Siemens SITRANS SL, Руководство по эксплуатации, Страница: 20 / 212

Description
2.5 Functional description
SITRANS SL
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Operating Instructions, 12/2010, A5E01132948-04
2.5 Functional description
2.5.1 Operating principle
SITRANS SL is a gas analyzer employing single line molecular absorption spectroscopy. A
diode laser emits a beam of infrared light which passes through the process gas and is
detected by a receiver unit. The wavelength of the laser diode output is tuned to a gas
specific absorption line. The laser continuously scans this single absorption line with a very
high spectral resolution (see following figure). The degree of absorption and the line shape
are used for the evaluation. The measurement is free of cross-interferences, since the quasi-
monochromatic laser light is absorbed very selectively by only one specific line in the
scanned spectral range.
①
Absorption line
②
Laser line
Figure 2-6 Typical spectral bandwidth of an absorption line compared to the bandwidth of the laser
light.
The field design of the SITRANS SL in-situ gas analyzer consists of a transmitter unit and a
receiver unit. The light which is not absorbed by the sample is detected in the receiver. The
concentration of the gas component is determined from the absorption.
The individual dependencies of concentration, pressure and temperature are application
specific.
An internal reference cell is used to constantly check the stability of the spectrometer, thus
assuring a continuous self-calibration of the analyzer.
The self-calibration of the analyzer is therefore valid for the time period specified in the
technical data without the necessity for external recalibration using calibration gases.