Figure 1 – Measurement Cell Theory
A gas filter correlation wheel is combined with this system. This wheel contains three parts to increase
measurement accuracy: CO, N
2
and the mask.
The CO window contains saturation (40%) of CO which acts as a reference beam –
absorbing a known amount of light.
The N
2
window, containing 100% N
2
, does not absorb IR at 4.7 microns at all and is used
during normal CO measurement.
The mask totally blocks the light source and is used to determine background signals, the
strength of other signals relative to each other and the background.
Figure 2 – Correlation Wheel
1.4.2 Kalman Filter Theory
The digital Kalman filter provides an ideal compromise between response time and noise reduction for
the type of signal and noise present in ambient air analysers.
The Kalman filter enhances measurements by modifying the filter time base variable, depending on
the change rate of the measured value. If the signal is changing rapidly, the instrument is allowed to
respond quickly. When the signal is steady, a long integration time is used to reduce noise. The system
continuously analyses the signal and uses the appropriate filtering time.
To Next Page
Loading...
