I thought initially it was a 'differential thermostat' but looking at these, does not fit the bill.
What do you call a controller that turns on/off solely on the DIFFERENCE of two sensors.
It has no setpoint in the conventional sense. It runs when sensor A registers X degrees above sensor B.
Application: I want to store cold. Insulated building. I want to run a small fan pulling outside are inside whenever the outside air temp is below the inside air temp, with the goal of freezing the top 8-10 feet of soil, as well as several hundred gallons of brine.