COSA Xentaur Xentaur XDTL-HT - Manual (Page 8)

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
f
f
f
1,
f
2
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.