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MEASURING PRINCIPLE
2 - 1
ETC00303(1) BINOS E e (2.0) 11/00
Rosemount Analytical
IR MEASUREMENT
2. Measuring Principle
BINOS E can employ up to four different measuring principles depending on the configuration
chosen. The methods are: NDIR, NDUV, paramagnetic and electrochemical Oxygen.
2.1 Non-dispersive Infrared (NDIR Measurement)
The non-dispersive infrared method is based on the principle of absorption of IR radiation by
the sample gas component 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 in principle consisting
of an infrared light source, a chopper wheel to alternate the radiation intensity between the
reference and measurement (sample) side, an analysis cell, filter cells and a photometric
detector. Due to a rotation chopper wheel, the radiation intensities coming from measuring and
reference side of the analysis cell produce periodically changing signals within the detector.
The detector signal amplitude thus alternates between concentration dependent and concen-
tration independent values. The difference between the two is a reliable measure of the
concentration of the absorbing gas component.
The principle photometer assembly is shown in Fig. 2-1.
Depending on the gas being measured, the application, the gas composition and the gas
concentration, one of two different measuring methods may be used as follows: