A good cover -- judiciously used -- goes a very long way in reducing ice
in dewars whether the dewar is made of foam, glass or stainless steel.
If you keep your dewar covered with a lid that extends on the outside of
the dewar down a few centimeters below the dewar edge, you will have
minimal icing in the dewar. It actually doesn't even matter what the lid
is made of. I use the covers of DVD/CDROM packs. I try to remove the
cover only for a second or two when plunging crystals and immediately
cover the dewar again. From my experience, most folks leave the lids off
too often and for too long. Typically, they walk away leaving the lid
off. It's no surprise that ice accumulates.
If you leave the dewar open to the moist air, you will get ice in the
dewar no matter what it is made of.
Some simple prophylactic habits will go a long way to practically
eliminate ice in the dewar.
Jim
