Instruction manual

NGA 2000 MLT Hardware
MEASURING PRINCIPLE
2 - 3
Emerson Process Management GmbH & Co. OHG
Instruction Manual
90002929
07/2006
2.1.1 Opto - Pneumatic Measuring Principle
For measurement a heating coil in the light source generates the necessary infrared radiation (1).
This radiation passes through the chopper wheel [light chopper wheel (3)].
Due to the special shape of the chopper wheel, the IR radiation passes through a filter cell (5) and
alternatively reaches the measuring side (8) and reference side (9) of the analysis cell [(7)
separated in the middle into two halves by an internal separating wall] with equal intensity.
The filter cell (5) screens interfering radiation areas out of the radiation spectrum.
Behind this analysis cell the radiation passes a second filter cell (10) towards the gas detector (12),
which compares the IR radiation intenisities from measuring side and reference side and converts
it into an AC voltage signal.
The detector (Fig. 2-2) consists of a gas-filled absorption chamber and a compensation chamber
which are connected together via a flow channel.
In principle the detector is filled with the infrared active gas to be measured and is only sensitive
to this distinct gas with its characteristic absorption spectrum. The absorption chamber is sealed
with a window which are transparent for infrared radiation [usually CaF
2
(Calcium fluoride)].
Fig. 2-2: Principle Design of Gas Detector
IR MEASUREMENT
Gas intake connection
Absorption chamber
CaF
2
window
Compensation chamber
Flow channel with
Microflow sensor