Calibration Defines Zero; Detection Measures Deviation
The useful baseline is the room's normal operating condition, not an imaginary perfectly quiet environment.
calibration
Calibration Is Not a Number; It Is a Contract With an Environment
Setting a calibration value does not guarantee that every future measurement will remain correct. The environment, reference, and operating range in which that value was created are part of its meaning. Calibration is a rule for interpreting a measurement.