Dewars
Sandfire Scientific has been manufacturing and repairing Dewars for a good fifteen years. So, when Dr. Ivan Keresztes, Director of the NMR Facility. Department of Chemistry and Chemical Biology, at Cornell University asked if we could repair or make him new units, we were happy to oblige. Dr. Keresztes sent us his broken Dewars, and the first thing we noticed was that they had a silver coating. In addition to repairing the units, the silver would have to be removed, which we did to properly measure the units. Then, the silver coating would have had to be reapplied. However, after discussing the issue with Ivan, it was decided to make new Dewars, but without the silver coating.
Customers have found that using a traditional silver layer deposited on the inside of the Quartz NMR Dewars, both for Varian Style and also for the Bruker Style, has led to some inconsistent readings due to the metallic layer. We therefore developed the insulation for both styles of Dewar out of material developed for the nose cones of missiles. The material is 99.999% SiO2 woven into a fiber, as you can see in the close up photo. The material is heated to around 1000 C before it is covered by the outer layer of the tubing. As it is being pumped on our vacuum system, it is heated to 600 C. This results is an NMR Dewar made entirely from 99.999% SiO2 with a superior vacuum which will not outgas from any contaminants.
The standard silver coated NMR Dewar has a coating that contains contaminants, and because the interspace is so small it is very difficult to clean the surface after the silver has been deposited on the inner surface. As well, the coating can only be heated to 300C when pumping as above that temperature the silver surface breaks down leaving a dull grey appearance. As this surface coating of silver, from a deposition process outgases over time, it can degrade the vacuum.
The pure quartz fiber, tight wound on the outside of the Dewar, serves well to stop radiation from disturbing the temperature of the Dewar, not quite as well as the silver layer, but the difference we believe is only between an increase of efficiency of around 30% for a silvered Dewar to an increase of efficiency of around 20% for the quartz fiber. There has not been any issues with the many Dewars that we have manufactured to date as the slight loss in efficiency has been made up by having a better vacuum and no metallic effects.
The photos show the old Dewars and the new ones. We can manufacture different lengths of both Varian and Bruker Style NMR Dewars and look forward to receiving your specific requirements for your applications. Please do not hesitate to contact us.
