It seems like you have a problem. At a constant ambient pressure, the
helium boil off rate is determined by heat transfer to the helium
reservoir in the magnet dewar. The liquid helium in the reservoir is at
the boiling point for helium at that ambient pressure. When the ambient
pressure drops, the boiling point of the liquid helium drops, so the
boil off rate increases until the bath temperature reaches the new
value. The boil off rate should then return to the steady state
value. Similarly, when the ambient pressure increases, the helium
boiling point increases and the boil off rate decreases until the heat
flow into the reservoir raises the helium bath temperature to the new
boiling point, at which time the boil off rate returns to the steady
state value. If the boil off rate is higher than usual after a couple
of days of steady barometric pressure, there has been an increase in the
heat flow into the helium reservoir.
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. This
can be remedied by pumping on the dewar. There are other possibilities
for increased heat flow, leading to increased boil off, which rerquire
more drastic action to correct. It sounds like you need to contact
Oxford pronto!
Superconductive magnets that drift often have a drift rate that is
dependent on barometric pressure for just these reasons, with the drift
rate dropping when the barometric pressure drops (our Oxford 800 is such
a case).
Ben Bangerter