Post by thread:%Vacuum Thermal Conductivity%

Hello, This is a bit of a long shot, but does anyone know any good references for information relating vacuum pressure to thermal conductivity in a Dewar? I am building a small Dewar with a multistage thermoelectric cooler inside and I need to calculate some theoretical values for the heat load at different pressures. I have tried numerous google searches, but basically all I have found is that people genearlly use pressures of 10 millitorr or less Thank you, Glenn ____________________________________________

On Mon, 18 Oct 2004, Glenn Jones wrote: {Quote hidden}> Hello, > This is a bit of a long shot, but does anyone know any good references for > information relating vacuum pressure to thermal conductivity in a Dewar? I > am building a small Dewar with a multistage thermoelectric cooler inside > and I need to calculate some theoretical values for the heat load at > different pressures. I have tried numerous google searches, but basically > all I have found is that people genearlly use pressures of 10 millitorr or > less > Thank you, > Glenn Afaik at about that pressure loss by radiation is significantly larger than loss by convection so vacuum counts less and less for heat loss. Peter ____________________________________________

i believe the thermal conductivity of the remaining gas will be proportional to the residual pressure and the thermal conductivity at atmospheric pressure. as others have noted, radiation is a far greater contributor at even modest levels of vacuum. omega.com has some excellent information on calculating thermal loads, though i don't know if they cover vacuum insulation. they also sell heating/cooling electronics and elements. the thermal conductivity of the supports will likely be much higher than that of the residual gas. note also that different gases have drastically different thermal conductivities so if you are considering a limited evacuation filling with the appropriate gas will also make a difference. the kurt j. lesker company sells a lot of vacuum equipment and also has a number of excellent tutorials and information like thermal conductivity of gases and general facts about the atmosphere and various levels of vacuum (for instance, at sea level an "air", i.e. nitrogen, molecule will on average travel about 4" before striking another molecule). it will also depend on how much moisture is in the residual air as i believe moisture greatly increases the thermal conductivity of air and any condensation may bridge the space between the walls. thermoelectric coolers don't generally go down to the range where a dewar is really worth while, i'd suggest that you also look at more conventional solid insulating materials which are also easier to work with. materials meant for high temperature insulation may still be appropriate at low temperatures, though they'll need to be sealed to prevent water absorption. without knowing how cold you need to go or what size container you are keeping cooled it's hard to say more. Glenn Jones wrote: > > Hello, > This is a bit of a long shot, but does anyone know any good references for > information relating vacuum pressure to thermal conductivity in a Dewar? I > am building a small Dewar with a multistage thermoelectric cooler inside ------ -- President George W. Bush, Vice President Richard B. Cheney, Secretary of Defense Donald H. Rumsfeld, and Attorney General John D. Ashcroft have committed violations and subversions of the Constitution of the United States of America. <http://www.VoteToImpeach.org> They should be charged with high treason and as leaders deserve the highest penalty. If there is no rule of law there can be no civilization. ____________________________________________