Specifications

DETECTOR METHODOLOGIES
Figure 2-1. Absorption Bands of Sample Gas and Transmittance of Interference Filters
2.1.2 Opto-Pneumatic Method
In the opto-pneumatic method, a thermal radiator generates the infrared radiation which
passes through the chopper wheel. This radiation alternately passes through the filter
cell and reaches the measuring and reference side of the analysis cell with equal
intensity. After passing another filter cell, the radiation reaches the pneumatic detector.
The pneumatic detector compares and evaluates the radiation from the measuring and
reference sides of the analysis cell and converts them into voltage signals proportional
to their respective intensity.
The pneumatic detector consists of a gas-filled absorption chamber and a
compensation chamber which are connected by a flow channel in which a Microflow
filament sensor is mounted. This is shown in Figure 2-2 below.
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 is transparent for infrared radiation. The
window is usually Calcium Fluoride (CaF
2
).
When the infrared radiation passes through the reference side of the analysis cell into
the detector, no pre-absorption occurs. Thus, the gas inside the absorption chamber is
heated, expands and some of it passes through the flow channel into the compensation
chamber.
Rosemount Analytical µCEM Continuous Analyzer Transmitter 2–2