Specifications

Detector Methodologies

2. Detector Methodologies

The µCEM can employ up to three different measuring methods depending on the

configuration chosen. The methods are: NDIR CO/SO2, Paramagnetic O

Electrochemical O

, and chemiluminescent NOx.

2.1 Non-Dispersive Infrared (NDIR)

The non-dispersive infrared method is based on the principle of absorption of infrared

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 component.

Depending on the gas being measured and its concentration, one of two different

measuring methods may be used as follows:

2.1.1 Interference Filter Correlation 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 µCEM analyzer

and the spectral absorption of the gases CO and CO

are shown in Figure 2-1 below. 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.

The signal generation is accomplished with a pyroelectrical (solid-state) detector. The

detector records the incoming infrared radiation. This radiation is reduced by the

absorption of the gas at the corresponding wavelengths. By comparing the

measurement and reference wavelength, an alternating voltage signal is produced.

This signal results from the cooling and heating of the pyroelectric detector material.

Rosemount Analytical µCEM Continuous Analyzer Transmitter 2–1