International Biomedical AeroNO 2.0, Руководство по обслуживанию, Страница: 38 / 105

Part No. 715-0088, Rev. B - 35 -
Gas diffusing into a cell becomes part of a reaction at the SENSE electrode:
oxidation (e.g. Nitric Oxide) or reduction (e.g. nitrogen dioxide). Each reaction
can be represented in standard chemical equation form.
Nitric Oxide (NO): NO + 2H
2
O HNO
3
+ 3H
+
+ 3e
-
(Oxidation)
Nitrogen Dioxide (NO
2
): NO
2
+ 2H
+
+ 2e
-
NO + H
2
O (Reduction)
The COUNTER electrode acts to balance out the reaction at the SENSE
electrode. If oxidation occurs at the SENSE electrode, oxygen will be reduced
to form water at the COUNTER. If, however, the SENSE electrode reaction is a
reduction, the COUNTER electrode reaction will be reversed (i.e. water will be
oxidized). The standard equation for this electrode can be written as:
½O
2
+ 2H
+
+ 2e
-
H
2
O
This overall equation demonstrates that the fuel for the reactions is gases
supplied to the sensor and the product is a gas emitted. In other words the
sensor is merely a catalyst for the reaction, and no part of it is directly
consumed.
3.10.2. Temperature Effects
A small signal known as the ‘baseline’ is present on the SENSE electrode of the
cell even when no reactant gas is present. The magnitude of the baseline
increases exponentially with temperature, but can be ‘zeroed’ out during
calibration. A large temperature change after calibration can cause a slight
shifting in an instrument’s zero. Even with wide temperature variations, such a
shift will normally be too small to be significant. However, if the application
requires very high resolution, at very low concentrations, a zero shift could
significantly alter the overall measurement. A fourth pin on these
electrochemical cells is used to compensate for temperature effects.
3.10.3. Nitrogen Dioxide (NO
2
) Analyzer Circuit
The nitrogen dioxide sensor and signal conditioning circuitry is located on the
Sensor Board. The signal conditioning circuitry provides cell bias and power off
support in addition to converting the cell output current to a voltage. The output
voltage includes compensation from the Auxiliary electrode on the
electrochemical cell, and is nominally
V(NO
2
) = 250mV/ppm . The voltage is
then sent to the control board where it is sampled by a dedicated Analog to
Digital converter and presented to the processor.
To maintain the cell in a ready to work state when an instrument is switched off,
the NO cell’s REFERENCE, AUXILIARY, and SENSE electrodes are shorted
together and the COUNTER electrode is disconnected from the amplifier circuit.
CAUTION: While shorted, it is important to avoid exposure to active gases
or solvent vapors. Exposure to active gases and solvents in
the shorted state will cause damage to the sensor cells.
This emphasizes the importance of “flushing” the sampling system with oxygen
or clean air for a few minutes before turning the instrument off after use.