Abstract: In mass flow rate measuring apparatus wherein the fluid is ionized at intervals at a predetermined point of the fluid passage, a voltage is sensed when the ionized flow mass is collected by an electrode located downstream from the ionization point. Electromagnetic radiation, generated in response to the generation of a strong electric field that ionizes the fluid, is prevented by means of a gate circuit from being sensed as a signal for triggering a time measuring device. The signal from the gate circuit is coupled to a voltage sensing circuit that detects a point near or at the peak value of successive voltage waveforms to signal the time measuring device as a point of ion collection.

Abstract: At least one engine parameter is used to estimate the state of the gas sensor while the frequency of the feedback control signal produced in the closed loop control system is measured to see if it is normal or not. A logic circuit produces a pair of signals respectively indicative of normal or abnormal functioning of the closed loop control system in response to the estimated state of the gas sensor and the frequency of the feedback control signal, to illuminate visual indicators. An additional gas sensor tester is additionally provided for detecting the internal resistance of the sensor.