Teledyne 9110E - Nitrogen Oxides Analyzer - Manual (Page 204)

Theory of Operation Model 9110E Instruction Manual
190 M9110E Rev B
must be simultaneously met to switch to the short filter. First, the instantaneous concentra-
tion must differ from the average in the long filter by at least 50 ppb. Second, the instanta-
neous concentration must differ from the average in the long filter by at least 10% of the
average in the long filter.
Figure 10-19: Schematic of Basic Software Operation
If necessary, these boxcar filter lengths can be changed between 1 (no averaging) and
1000 samples but with corresponding tradeoffs in rise time and signal-to-noise ratio. Signal
noise increases accordingly when in adaptive filter mode, but remains within the official
M9110E specifications defined by the EPA as long as the filter size remains at or above 3
samples.
In order to avoid frequent switching between the two filter sizes, the analyzer has a delay
of 120 s before switching out of adaptive filter mode, even if the two threshold conditions
are no longer met.
10.4.2. Calibration - Slope and Offset
Aside from the hardware calibration of the preamplifier board (Section 11.6.5) upon factory
checkout, calibration of the analyzer is usually performed in software. During instrument
calibration (Chapters 7 and 8) the user enters expected values for span gas concentration
through the front panel keypad and supplies the instrument with sample gas of know NO
and NO
X
concentrations. The readings are then compared to the expected values and the
software computes values for the new instrument slope and offset for both NO and NO
X
response. These values are stored in memory for use in calculating the NO, NO
X
and NO
2
concentration of the sample gas. By default, the iDAS stores 200 software calibration
settings for documentation, review and data analysis.
Instrument slope and offset values recorded during the last calibration can be viewed on the
front panel. NO SLOPE, NOX SLOPE, NO OFFS and NOX OFFS are four of the test
parameters accessible through the buttons.