1-2 Chapter 1: Description of functions
SDE_U14
Physical principles
Measurement
principle
• NDIR technique
(Nondispersive infrared analysis)
• The measurement effect is based on resonance absorption of gas-specific
vibration-rotation bands of differing atom gas molecules in the median infrared
spectrum at wavelengths between 2.5 and 8 µm.
• The individual gases to be measured are identified by their specific absorption
bands. Each gas has such an absorption spectrum (fingerprint).
Exceptions:
- Monoatomic gases, such as inert gases
- Symmetrical gases, such as N
2
, O
2
and H
2
- These types of gases cannot be measured with this method.
• The relationship between measured infrared radiation absorption and the
measurement component is based on the LAMBERT-BEER law:
A = (I0 - I1) / I0 = 1 - e
-ε(λ)⋅ρ⋅l
where
A= Absorption
I0 = Radiation entering the cell
I1 = Radiation leaving the cell
ε(λ) = Sample component extinction factor
ρ = Sample component density
l = Sample cell length
The relationship between sample component density
ρ and its volumetric
concentration c is
ρ = ρ
0
⋅ c ⋅ p/p
0
⋅ T
0
/T
where
ρ
0
= Pure gas density
p
0
= Pressure
T
0
= Temperature
under standard conditions (1013 hPa, 0°C).
The second equation shows that the sample component's volumetric
concentration depends on the sample cell pressure and temperature.
The first equation finds a non-linear relationship between absorption and
volumetric concentration.
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