I am in the process of building an incubator from a small fridge. I am using the bottom section as an incubator with articulating trays and the upper freezer section for hatching. This should allow me to run three different sets at a time. I have digital controllers for both sections and did a test run tonight with nothing but an empty cabinet. The temp stabilized between 99.1 and 99.5 within about 15 min. I had planned on adding some containers of sand to help balance out temp swings and provide a little security for short power outages. Temp swing does not look like its going to be an issue because if I open the door and let it drop 5-6 degrees it's back to a stable temp in just a couple of minutes. Is their a recommended formula for how much mass to add for a given volume of space in an incubator? And if I do add thermal mass does it create more of a liability than it's worth should an overheat situation occur? I plan on doing my first set on Monday or Tuesday so I can have chicks during my daughters visit Thanksgiving week. The incubator will be able hold about 90 eggs at any given time. I have around 4 dozen eggs to choose from on my desk ready to go. and will probably try to do three sets of 20 next week. Any last minute advice would be appreciated.