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3-1
Instruction Manual
HASAxE-IM-HS
05/2006
Emerson Process Management GmbH & Co. OHG
3 Principles
Chapter 3
Measuring Principles
3-1 Infrared Measurement (IR)
Ultraviolet Measurement (UV)
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series analyzers support several
measuring principles or combinations of
principles, depending on measured gas
components. This provides best possible
results as the principle always is adjusted to
meet the specific characteristics of the
particular gas.
The following sections introduce the available
measuring principles and show their specific
characteristics.
This principles make use of infrared/ultraviolet
light absorbed by the sample gas. The wave
length of the absorbed portion of IR/UV light
characterizes the gas component whereas the
intensity of absorption is a measure of
concentration.
Two different IR measuring principles are
available, both comparing concentration
depend and concentration independ signals.
The difference of these two signals results in a
measure of concentration.
One of these principles (as described in detail
in the following sections) is adapted for UV
measurements: The absorption measurement
in the UV spectral range is based on the same
principle as the IR measurement, but a glow-
discharge source is used instead of an IR
source.
As the glow-discharge source needs a specific
and as constant as possible temperature, it is
either thermostatted to about 55 °C or built into
a thermostatted box, covering all physical
components and completely thermostatted.
Section 3-1-2 covers both IR and UV measure-
ment.
3-1-1 Interference Filter Correlation (IFC)
This is an IR principle only and suitable for
applications not requiring high performance
parameters.
An undivided analysis cell is alternately passed
by light of two different wave lengths, filtered
by interference filter out of the spectrum of an
IR source: One wavelength covers the
absorption band of the measured gas
component, the other is selected to cover a
region where no absorption occures.
Fig. 3-1 shows an example of interference filter
transmissions and absorption bands of the gas
components CO and CO
2
. It‘s easy to see that
the interference filters‘ transmissible spectral
bands overlap the absorption bands of the
gases, whereas within the bandwidth of the
reference filter no absorptin takes place. Other
gases (CH
4
) and HC do not affect the measu-
ring result because they do not absorb IR light
of these wavelengths.
The decision about which measurement (UV
or IR) is selected for a specific application
depends on the gas component to be
measured. The decision about which IR
measurement to be used is based on the
required performance.