International Biomedical AeroNO 2.0, Руководство по обслуживанию, Страница: 42 / 105

Part No. 715-0088, Rev. B - 39 -
3.10.13. Oxygen Analyzer Circuit
As the O
2
sensor also contains the output resistor, minimal signal conditioning
is required. This circuitry is located on the sensor board and provides an output
voltage proportional to the % oxygen. This voltage is sent to the control board
where it is read directly by the processors internal Analog to Digital Converter.
Sensor output drops as the cell ages. Like the NO and NO
2
circuits, O
2
circuit
calibration is handled in software and requires no mechanical adjustments by
the user.
3.10.14. Linearity
The signal from an Oxygen cell is slightly non-linear and follows the law:
S = K ln[1/(1-C)]
NOTE: If the sensor is calibrated in dry air to read 20.9% (S=20.9,
C=0.209), then K=89.14. The maximum error then occurs at
about 10% Oxygen when the cell output is approximately 0.5%
lower than a linear response would indicate. In most
circumstances this error is insignificant.
3.10.15. Carrier Gas Effects
For most purposes, the Oxygen cell may simply be calibrated in ambient air. In
the presence of high concentrations of gases other than air, however, the effect
of the carrier gas (i.e. mixture less oxygen) on the output signal becomes
important. The rate of diffusion of oxygen, and hence the signal from a cell, is
proportional to the molecular weight of the carrier gas (Graham’s Law).
Dry air may be considered to consist of 20.95% oxygen in nitrogen, which has a
molecular weight of 28 g/mole. When using the sensor with a different carrier
gas, with a significantly different molecular weight from nitrogen, the signal from
the cell will be affected.
3.10.16. Cross Sensitivity
Toxic gases at Threshold Limit Value levels will have no cross-sensitivity effect
on the Oxygen cell. At very high levels (i.e. percent levels), highly oxidizing
gases (e.g. ozone and chlorine) will interfere to the extent of their oxygen
equivalent, but most other commonly occurring gases will have no effect.
For example:
Methane 100% 0
Hydrocarbons 100% 0
Hydrogen 100% < -2%
Carbon monoxide 20% < -0.5%
Acid gases such as CO
2
and SO
2
will be slightly absorbed by the electrolyte
and tend to increase the flux of oxygen to the electrode. This gives an
enhanced oxygen signal of about 0.3% of signal per 1% CO
2
. Oxygen cells are
not suitable for continuous operation in concentrations of CO
2
above 25%.
NOTE: Sensors should not be subjected to prolonged exposure to highly
corrosive atmospheres as this will cause premature failure.