Any of ya'll been coveting a vacuum oven for doing your final purging and processing, but have been stopped by the high cost of new ones and the poor temperature control of used ones?
Here is a simple fix, though we of course had to make it complicated getting to our final stage, so I will share with you the steps that we took to arrive where we did.
First we bought a used Napco 5851 Vacuum Drying Oven, listed as working, but that actually meant that it heated up when you turned it on.
The Napco design consists of an inner stainless steel can that is under vacuum, and an outer shell with glass insulation in between.
Heating was accomplished using two 500W strip heaters bolted to the bottom of the can and controlled by a capillary tube mechanical thermocouple clipped to the side of the inner can.
I first replaced the mechanical thermocouple and control with a Type K thermocouple and a Panasonic AKT4112200 SP20 self teaching temperature controller.
I inserted the Type K thermocouple though a compression fitting in the existing port in the rear of the oven and added a Crydom solid state relay between the oven heat strips and the 12V DC control output.
Growgeek programmed the puppy and tests were made, but alas, while every thing worked, it wouldn't maintain closer than minus 5, plus 60 degrees F of set point, even after being in self teach mode.
Sooooo, I took the oven completely apart and rebuilt the outside stiffeners enough to allow me to relocate one of the heating elements to the top of the oven and center the other one on the bottom. I also had lugs welded on for the heat strips at their new location, as well as for Type K thermocouples on both the top and bottom of the oven can.
I initially started out with two Panasonic Temperature Controllers actively controlling one heat strip each and getting feedback from its on dedicated thermocouple, but alas the swing was still excessive, soooooo, I disconnected the top heating element completely and heated with only the lower element.
I hooked the original Type K thermocouple in the center of the oven space to the second Panasonic, in read only mode, so that the lower controller controlled the temperature of the oven shell and the second Panasonic read the actual temperature of the work area.
Hee, hee, hee, if you take enough stabs at it, you can get close, and it looks like our final arrangement will fill our needs in the 85F to 115F control ranges, that have thus far eluded us with this oven.
The other thing we found of course, is that if we start at ambient and set the controller at 115F, by the time the temperature hits 115F, there is excessive hysteresis in the heating element and oven can. As a result it will overshoot wildly and take a while to settle back down. We therefore only raise the temperature 5 degrees at a time until we settle in at set point.
I also added two switches between the controllers and the solid state relays, so that I can turn off either element or both. We reserved the ability to use the upper element, and controlling it with the lower controller, in case we need to run some processes later at higher temperatures.
Lu lu lu lu lu lu lu lu lu lu lu!!!!!!!!!!!!!!!!!1 Hee, hee, hee, snicker, snark, snort.............................. Just ran trials, and the controls hold the oven dead on what ever we set it at. Plus nothing, minus nothing.
There is about 5F delta T between the Type K can ring thermocouple and the Type K sticking the center of the oven at 100F. The second thermocouple is of course reading radiant energy!
Here are some pictures of our progress and final oven: