside. So I’m wondering if a dewar is more effencent when used with somthing that’s 3 times warmer. If so then the dryice could be cheaper by a large margin especially considering I would only have to keep a small layer in the bottom. Seams like I test is in order.
Click to expand...

That sounds odd to me. The place where I buy my LN will sell me 1 liter, the full 30 liters, or any amount in between. They will also top it off if I only have a few liters left and want to get more. They're really laid back at this place and are always quizzing me about how to make knives. They hardly ever charge me the full amount.
 
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