1. A configurable thermometer comprising: a controller for operating the thermometer, said controller having a memory for storing one or more software instructions implementing a plurality of functions of the thermometer; and a model selection device identifying a selected one of a plurality of operating configurations of the thermometer, each of said configurations of the thermometer being defined by said model selection device being connected to the controller, said controller being responsive to the model selection device connected thereto for operating the thermometer in the selected configuration according to software instructions selected from the one or more software instructions, said selected software instructions corresponding to the model selection device, wherein the model selection device comprises one of a plurality of interchangeable user input devices, wherein each of said user input devices comprises a keypad overlay comprising one or more buttons, said overlay, when connected to the controller, selectively enabling execution of the selected software instructions by a user of the thermometer, each of said user input devices and said controller having mating connectors for connecting a selected one of the user input devices to the controller to configure the one or more software instructions within the thermometer according to the selected configuration.