Instruction manual

NGA 2000 MLT Hardware

MEASURING PRINCIPLE

2 - 5

Emerson Process Management GmbH & Co. OHG

Instruction Manual

90002929

07/2006

IR MEASUREMENT

Fig. 2-3: Absorption Bands of Sample Gases and Transmittance of the

Interference Filters used

The interference filter, used for generating a reference signal, has its passband in a spectral region

where none of these gases absorb. Most of the other gases of interest also do not absorb within

the passband of this reference filter.

The photometer assembly is similar to the assembly with “gas detector” (Fig. 2-1) with the

exception of the analysis cell. For analysis cell a non-sparated version is used. Behind this analysis

cell the radiation passes a second filter cell (10) to the pyroelectrical detector (12). In front of this

filter cell there are the individually optical filters.

The detector records the incoming IR radiation. This radiation will be reduced by the absorption

of the gas in the analysisi cell at this wavelength. The comparison between the measuring

wavelength and reference wavelength leads to an alternating voltage signal. This signal results

from cooling and heating the pyroelectrical material.

Wave Length [nm]

4400 460042004000 48003000 3200 3400 3600 3800 5000 5200 5400 5600 5800 6000

Interference

Filter

Absorption band

18 36 54 72 900

Transmittance [%]

9075604530150

Transmittance [%]

Reference