A friend of mine is taking over his dad's chemical company and I'm helping him out. Part of the process involves using an old Hoskins crucible furnace to maintain a static temperature of 500c +/- 5 degrees for a reaction. He's currently using a variac to control the furnace temperature because his father told him the furnace would last longer.

The problem is that regardless of the setting, the temperature won't remain stable. The temperature continually ramps up or down no matter how long you wait for it to stabilize or which way you nudge the variac. The result is that the system has to be checked every 1-1/2 to 2 hrs when running and it needs to be run for days nonstop. I'm getting awfully sleepy lately and this is only one system. We should be running two simultaneously.

Q: Will cycling the power on and off with a temperature controller actually reduce the furnace's useful life? If so, is there a way to control the variac output automatically, based on temperature? Or is there a better way?

My drooping eyelids can't thank you enough.