I've been looking at various papers which describe the mechanisms and provide calculations describing heat flow into helium. I'm trying to minimize helium loss and am in the market for a new dewar flask/cryostat. I plan on asking the supply companies directly what the proper dimensions are to reduce helium loss while still maintaining low temperatures for our needed time period (~10 min), but I would like an idea of the physics behind these dimensions. Currently, we have a glass dewar flask containing LHe sitting in a glass dewar flask containing LN2. Purchasing a metallic cryostat is an option, but I don't know much about them. Here is one of the papers I found: http://jap.aip.org/resource/1/japiau/v22/i12/p1463_s1#tabs_1_113_1274104113_tab3 [Broken] (Evaporation Rate of Liquid Helium. I). It provides good qualitative description, but the dewar flask I was looking at doesn't have a reduced-diameter neck as the one they describe does. Any input would be valuable! Thanks!