Cryogens:
Having worked daily with crygenically trapped high vacuum lines, maintained liquid helium cryostats for super conducting magnets, and employed various cold baths, I can offer the following comments.
Dry ice + solvent:
Basically any cheap solvent will do the job, as long as it doesn't freeze.
Acetone is commonly used because it is cheap, and doesn't get viscous when it is cooled. Also, if it gets slopped around, it readily evaporates. Downside, is it dissolves plastics and being quite volatile, smells a lot.
Isopropyl alcohol gets almost syrupy when used, and spillage doesn't evaporate as readily. Foaming is a little less violent if dry ice is added to a bath that has warmed up and consumed all the dry ice. Isopropyl alcohol is compatable with a greater variety of containers like insulated picnic coolers, etc.
Haven't used ethanol, isopropyl was always more readily available, and there aren't any tax stamp, de-naturing issues. Build up of water ice is minimized, because the water dissolves in these solvents. You don't want something to get stuck in a bunch of water ice.
There many cold bath concoctions between dry-ice temp and ice temp. They are basically slushes of frozen solvents which have been prepared by adding a judicious amount of dry ice or liquid nitrogen to the liquids. These must be consantly tended, and most of the solvents are not very nice.
Liquid nitrogen is pretty simple to use, (be warned that this causes instant frostbite...duh) if the following are considered:
Only use/store in appropriate containers that can't build up pressure.
Be aware that liquid nitorgen is very slightly colder than liquid oxygen. While unlikely in knife-treating, it is possible to condense liquid oxygen from the air.
What dissolves into a container of liquid nitrogen is little problem. But if some open, empty container gets cooled to liquid nitrogen temperature it is quite possible to condense relatively pure liquid oxygen. You definately don't want to do this.
For temperatures between liquid nitrogen, and ice or dry ice, a common procedure is to boil off the liquid nitrogen with a heater and use a thermostated heater coil to maintain the steam of gas at the desired temperature. This is easily adjustable, and avoids the hassles of frozen slushes. The same procedure employing liquid helium is the only way that I know off to maintain temperatures between liquid helium and liquid nitrogen.
Thermostatted heavy-duty immersion refrigerator coils are also used to maintain baths between dry ice and ice temperatures.
Course this equipment ain't cheap.
Forget about using liquid helium, that is a whole 'nuther ball game, and unless demonstrably qualified and properly equipped I doubt that any vender will provide it. Seriously it is not a simple extension of using liquid nitrogen.
Hope this helps.
