Instruction Manual

748467-A January 2002

Model MicroCEM

Other application-dependent options include a wide range of sample cell materi- als, 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 ab- sorption, modulation by the chopper motor and the fill gas of the detector chambers. The gas flow/sensor output is proportional to the measured gas con- centration. 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 as- sembly. This design incorporates on board “intelligence” to provide continuous “self test” diagnostics.

c.Paramagnetic O2

The determination of oxygen is based on the measurement of the magnetic sus- ceptibility of the sample gas. Oxygen is strongly paramagnetic, while other com- mon gases are not. The detector used is compact, has fast response and a wide dynamic range. The long life cell is corro- sion resistant, heated and may be easily cleaned. It has rugged self-tensioning suspension and is of welded non-glued construction.

1-4 DETECTOR METHODOLOGIES

The MicroCEM can employ up to three different measuring methods depending on the configuration chosen. The methods are: NDIR, Paramagnetic O2, Electrochemical O2, and Chemiluminescense.

a.Non-Dispersive Infrared (NDIR)

The non-dispersive infrared method is based on the principle of absorption of in- frared radiation by the sample gas being

measured. The gas-specific wavelengths of the absorption bands characterize the type of gas while the strength of the absorption gives a measure of the concentration of the gas component being measured.

An optical bench is employed comprising an infrared light source, two analysis cells (reference and measurement), a chopper wheel to alternate the radiation intensity between the reference and measurement side, and a photometer detector. The detector signal thus alternates between concentration dependent and concentration independent values. The difference between the two is a reliable measure of the concentration of the absorbing gas com- ponent.

Depending on the gas being measured and its concentration, one of two different measuring methods may be used as fol- lows:

Interference Filter Correlation (IFC) Method

With the IFC method the analysis cell is alternately illuminated with filtered infrared concentrated in one of two spectrally separated wavelength ranges. One of these two wavelength bands is chosen to coincide with an absorption band of the sample gas and the other is chosen such that none of the gas constituents expected to be encountered in practice absorbs anywhere within the band.

The spectral transmittance curves of the interference filters used in the MicroCEM analyzer and the spectral absorption of the gases CO and CO2 are shown in Figure 1-1. It can be seen that the absorption bands of these gases each coincide with the passbands of one of the interference filters. The forth interference filter, used for generating a reference signal, has its passband in a spectral region where none of these gases absorb. Most of the other gases of interest also do not absorb within the passband of this reference filter.

1-2

Description and Specifications

Rosemount Analytical Inc. A Division of Emerson Process Management