Stray Helium in NMR Suites

A possible detriment to instrument seals.

Published April 2006 Stray helium gas may degrade the instrument's vacuum and could lead to a quench.

There is reason to believe that NMR rooms should be periodically pumped free of stray helium gas – the thinking is that stray helium leaks past instrument seals and degrades the instrument’s vacuum and lead to a quench. It appears that the accumulation of low concentrations of helium (produced during the normal operation of an NMR) may penetrate the instrument's rubber seals and subsequently degrade the instrument's vacuum, which in turn may lead to a quench. This was advice from a vendor during the lead up to the installation of a 900 NMR and seemed to be relatively recent information. A passive exhaust system is recommended to remove this helium build up. The solenoid of the magnet is encased in a liquid helium dewar that is also encased in a liquid nitrogen dewar. In the case of an 800 or 900 MHz magnet, these higher fields are achieved by evacuating the solenoid thereby affording a lower temperature and allowing increased current. These dewars have seals that can deteriorate with time. There is preliminary evidence that gaseous helium caused by simple evaporation of the liquid helium might contribute to the erosion of the seals over time by some unknown mechanism. Therefore it might be beneficial to have a mechanism to purge the system of gaseous helium. However, the timeframe of this deterioration is quite slow and may not be an important consideration in design of the magnet room.

- From Tradeline's 2006 Summit on Facilities for the Emerging Sciences.