Chapter 1
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BE6020-14
1316A-1 Multi Gas Monitor
LumaSense Technologies A/S
Instruction Manual
Page 14 of 98
µm (2260 cm
-1
), and the filter associated with Carbon Monoxide is
centred on 4.65 µm (2150 cm
-1
).
In the measurement chamber the three beams, which differ in
wavelength frequency, each excite their respective gas (if present).
The absorption of the incident light will be registered by the
Detector.
System Board Assembly
The system board assembly is a surface-mount printed circuit board
that includes the digital and analogue electronic components and the
IR detector assembly. The IR detector assembly includes thermopile
IR detectors for the HC, CO, CO
2
, and reference channels. Light
passed through the band pass filters is directed through the
thermopile window to a detector substrate. The substrate has a light-
sensitive coating, which produces a voltage that is proportional to
light intensity. A reference voltage is produced by the reference
detector, which provides an output that varies with light intensity
and is not affected by the sample gas. The HC detector includes a
thermistor. The detector block temperature measurement facilitates
compensation for IR filter and IR detector temperature variances.
The detector outputs are amplified and sent through a multiplexer to
an ADC converter. The microprocessor samples the ADC converter
output at 200 Hz and derives an uncompensated digital value
representing each gas concentration. The data are subjected to a
compensation routine that provides adjustment for temperature and
pressure variances. The digital value representing each gas
concentration is derived.
1.4.4 The NDIR Temperature Measurements.
The analyzer incorporates several thermistors for the measurement
of temperatures.
IR Detector/Filter Temperature (HC Temperature):
A thermistor located in the optical assembly measures the IR
detector and IR filter temperature. It is used as follows:
“Gas law compensation” and detector response compensation during
HC, CO and CO
2
gas concentration measurement.
Span calibration (via the use of temperature compensated HC, CO
2
and CO data).
An “out of range” IR detector/filter temperature condition causes HC,
CO
2
and CO data to be declared “data invalid.”
