Part No. 715-0088, Rev. B
- 36 -
3.10.4.
Nitric Oxide (NO) Analyzer Circuit
The Nitric Oxide analyzer circuit is similar in design to the NO
2
circuit. The
amplifier circuit and standby cell conditioning are different due to the nature of
the electrochemical cell. The signal conditioning, cell bias, and output amplifier
are contained on the sensor board. The output voltage includes compensation
from the Auxiliary electrode on the electrochemical cell, and is nominally
V(NO)
= 25mV/ppm
. The output voltage is sent to an additional dedicated Analog to
Digital on the control board, controlled and read by the processor.
The NO electrochemical cell requires a 300mV bias between the COUNTER
and REFERENCE electrodes at all times. Losing cell bias or installing a new
unbiased sensor will produce a large, rapidly decreasing baseline which will
require 48 hours to stabilize before stable measurements can be made. The
baseline will take about three weeks to fully stabilize. For this reason, the power
supplies associated with the sensor circuits are ON anytime the battery is
installed.
This emphasizes the importance of keeping the unit plugged into the charger in
between uses. Doing so will maintain the unit in a ready to work state.
3.10.5.
NO / NO
2
Circuit Calibration
For maximum accuracy, cells should be calibrated using a gas mixture in the
range where most measurements are to be made or where this is not possible,
choose a mixture towards the top of the cell range. Calibration gases exceeding
the range of the cell must not be used, as this may not provide an accurate
calibration.
As calibration normally involves exposing the sensing face of the cell to gas for a
relatively short period, a calibration gas need not contain oxygen - sufficient is
supplied from ambient air, for a limited time, through the side access paths. In
most cases, a five-minute exposure time is sufficient to achieve a stable
calibration signal. Depending on the equipment used, however, NO
2
cells may
need a longer exposure time due to surface adsorption.
3.10.6.
Pressure Effects
Cells will give a transient response when exposed to sudden changes in
pressure in the presence of a measured gas; however the peak signal decays in
only a few seconds.
3.10.7.
Temperature Dependence
Both the span signal and the baseline (zero gas current) are affected by
temperature. The output from a cell will vary only slightly with temperature.
Rapid changes in temperature will create a transient response, which will die
away after 20 - 30 seconds. The graphs show the typical variation in span
output with temperature for cells calibrated at 20 C to a reading of 100% from a
suitable test gas.
Содержание AeroNO 2.0
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