We have a 60L dewar and are conserned that there may be uneven sample freezing conditions within the rods. Rather than buying an expensive cryo therometer we thought someone may have prior experience with this? Thanks in advance..
Tony,
Depending on where the major heat input comes from (wall? floor? liquid surface?) you may have stratification with no overturn. For example, a neck-suspended vacuum jacketed Dewar will have heat input at the top of the liquid column - rather than the bottom (cork supports on a stainless steel non-jacketed vessel). In that case, where heat is not put in predominantly at the bottom of the liquid, you may not have convection - and so the liquid will stratify with warmer LN2 (boiling) at the surface, and colder nitrogen at the bottom. The easiest way to confirm this is to poke a thermometer in.
A Pt100, or T-type thermocouple will do the trick. The Pt100 is the more accurate here, but I really doubt that you'll see more than the error of the thermometer as an answer.
TLDR: Chill. You're probably isothermal. But it's nice to check.
All depends how sensitive your samples are. We have in the lab dewars of several sizes (glass and cryomoovers) and as long you keep them flooded the temperature within the liquid is "homogeneous". In the vapor phase we observed a T gradient towards the opening but the T-profile is dewar specific. To get some detailed profile for your dewar you would have to measure it with some well calibrated sensor like cernox with calibration curve for Lakeshore. With some effort you can measure profiles with 0.01K resolution.
Great thanks guys! My mind is at ease for now.. When we do check with the thermocoupler I will let you know the results! Cheers
Hey Tony,
The liquid nitrogen is in the boiling point so no meter the amount of liquid it will always be at the same temperature, only the gas will have a temperature gradient.
concerning the thermal gradient on the gas, it will depend on the Dewar profile and walls and how full is the Dewar. Cheers
Hi Tony,
just keep your sample inside liquid nitrogen. You have the thermal gradient due to heat flux through Dewar walls and the sample support in the gas phase only.
Hello,
the temperature gradient depends on the dewar diameter, the height above the liquid nitrogen surface, and the heat flux value.
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