083730300A DCN8101 Theory of Operation 167
6.5.2 CALIBRATION - SLOPE AND OFFSET
Calibration of the analyzer is performed exclusively in software.
During instrument calibration (see Section 4) the user enters expected values for zero and
span via the front panel control buttonand commands the instrument to make readings of
calibrated sample gases for both levels. The readings taken are adjusted, linearized, and
compared to the expected values. With this information the software computes values for
instrument slope and offset and stores these values in memory for use in calculating the
CO concentration of the sample gas.
The instrument slope and offset values recorded during the last calibration are available
for viewing from the from the front panel (see Section 2.3.4.3).
6.5.3 MEASUREMENT ALGORITHM
Once the IR photo-detector signal is demodulated into CO MEAS and CO REF by the
sync/demod board and converted to digital data by the motherboard, the T300/T300M
analytical software calculates the ratio between CO MEAS and CO REF. This value is
compared to a look-up table that is used, with interpolation, to linearize the response of
the instrument. The linearized concentration value is combined with calibration slope
and offset values, then normalized for changes in sample gas pressure to produce the final
CO concentration. This is the value that is displayed on the instrument front panel
display and is stored in memory by the analyzer’s DAS system.
6.5.4 TEMPERATURE AND PRESSURE COMPENSATION
Changes in pressure can have a noticeable, effect on the CO concentration calculation.
To account for this, the T300/T300M software includes a feature which allows the
instrument to compensate for the CO calculations based on changes in ambient pressure.
The TPC feature multiplies the analyzer’s CO concentration by a factor which is based on
the difference between the ambient pressure of the sample gas normalized to standard
atmospheric pressure. As ambient pressure increases, the compensated CO concentration
is decreased.
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