System Description
Software
Thermo Fisher Scientific
Model 42
i
Trace Level Instruction Manual
8-5
stabilize. After those three seconds, it accumulates the signal counts for
seven seconds before again switching the solenoid.
The calculation of the NO and NO
x
concentrations is lengthy and uses the
high-level processor to provide the most accurate readings. The calculation
begins by subtracting the appropriate electronic offset from the seven-
second count accumulation. Following this correction, the raw
accumulated counts are scaled according to the gain setting of the input
board.
Next, the uncorrected NO and NO
x
values are determined according to a
unique averaging algorithm which minimizes errors resulting from rapidly
changing gas concentrations. This algorithm results in NO and NO
x
values
which are stored in RAM in a circular buffer that holds all the ten second
data from the previous five minutes. This data is averaged over the selected
time interval, which can be any multiple of ten between 10 and 300 (the
manual modes have additional intervals of 1, 2, and 5 seconds).
The background values for NO and NO
x
, which are corrected for
temperature, are subtracted from their respective averages. The NO reading
is corrected by the stored span factor and by the temperature factor. The
NO
x
reading is partially corrected by the span factor, temperature factor,
and balance factor. The corrected NO value is subtracted from the partially
corrected NO
x
value to yield an uncorrected NO
2
value. The NO
2
value is
then corrected for converter efficiency to give a corrected NO
2
reading.
Finally, the corrected NO
2
reading is added to the corrected NO reading to
yield a fully corrected NO
x
value.
The front panel display, serial and Ethernet data ports, and analog outputs
are the means of communicating the results of the above calculations. The
front panel display presents the NO, NO
2
, and NO
x
concentrations
simultaneously. The display is updated every 1-10 seconds, depending on
the averaging time.
The analog output ranges are user selectable via software. The analog
outputs are defaulted based on the measurement range. The defaults are
calculated by dividing the data values by the full-scale range for each of the
three parameters and then multiplying each result by the user-selected
output range. Negative concentrations can be represented as long as they
are within -5% of full-scale. The zero and span values may be set by the
user to any desired value.
Measurement
Calculations
Output Communication
