Specifications

INTRODUCTION
1.3 Theory of Operation
1.3.1 NOx
The NOx analyzer continuously analyzes a flowing gas sample for NOx [nitric oxide
(NO) plus nitrogen dioxide (NO
2
)]. The sum of the concentrations is continuously
reported as NOx.
The µCEM NOx Analyzer Module uses the chemiluminecence method of detection.
This technology is based on NO’s reaction with ozone (O
3
) to produce NO
2
and oxygen
(O
2
). Some of the NO
2
molecules produced are in an electronically excited state (NO
2
*
where the * refers to the excitation). These revert to the ground state, with emission of
photons (essentially, red light). The reactions involved are:
NO
2
+ O
3
→ NO
2
* + O
2
NO
2
* → NO
2
+ red light
The sample is continuously passed through a heated bed of vitreous carbon, in which
NO
2
is reduced to NO. Any NO initially present in the sample passes through the
converter unchanged, and any NO
2
is converted to an approximately equivalent (95%)
amount of NO.
The NO is quantitatively converted to NO
2
by gas-phase oxidation with molecular
ozone produced within the analyzer from air supplied by an external source. During the
reaction, approximately 10% of the NO
2
molecules are elevated to an electronically
excited state, followed by immediate decay to the non-excited state, accompanied by
emission of photons. These photons are detected by a photomultiplier tube which
produces an output proportional to the concentration of NOx in the sample.
To minimize system response time, an internal sample bypass feature provides high-
velocity sample flow through the analyzer.
1.3.2 CO
The optical bench can selectively measure multiple components in a compact design by using a
unique dual optical bench design. Depending on the application, any two combinations of NDIR
channels can be combined on a single chopper motor/dual source assembly.
Other application-dependent options include a wide range of sample cell materials, optical filters and
solid state detectors. The NDIR Microflow detector consists of two chambers, measurement and
reference with an interconnected path in which an ultra low flow filament sensor is mounted. During
operation, a pulsating flow occurs between the two chambers which is dependent upon: sample gas
absorption, modulation by the chopper motor and the fill gas of the detector chambers. The gas
flow/sensor output is proportional to the measured gas concentration. The optical bench is further
enhanced by a novel “Look-through” detector technique. This design allows two detectors to be
arranged in series --- enabling two different components to be measured on a single optical bench.
The optical bench contains a unique eddy current drive chopper motor and source assembly. This
design incorporates on board “intelligence” to provide continuous “self test” diagnostics.
Rosemount Analytical µCEM Continuous Analyzer Transmitter 1–5