2 - 1
PHOTOMETER ASSEMBLY
90002927(1) BINOS
®
100 4P e [1.00] 17.06.97
Rosemount Analytical
2. Photometer Assembly
Depending on gas component and measuring range, different photometer assemblies may be
realized in BINOS
®
100 4P. Optionally the photometer can be sealed against 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 mounted.
2.1 Photometer
To enable both types of measurement - the standard absolute and the additional differential
measurement (while taking the base concentration level into consideration) - a special photometer
assembly has been developed (see Fig. 2-1).
The basic part of the photometer assembly is the chopper housing (01). The light source (thermal
radiator, 02), the analysis cell (03), and the signal detection unit [including a filter cell (04) and a
detector (05/07)] are all mounted on this chopper housing.
The chopper housing also incorporates the interference filters for the selection of spectral
bandpass 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,
there is the chopper blade which is driven by a stepping motor. Both the chopper housing and the
motor encapsulation are hermetically sealed against ambient in order to prevent any gas from
entering, such as atmospheric CO
2
, which could lead to background absorptivity (preabsorption)
thus causing drift effects. An absorber provides for constant removal of any traces of CO
2
which
may enter the interior of the chopper housing via diffusion.
In addition the chopper housing incorporates a photoelectric gate for providing a reference signal
for the phase angle of the chopper blade, and a temperature sensor (10) for monitoring
continuously the temperature of the photometer assembly. The information thus obtained is used
by the signal processing electronics to compensate thermal effects.