To provide a level switch with verification capability.

The level switch, which can be used to detect liquid level in a process vessel, requires no manual testing for proper operation and switch calibration because of its own self-validating function. The switch contains a set of sensors for measuring fluid pressure at two points in a vertically-separated vessel, and a processor for calculating fluid pressure difference between the two points to generate a differential pressure signal. Additionally, the processor combines the differential pressure signal with a steady-state water head signal from one of the two points to generate a synthetic output signal. Thus, it also generates an out-of-range signal for fluid level based on a certain variation in the synthetic output signal.