Liquid level sensors for LN2 in a dewar?

[jdspear]

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

http://www.lbl.gov

http://bcsb-web.als.lbl.gov/Staff/Newstaff2.htm

 

[rerickson]

I have no idea what the capacitance of ice is but if it is even close to water, it should register overfull, not empty. Maybe there is another problem. Either way, if you don't need to know the actual level at all times and just when it is full or empty, I would switch to a temperature sensing device (2 RTD's would be my choice). You can buy RTD switches from OMEGA or ???. I'm not sure whose dewar you are using, but CHART and Taylor Wharton probably have a liquid level switch they offer for the dewars you're using. They would probably be RTD's (or Differential Pressure if it's a big dewar). You probably have to call to find out about them, I doubt they are listed on their websites.

Good luck,
Ryan

 

[iainuts]

You might also consider using a weight scale and simply weighing how much LIN is in your dewar.

 

[scooter911]

Weight scale is a great idea, if not try an all stainless float switch, not sure this would work due to LN2's lower density. Solenoids leak all the time, the Parker Gold Seal valve is petty good. I once bought five Asco's and they all leaked. Good Luck

Scoot

 

[stevene]

Try using a pressure difference level sensor which is designed for open systems however is also suitable for closed containers.

DeMaCo Holland manufacture this equipment you can visit their website

http://www.demaco.nl

 

[unclesyd]

I used a cork float with a wooden rod as a primitive level gauge many years ago when LN2 first showed up on the industrial scene. Today you would probably have to use one of the foams instead of cork.