Part No. 715-0088, Rev. B
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3.10. Environmental Effects
3.10.1.
Nitric Oxide and Nitrogen Dioxide Sensors
The Nitric Oxide (NO) and Nitrogen Dioxide (NO
2
) sensors utilized in the
AeroNOx 2.0 are amperometric (also called electrochemical). They can be
thought of as micro fuel cells, as they produce a measurable electric current
while oxidizing or reducing traces of electroactive gases. The simplest form of
sensor operating on electrochemical principles has two electrodes - SENSE and
COUNTER - separated by a layer of electrolyte and connected by a low
resistance external circuit. Gas diffusing into the sensor is either oxidized or
reduced reacted at the surface of the SENSE electrode, causing a current to
flow between the electrodes through the external circuit. The current is
proportional to the concentration of gas and can be measured across a load
resistor in the external circuit.
Figure 1
Schematic and Conventional Representations of Electrochemical Gas Sensor
For reaction to take place, the SENSE electrode potential must be within a
specific range. As the gas concentration increases so does the current flow,
causing a change in the potential of the COUNTER electrode (polarization).
With the electrodes connected together by a simple load resistor, the SENSE
electrode potential follows that of the COUNTER. Using a third REFERENCE
electrode and an external potentiostatic operating circuit, the range of
concentrations a sensor can be used to measure is much greater. With this
arrangement, the SENSE electrode is held at a fixed potential relative to the
REFERENCE electrode. No current is drawn from the REFERENCE electrode,
so both maintain a constant potential. The COUNTER electrode is still free to
polarize, but this has no effect on the SENSE electrode and so does not limit the
sensor in any way. By controlling the potential of the SENSE electrode, the
potentiostatic circuit also allows greater selectivity and improved response to the
target gas. The same circuit is used to measure the current flow between the
SENSE and COUNTER electrodes.
Sense Electrode
Counter Electrode
Sense
Electrode
Conventional Symbol of Sensor
Electrolyte
Simplified Representation of Sensor
Porous
Membrane
Sample
Gas
Sealed
Container
Sensing Electrode
Catalyst Material
Counter Electrode
Reference Electrode
Reference Electrode
Содержание AeroNO 2.0
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