Being the brave souls that we are we thought we would attempt to make a sample dewar without a lid. Our main concern is ice. We are concerned with the build up of ice around the rim of the dewar, but more important is the prevention of ice accumulating in the liquid nitrogen in the dewar. In order to prevent ice around the rim we have completely covered the top of the dewar with flexable silicon heaters as seen in the following picture.
This design works quite well. We have 240 Watts of heaters being powered by a 48VDC power supply and controlled with a small Minco controller. The prevention of ice building up in the liquid nitrogen is a much more complicated matter. Our present thinking is that a laminar flow of air across the surface of the nitrogen may be enough to draw off any accumulating ice crystals in the air above the nitrogen before they have time to reach “snowflake” size and fall into the liquid nitrogen…. pics and results to follow.