Cosa-xentaur Xentaur XDTL-HT Manuel d'utilisateur télécharger pdf (Page 8)

Theory of Operation

Atoms and molecules have absorption spectra that are distinctive to their species. The XTDL-HT™

measures water vapor in a sample by measuring an absorption line (wavelength) from the water vapor

absorption spectrum. As a near-infrared laser beam passes through the sample cell, a photodetector

measures the reduction in light intensity due to interaction with the sample. By Beer-Lambert law,

where

is the intensity of the transmitted light

is the intensity of the incidental light

is the instrument-dependent absorption coefficient

is the beam pathlength

is the density of the absorbing particles

the instrument can calculate the water concentration. To improve the signal-to-noise ratio, the XTDL-

HT™ employs the wavelength modulation spectroscopy (WMS) technique. In WMS, wavelength of the

laser light is continuously modulated with a sinusoidal waveform with the target wavelength at the

center, and the signal is detected as harmonics of the modulation frequency. The measurement value

is computed using the harmonic amplitudes over the stored calibration curve.

Laser Diode

Sinusoidal

Generator

Sample Cell Photodetector

Demodulator

Calibration Curve

Reading

The absorption coefficient in the Beer-Lambert law is a function of sample temperature and pressure.

To ensure stable and accurate reading, the XTDL-HT™ maintains the sample temperature with a heated

enclosure and the sample pressure is fixed with a back pressure regulator.

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Cosa-xentaur Xentaur XDTL-HT Manuel d'utilisateur Page 8