Here are some numbers on the heat radiated away from the vacuum can by leakage through the foam insulation and from the exposed metallic parts. The heat loss is dominated by the foam as it has an larger surface area. However, these numbers are maybe a little rough as they don't account for the Al foil cladding on the outside and ignore the details of the cylindrical geometry.
I've been working on documenting the thermal aspects of the vacuum can. Info is spread across the elog in various places but not in one place. This stuff is gradually getting added to the wiki which will be the central collecting point for information to avoid this iterative amnesia. I am also almost finished on a graphic that summarizes the setout of the vacuum can and its sensors. This is just the heat loading calculations.
The tank is 22" long with 8.3" tube diameter with two 10" flanges on the ends. This apparatus is clad all around by 2" (average) thick CertiFoam 25 see post PSL:178 for characteristics (note that Frank's values are for 1" thickness only, this must be scaled to the thickness used). Total dimensions of the foam box are 12x12x36" giving a surface area of ~1.3 m^2. For a tank held at 35 C above room temperature 20 C this is 11.3W of heat loss.
At the top of the vacuum can there are three half nipples welded along the top, these hang out above the insulation as access is needed for the turbo/roughing pump connection, the ion pump and the subD 9 feedthrough. The exposed surface is a mix of shiny stainless steel and matt/sandblasted bits. Shiny and matt exposed areas are respectively 0.0296 m^2 and 0.0168 m^2 (not including the ion pump) which is not big. With emmisivities of 0.09 and 0.18 for these two surface types we get a total of 0.5265 W radiative heat dissipation for a tank held at 35 C above 20 C room temperature.
Thus total estimated heat for a 35 C tank is 11.78 W. We don't need to run it at this temperature but I use it as a reference value. See attached graph for heat loss as a function of vacuum can temperature.
A summary of these numbers and details is in the attached matlab file.

For reference the tank has 4 resistive heating mats wrapped around it. The small ones near the ends are 30 Ohm and the two larger ones near the center are 70 ohms each. These have been connected in parallel+ series network that gives a total resistance of 50 Ohms and can be driven with up to 115 V. To just maintain the tank at 35 C we would need 24.5 V with ~0.5 A. This seems like a lot but is almost doable with available OPAMP buffers.

The next step is to work out heat capacity. I can't find design drawings for the tank itself on the elog/wiki, maybe its too far back in ancient history. There is a solidworks drawing on the SVN but was made in the student version of solidworks so is very buggy. I will try an extract numbers to get an idea of the mass of metal in the tank.
Also in progress is a step function measurement of tank cooling. I spent some of this week working out how to integrate a new RTD acromag card into our existing EPICS setup so we can log the temperature drop after heating is turned off. This took a while as I was unfamiliar with this kind of setup and also the power supply turned out to be not doing what I thought it was. The voltage current source is now hooked up and a thermister fitted for logging temperature. This measurement should give us some more grounded numbers on the real characteristics of the tank thermal decay rates.
