A dewar is a type of cryostat named after Sir James Dewar, the researcher who first developed the concept of a vacuum insulated container with silvered walls to reflect thermal radiation. Dewar was the first to liquefy hydrogen, and he created the device to store his discovery.

The thermos bottle is a rudimentary form of a dewar. It has an insulating vacuum and silvered walls to reduce heat transfer between room temperature and whatever consumers put into it.

While “dewar” and “cryostat” can be thought of as synonyms, many engineers tend to use “dewar” when referring to cryostats designed principally to store or transport cryogenic liquids. This is not universally true, however, and one occasionally hears researchers referring to test dewars that contain experimental equipment. Further complicating the nomenclature is the fact that many large storage dewars, particularly those used for LNG, do not use vacuum insulation as part of their design. Figure 1 shows a liquid helium storage dewar.

The design of cryostats and dewars is covered in R. Scurlock, Low Loss Dewars and Tanks, Cryogenic Society of America, 2008, and J.G. Weisend II (ed.), Cryostat Design, Springer, 2016. The use of non-vacuum insulation systems for storage tanks is addressed in R. Barron, Cryogenic Systems, Oxford, 1985; and T. Flynn, Cryogenic Engineering, Dekker, 1997. ■

This is the last “Defining Cryogenics” column. I hope that everyone has enjoyed reading them as much as I have enjoyed writing them. I wish to thank Brian Dudley, Laurie Huget and the entire CSA and Cold Facts team for their help and support with this column. I also wish to thank the many colleagues who have helped me over the years with questions, comments and suggestions for this column.

In the next issue of Cold Facts, I will be starting a new column that explores the lives of people who have played a key role in the development of cryogenics. This new column titled “Cryo Bios” will begin by looking at the life and contributions of Sir James Dewar.