A Metal-Encased Dewar Flask Glass Dewar flasksare susceptihle to implosion by either mechanical or thermal shocks. A previous article in this Journal describes using polyethylene battles to protect Dewar flasks from breakage ( I ) . Far even greater protection, Dewar flasks may be purchased encased in m e t . cylinders, but these are expensive. In order to hold down costs while improving safety, a homemade metal-encased Dewar was constructed. The cast of the materials is about one-tenth the cost of a commercial flask. The homemade flask has held up well to use with liquid nitrogen. The &el casing is made from two 13-az coffee eans. One can has both the top and bottom removed. The other can has its top removed and four small holes punched in its bottom. The two eans are taped together to form a steel cylinder. AThermos brand size 74F or 75F borasilieate glass replacement liner will fit into the cylinder with approximately 3116-in. radial clearance. Two small pieces of cardboard are used to center the Dewar in the cylinder. A commercial spray p o l y u r e t h a n e i s a t e foam is then injected through the holes in the bottom and along the sides to completely fill the gap between the Dewar and the cylinder. After the foam cures overnight, trim off any excess. Although quantitative tests have not been conducted, the foam is expected to provide shock resistance, supplement the insulation, and help to secure the outer wall of the Dewar in the event of an implosion.
Literature Cited 1. Gyrniib, M.J Chem. Educ 1971.48 397
David Ritter Southeast Missouri State University Cape Girardeau, MO 63701
70
Journal of Chemical Education
