Hi,

I recently joined a research group that is using a dewar for containing samples that need to be immersed in liquid nitrogen.  An automated mechanical gripper occasionally takes samples into and out of the dewar. The dewar is automatically refilled by a system that pipes in liquid nitrogen. A capacitive level sensor triggers a solenoid valve in the LN2 supply line, to maintain a sufficient level of LN2 in the dewar.

I am told that we are having problems with the level sensors because they get iced up and give a false zero reading, which causes the dewar to overflow. Obviously, we must do our best to keep the moist room temperature air out of the dewar, but some will inevitably accumulate.

My question is, what sensors exist for determing the level of liquid nitrogen in an open container?  

If anyone can give practical reference info for cryogenic techniques, I would appreciate that. I see that last year, 'eigenstate' posted a couple of books,  "Cryogenic Engineering" by Hands (1986) or "Matter and Methods at Low Temperatures" by Pobell (1991).  Do these books cover practical tachniques of LN2 system design?


Thanks in advance,
Jon Spear
jdspear@LBL.gov

Jon Spear
Berkeley Lab
www.lbl.gov
http://bcsb-web.als.lbl.gov/Staff/Newstaff2.htm