> Often this is due to helium gas in the evacuated region of the dewar,
> due to helium gas getting past the O rings that seal the dewar"
Are you saying that changes in the barometric pressure of the atmosphere
has an effect on the Helium that has leaked into the vacuum space. If there
are that many off the littlecritters in the vacuum space the boil off rate
should be fairly impressive.
I always thought the increase in boil off gas was caused by a thermal
oscillation created between
the liquid in the dewar and the gas. These pressure waves of the oscillation
allow energy into the
system and the boil off goes up.It has been my observation that the
magnitude of the change of boil
off is related to the speed in which the barometric pressure changes. Does
the Helium in the magnet
dewar have a gradient ( A helium fog that turns to liquid?) and that
establishes a certain heat loss
pattern and the resulting barometric pressure change create a change in this
flow and create heat .
It has been my experience the magnitude of the boil off change due to the
weather and jumps in
the barometer is greater with a system that is low on liquid. A full dewar
does not do this.
Will a pressure relief valve with a constant pressure spring prevent this?
A.S.Chesnick
Laboratory of Cardiac Energetics
NIH / NHLBI
Bethesda, MD