In order to expedite characterization of on-line detection systems, we have developed a flow rate scanner comprising a pump, an inexpensive single-board computer, and an associated Python program. The program controls the pump, triggers the detector, acquires raw data, filters/smooths the data, calculates the first derivative of the temporal signal trend, and determines the sensitivity regime threshold in real time. The slope of the curve relating the detector response with the sample flow rate is used to determine the sensitivity regime for a given analyte and a given detector. The key part of the data analysis is on-the-fly calculation of the first derivatives using a fourth-order polynomial fit to the raw data by the Savitzky-Golay algorithm. The device has been tested by determining the flow rate profiles for an ultraviolet absorption detector and a mass spectrometer operated with electrospray ionization, atmospheric pressure chemical ionization, and a dual ion source. The sensitivity regime thresholds for various test analytes have been determined automatically and robustly in time as short as a few minutes.