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PHOTOMETER ASSEMBLY
ETC00781(4) Series 100 e 02/2004
3. Photometer Assembly
Depending on gas component and measuring range, different photometer assemblies will be
realized in 100 series.
Optional the photometer can be sealed to ambient air. In this case all parts are sealed with O- rings.
The entire photometer assembly is mounted as a unit on the main circuit board (BKS) by means
of a bracket. The main circuit board is inserted into guide rails in the analyzer housing, to which
the front panel (membrane keypad) and the rear panel are assembled.
3.1 Photometer with Pyroelectrical Detector (Solid-state detector)
Fig. 3-1 shows the schematical photometer assembly for dual channel operation.
The base element for the photometer assembly is the chopper housing (03), upon which the light
source (thermal radiator, 07), the analysis cell (cuvette, 09), and the signal detection unit [filter cell
(14/15), pyroelectrical (solid-state) detector with integrated preamplifier (16)] are all mounted.
The chopper housing also incorporates the duplex filters (04/05) for the selection of spectral band-
pass ranges from the broadband emission of the light sources.
Between the two halves of the chopper housing (03), which are sealed together with an O-ring,
is the chopper blade, driven by a stepping motor. Both the chopper housing and the motor
encapsulation are hermetically sealed with respect to the ambient in order to prevent entry of
gases, such as atmospheric CO
2
, which could produce background absorptivity (preabsorption)
leading to drift effects. An absorber material provides for constant removal of any traces of CO
2
which may enter the interior of the chopper housing via diffusion.
The chopper housing additionally incorporates a photoelectric gate for providing a reference
signal for the phase angle of the chopper blade, plus a temperature sensor (28) for monitoring
continuously the photometer assembly temperature. This temperature information is used by the
signal processing electronics for the compensation of thermal effects.
The analysis cells are merely aluminum tubes equipped with sample gas inlet and outlet fittings.
PYROELECTRICAL DETECTOR