Has anyone used dry ice + kerosene in an LN2 dewar? What 'shelf life' did you get?

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direstrats220

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Sep 29, 2012
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I don't have a good local source for liquid nitrogen, but can get dry ice and kerosene 5 miles down the road. Is there any safety issue or something I am missing? I'm curious how long the dry ice would last in the dewar, presumably longer than LN2 due to the smaller temperature gradient?
 
How big is your dewar? Either way, the LN is going to last significantly longer than any kind of dry ice or dry ice slurry. Even with a large, well insulated dewar, I'd be surprised if the dry ice lasted more than a couple of days before it fully sublimated. You almost certainly wouldn't get more than 3 or 4 days. The last time I filled my dewar, it lasted a few weeks before it was completely evaporated. The last dry ice slurry I made (in a very well insulated ventable container) lasted about 1 day or so, though it was only about 2 gallons of a dry ice / acetone slurry.
 
I have used 5# of DI and 4L alcohol. It lasted about two days. I also tried just 5# of DI pellets. It lasted about the same. My dewar was 20L.

I normally just use a deep pan of alcohol and a few pounds of DI. That lasts about 3-4 hours. Since there is no required soak time for the sub-zero quench, 30 minutes is more than long enough to reach Mf. Remove from DI bath, allow to warm up to room temp for about 15 minutes, and temper immediately for best results.
 
Stacy E. Apelt - Bladesmith said:
I have used 5# of DI and 4L alcohol. It lasted about two days. I also tried just 5# of DI pellets. It lasted about the same. My dewar was 20L.

I normally just use a deep pan of alcohol and a few pounds of DI. That lasts about 3-4 hours. Since there is no required soak time for the sub-zero quench, 30 minutes is more than long enough to reach Mf. Remove from DI bath, allow to warm up to room temp for about 15 minutes, and temper immediately for best results.
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My issue is that I have limited space in my HT furnace/knife tray, and even more limited time to make and HT blades (young kids, demanding job), so I would want to do batches over a couple days if possible so I can go right from quench to cryo.
 
Drew Riley said:
How big is your dewar? Either way, the LN is going to last significantly longer than any kind of dry ice or dry ice slurry. Even with a large, well insulated dewar, I'd be surprised if the dry ice lasted more than a couple of days before it fully sublimated. You almost certainly wouldn't get more than 3 or 4 days. The last time I filled my dewar, it lasted a few weeks before it was completely evaporated. The last dry ice slurry I made (in a very well insulated ventable container) lasted about 1 day or so, though it was only about 2 gallons of a dry ice / acetone slurry.
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I could be wrong, but my understanding is that dry ice should last longer than LN2 in a dewar. I recall seeing 'static hold time' equations, and heat loss out of a dewar is a function of radiative heat transfer, which scales with temperature difference, i.e. the temperature delta between the liquid and the atmosphere. I'd be very surprised if dry ice didn't last longer than LN2! I guess I'd test it, but LN2 is such a pain for me to get. Maybe the local university would be the best source.

My dewar is 10L I believe.
 
I checked my supplier info page and it says a 20L dewar of DI pellets lasts 18-24 hours.

As for Ultra-Low-Temp freezers - I looked into them before. An Ultra-Low-Temp freezer will work for small to medium blades. The chamber on a countertop unit is 13x12X8". Cost is $1300 plus shipping. Next size up is a floor model at $2300. That unit has 14X11X11 tank dimensions. They also draw 10 amps 24/7. At .20/Kwh that is $1750 a year to keep it cold. I suppose turning it off between sessions would work, but it takes a day to cool down again. I asked the local university if they every get rid of one and they said they can't give them away even if they replace one with a new unit.

My sub-zero setup is a "Whole Fish Pan" 20" long with a lifting rack. I pour in a gallon of paint grade denatured alcohol ($15) and break up a 3# block of DI ($ 5-7 at the grocery store). Slowly add the DI. It will boil violently at first and then settle down to a slow bubbling as it reaches around -95°F. Put rack in the pan and then set the blades on the rack. Let sit as long as you want, but after 15 minutes it has reached Mf. You can take those out and put in the next batch of blades then. If needed, add some more DI. As long as the alcohol is bubbling it is cold enough to do sub-zero quench. When done with the bath, let the alcohol sit for several hours after the bubbling stops, pour it back in the can for the next time. Put the cap on a little loose for a day to let the last CO2 gas off.

NOTE:
Sub-Zero quench and Cryo are not the same thing. Sub-zero is considered to be cooling to around -100°F and is the lower Mf for most stainless and high alloy steel. Cryo is done in Ln at -320°F. The steel reaches it's Mf as it drops below -100°F and after reaching -320°F and holding for a period of hours causes some other changes to the structure of the steel and adds an extra 1-2 points of hardness. While cryo is very nice, sub-zero is sufficient for most knives. Both methods create a fully hardened blade.
IIRC, only a very few steels (niobium alloys?) have an Mf below -100°F.
Dry Ice is cheaply available from many grocery stores, in pellet form welding supplies (still pretty cheap), and even delivered from Amazon.
 
You have to read into those descriptions on Amazon, most all that they describe as dry ice is really those gel pack you keep in a freezer then toss them in a cooler just to keep food and drinks cold. I did see one add for real dry ice but it was expensive and the reviews weren’t very good, like a lot of product sublimating before it gets to you.
 
Stacy E. Apelt - Bladesmith said:
I checked my supplier info page and it says a 20L dewar of DI pellets lasts 18-24 hours.
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I did a little more research and I'm fairly certain that is just not accurate for hold times for a 20L dewar. I've gotten shipments of ice cream in a cheap foam cooler with half the dry ice still there after 2 days of shipping.

"Dry-Ice (solid CO2) is also frequently used in Dewars. This exists at the relatively warm temperature of around -90°C. Our tests suggest that holding times for CO2 are about 25% longer than those listed above for LN2."

I guess there's really nothing to do but try it.
 
direstrats220 said:
I did a little more research and I'm fairly certain that is just not accurate for hold times for a 20L dewar. I've gotten shipments of ice cream in a cheap foam cooler with half the dry ice still there after 2 days of shipping.

"Dry-Ice (solid CO2) is also frequently used in Dewars. This exists at the relatively warm temperature of around -90°C. Our tests suggest that holding times for CO2 are about 25% longer than those listed above for LN2."

I guess there's really nothing to do but try it.
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Back in my music festival days, I used dry ice in the bottom of my cooler under regular ice. In a large Walmart cooler, 20lbs of dry ice would last 4-5 days in 90 degree weather.
 
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