Wow, vacuum ovens are sure neat, albeit expensive for a new one and the old used ones frequently can't hold the temperature in the low ranges that we use for purging and refining cannabis essential oils. What to do?
A simple vacuum pot sitting on a hot pad does the trick, so our inclination to spend much money was low, and we decided to refurbish a used Napco 5851 vacuum oven, which we scored for $550.
As received, it wouldn't hold within -7F/+60F of the 115F set point, so I first replaced the capillary tube type thermostat with a Panasonic AC100 self teaching Temperature Controller, in conjunction with a Type K thermocouple. To my chagrin, while that helped, the temperature still swung wildly, because of the overall oven design.
The Napco oven is a simple vacuum can, wrapped in insulation, with an outer metal surround. The inner can has two 500W strip heaters bolted to the bottom, and they heat the entire can through convection. That clearly leaves a wide disparity in temperature within the oven, plus the inertia of the two 500W heaters significantly overshoot the upper control end.
Soooo, I took the oven completely apart, and after replacing the stiffeners that were in the way, I relocated one 500W heat strip to the top of the oven and centered the remaining one under the bottom. I added Type K ring thermocouples to the upper and lower can surfaces, and added another Panasonic AC100 to control the upper heating element.
I inserted my first Type K thermocouple into the center of the oven, through a compression fitting in the rear oven port, but left it to just hang at this point.
In both cases, I added Crydom solid state relays, to control the oven current with the low Panasonic controller voltage.
Alas, still too much heat strip inertia to control at 115F, so next I disconnected the upper heat strip completely, and hooked the thermocouple in the center of the oven to the Panasonic controller, in read only mode.
Finally I was able to control the oven in the 85F/115F range that we were shooting for, so next I will pick up a nice box for the two Panasonic controllers and finish up the oven. In addition to using one controller to run the lower heating element, and the other to read out the actual inside oven temperature, I will add individual switches to the upper and lower heat strips, so that I can switch one or both on or off at will.
That will allow me to use the upper heating element to speed up startup, and switch it off during actual processing.
Here are some pictures. I will follow up when we complete final packaging.