25/35 9.4 Chemical Composition-Oxygen Sensors Principle:Above about500℃the sensor becomes permeable(有渗透性的) to oxygen ions,and a solid electrolyte effect produces a voltage eo millivolts between the two electrodes,given by the Nernst equation: eo =0.0215T In- 02r 02m (9.7)） where: T is sensor temperature in Kelvins; Ozr is the oxygen concentration in percent of a reference gas (often atmospheric air,O2r=20.9%)at the sensor inside surface; O2m is the oxygen concentration of the measured gas at the sensor outside surface; Sensor normally include a built-in RTD or platinum/rhodium thermocouple(铂铑热电偶)since T must be known to relate e。toO2m' 26/35 9.5 Current and Power Measurement Many familiar voltage-measuring instruments:meters,potentiometers, strip-chart recorders,oscilloscopes,etc. Only one instrument dedicated to current measurement,the analog or digital ammeter.(The digital ammeter is in fact a voltmeter with a built-in current sensor.) Ammeter are useful only for steady current,while most sensors can also deal with dynamic current. Current sensors transduce a current into a voltage,which is then measured by one of the voltage-sensing instrument. The simplest current sensor is just a resistor since it transduces the current through it to a voltage across it.Be careful that the resistor should has low ohm value and small inductance capacitance. For measuring small currents,the op-amp current to voltage converter may be useful9.4 Chemical Composition - Oxygen Sensors Principle: Above about 500℃ the sensor becomes permeable(有渗透性的) to oxygen ions, and a solid electrolyte effect produces a voltage e0 millivolts between the two electrodes, given by the Nernst equation: ݁଴ = 0.0215ܶ ln ைమೝ ைమ೘ (9.7) where: T is sensor temperature in Kelvins; O2r is the oxygen concentration in percent of a reference gas (often atmospheric air, O2r =20.9%) at the sensor inside surface; O2m is the oxygen concentration of the measured gas at the sensor outside surface; Sensor normally include a built-in RTD or platinum/rhodium thermocouple（铂铑热电偶）since T must be known to relate eo to O2m. 25/35 z Many familiar voltage-measuring instruments: meters, potentiometers, strip-chart recorders, oscilloscopes, etc. z Only one instrument dedicated to current measurement, the analog or digital ammeter. (The digital ammeter is in fact a voltmeter with a built-in current sensor.) z Ammeter are useful only for steady current, while most sensors can also deal with dynamic current. z Current sensors transduce a current into a voltage, which is then measured by one of the voltage-sensing instrument. z The simplest current sensor is just a resistor since it transduces the current through it to a voltage across it. Be careful that the resistor should has low ohm value and small inductance & capacitance. z For measuring small currents, the op-amp current to voltage converter may be useful . 9.5 Current and Power Measurement 26/35