Making Dry Ice -

There is a bag-type device we used in the lab to make small amounts of dry ice attaching it to a commercial tank of CO2. It was available from Aldrich Chemical in Milwaukee, and probably other scientific supply houses. Terribly inefficient, plus it made a fine flaky material which because of the high surface area sublimed way too fast. We had the stuff delivered in pellets the last 15 years or so and it was way better for acetone baths. This could be an interesting thread, though, cuz the stuff's $13 a brick here locally, and I can't waltz into a place I retired from and grab a couple handfuls for a "government job" anymore. I wonder if bubbling CO2 thru an appropriate liquid shouldn't have the same effect, though this would be a touchy process at best.
 
Just out of curiousity, what are you folks doing with the dry ice, and what sort of temperature do you need to reach?
 
There is a deposition type box made which is much, much more efficient than the bag type device that fitzo mentioned. They are quite a bit more $$, but it will pay for itself over time and you will get blocks, not a bag of CO2 flakes.
 
Interesting.

Actually I found a source on the net for a little box shaped machine that will make blocks from 1 to about 3 lb., depending on the model. The 1 lb. model is about 430 bucks but does include the valve and gauge (whatever). The 3 lb. model is about 130 bucks more. The 1 lb. block of dry ice is made in about 1 minute! You still got to go fetch the CO2 though, of course.

I have aged steel with dry ice but hate the 24 mile round trip to the vendor. I am kinda wondering about whether to just drive the 24 miles round trip each time I need to cryo, go with liquid nitrogen (that cost bucks to get started too), or make my own dry ice. LN cryo is faster.

PAUL: dry ice is one way to age the blade steel (transform remaining austinite into martensite). Martensite is what we strive for in the heat treating process. Dry ice has a surface temperature of about -109 F. (-109.3 F.) Doing a 'dry ice' or 'cryo' search on this forum will reveil more indepth info about the subject.

Thanks guys. I hope someone who is making their own dry ice in-house will see the post.

Roger
 
If I had a gadget that could freeze CO2 to make dry ice I would use that directly to provide cryogenic temperatures rather than bothering to make the dry ice. I would circulate the acetone through the gadget if it worked like a refrigerator and just use chilled acetone. If it relied on expanding liquid CO2 to create cryogenic temperatures I would just use the liquid CO2 directly. If you properly meter expanding liquid C02 you can probably reach lower temperatures than you would with your dry ice and acetone slurry.
 
Liquid CO2 is accomplished by pressure, not by refridgeration. As the liquid evaporates the remainder turns solid. The liquid state is warmer than the solid state of CO2.

RL
 
Guys, for the price and hassle of these little gadgets you could get a LN2 dewer. I picked up a 30 liter dewer on Ebay for $125 including shipping. LN2 is not that costly, I get it for $2.50 per liter. I can get deep cryo on up to 75 blades per fill.

If you are just a hobbiest (not doing 40 knives per month) then you can get by quite well with a smaller unit. These can also be found on Ebay for under $50.

Most welding supply centers will rent dewers also. They normally run something like $5 per day (not counting LN2). In one day you can do a real deep cryo on 20 blades.
 
 
Try cruising some of the local welding gas suppliers, not the national chain guys but the independent ones.

One here has a loading dock with several used dewars in several styles/sizes basking in the shade waiting for a buyer, $65. each, your pick. And, thats their asking price.
 
Universities with active research programs often have a surplus store that sells stuff to anybody. Dewars can be found its the undewars that are hard to track down.

Lynn
 
I just use a small lab dewar
I pay by the pound at $1.17 at about $10.00
I can do at least
8 blades with it. I got mine from E-bay also.
with co2 your at about -90 deg
LN about -300 deg the lower the temp the better.
 
RLinger, could you please post the url for the device that made dry ice blocks? Thanks.
 
R.W., Thanks for all those ebay links.

I think the best way for me to go is LN. I will continue to purchase dry ice until I get better settled into knife making; then start shopping for a dewar and whatever else needed.

MIKE, here is the link to the dry ice machine I mentioned within these posts: http://www.polyfoam.com/dry_ice_makers.html

Thanks to all.

Roger
 
Thanks, Roger, for posting that link! I am eventually going to get a more permanent cryo setup at home, and this will help in making cost comparisons. Getting tired of running to the only folks who sell dry ice in town and finding out "it'll be two days before we get a shipment". LN is available, but I just hate working with that stuff. Had a spill in the lab once and it had quite a high "pucker factor". LN on yer tennis shoe makes for an interesting moment! LOL Thanks, again.
 
Rlinger, You make dry ice from liquid CO2 by letting it evaporate and expand. This cools the liquid until some of it freezes. You are wasting most of the refrigeration potential of the liquid CO2 if you only collect the solid CO2. It is a common cryogenic technique to compress a gas (which heats it in the process) then let it cool to room temperature, then let it expand at an extremely high rate. The temperature of the compressed gas is a function of the average kinetic energy of the gas molecules. Since KE= .5*m*v-squared the temperature of your gas is proportional to the square of the speed of the molecules. When you open a valve and let gas escape quickly the fastest molecules are the first to go. You selectively extract the hot (fast) molecules first and as time progresses the remaining gas gets colder and colder. When I was working on heat seeking missiles we used expanding nitrogen to supercool our infrared seekers.

So I am serious, you can use liquid CO2 that isn't cold before you open the valve as a refrigerant. The nice thing is that it doesn't get warm or evaporate between uses.
 
Jeff,

Thank you for that thorough description. So; as I think I understand, the liquid CO2 can achieve cryogenic temperature. Please correct me if I misunderstood. My question is what lowest temperature can one expect from the liquid for the purpose of aging steel by heat exchanging the CO2 with another liquid such as the one you mention in your first post?

Thanks again; Roger
 
After reading up on the dry ice maker, Jeff has got it right on the head. From a 50 pound liquid CO2 charge the company says you get 9-11 1-lb blocks; very inefficient. The use of the gas expansion to cool is what I was alluding to in an earlier post. I'm wondering if you can't use a non-siphon tube tank to take it directly out as gas, bubble the cold gas thru a solvent such as acetone or kerosene. Using a gizmo called a filter candle, which is a sintered glass membrane, the gas may bubble fine enough to get pretty good heat transfer. Have to use it in a ventilated room, of course. Interesting.
 
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