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3.
Brief description of unit functions and
Servicing the unit
3.1
Exhaust-gas analyzer module
The exhaust-gas analyzer module is designed for the following
measuring ranges:
Carbon monoxide (CO) 0...10.00 % vol.
Hydrocarbons (HC) 0...9999 ppm
Carbon dioxide (CO
) 0...18 % vol.
2
Oxygen (O
) 0...22 % vol.
2
Excess-air factor (lambda) 0.500...9.999
Nitrogen monoxide (NO) 0...5000 ppm vol. NO
3.1.1 Analyzer part (HC, CO and CO
The measuring channels for HC, CO and CO
infrared pulsating-light method.
This technique makes use of the ability of different gases to
absorb infrared rays of a certain wavelength.
An infrared ray is produced by a lamp and interrupted in cycles by
a chopper wheel, then it passes through analysis systems for CO,
HC and CO
in succession. The individual analysis systems are
2
sensitive to infrared light of different wavelengths and can there-
fore be mechanically arranged one behind the other.
Each of these systems consists of an analysis chamber through
which test gas flows, and a receiving chamber filled with a suitable
gas mixture.
The systems are constructed in such a way that when zero gas
(air) flows through the analysis chamber, a maximum electric
alternating-voltage signal – the measured-value signal – is gene-
rated in the receiving chambers.
The signal is amplified by the appropriate channel amplifier,
rectified and conveyed on the motherboard to an analog-to-digital
converter (ADC). The digitalized signal is read and stored by the
MPU.
When test gas containing the component to be measured flows
through the analysis chambers, component-specific wavelengths
of the infrared ray are attenuated accordingly. A smaller measu-
red-value signal is generated in the receiving chambers (see gas
circuit diagram, Section 12.5).
3.1.2 System calibration
System calibration is always started automatically when the
system switches to a measuring mode and the infrared measured
values at this moment differ from zero.
During system calibration, a solenoid valve switches the zero-gas
inlet (with carbon canister) into the test-gas duct for the duration
of calibration. During this process, zero gas (air) is used to
determine the actual system sensitivity of the HC, CO and CO
measuring channels. This value is then stored as the zero point.
If the exhaust-gas analyzer module is still in analysis mode after
15 minutes, system calibration is repeated. Here, the analyzer
module automatically checks whether analysis is currently taking
place and, if it is, delays system calibration until all measured
values have fallen to zero.
8
measurement)
2
function using the
2
2
3.1.3 Self-test
The BEA performs a self-test during which it checks all its most
important functions. Any malfunctions detected result in an error
message (Section 8).
3.1.4 Adjustment (HC, CO and CO
During 1-point adjustment (recalibration) with calibrating gas, the
measurement effect (difference from zero) is measured. The
analyzer module is informed by means of a nominal value as to
which concentration of calibrating gas corresponds to this
measurement effect.
You may preselect different adjustment options which vary in type
and frequency, to suit your application.
3.1.5 O
measuring channel
2
The sensitivity of this measuring channel is adjusted automatically
during each system calibration. During this process, the measu-
red oxygen content is set against the compensating air = 20.9 %
and the condition of the O
2
zero point of the O
measuring channel is recognised automati-
2
cally.
The oxygen measurement function can be deactivated
(Section 4.5, Write parameters menu).
3.1.6 Compensation of pressure influence
Different levels of atmospheric pressure mean that different
concentrations are measured in the analysis system. In order for
the exhaust-gas analyzer module to achieve a correct measure-
ment, the atmospheric pressure to be taken into consideration is
determined by means of an integral pressure sensor. The exhaust-
gas analyzer module can then calculate the correct displayed
value itself.
3.1.7 Compensation of temperature influence
Temperature sensors are situated on the receiving chambers and
on the infrared lamp. The operating temperatures measured there
are automatically taken into account during the adjustment of the
exhaust-gas analyzer module and the conversion of the measured
value to the displayed value.
3.1.8 Measurement of excess-air factor (lambda)
The exhaust-gas analyzer module calculates the excess-air factor
(lambda λ) from the measured concentrations of CO, HC, CO
and O
. The lambda value is indicated on the display if the
2
measured CO
content exceeds 2 % and both the oxygen
2
measurement and excess-air measurement functions are activa-
ted. The oxygen measurement is extremely important for calcula-
ting the excess-air factor. An imprecise oxygen measurement
leads to the incorrect calculation of the excess-air factor. The
lambda measurement function can be deactivated (Section 4.5,
Write parameters menu).
measuring channels)
2
measuring sensor is monitored. The
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