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How TDL Technology Works


How It Works
SpectraSensors analyzers utilize a laser absorption spectroscopy technique to detect and measure the concentration of analyte molecules (H2O, H2S, CO2, NH3, and C2H2) in process gas streams. This basic technique has been applied to gas measurements since the invention of the laser over 40 years ago. Recent advances in semiconductor lasers have made the development of tunable diode laser analyzers for process gas monitoring applications technologically and economically viable.

SpectraSensors pioneered the use of tunable diode lasers to design analyzers for on-line, real-time measurement of H2O, H2S, CO2, NH3, and C2H2 in process gas streams. Our expertise in this field is underscored by our extensive portfolio of patents.


TDL Analyzer Design and Operation
The basic design of a TDL analyzer is depicted in the diagram below. The principal components of the analyzer are; an optical head housing the laser with thermo-electric cooler and a solid state detector, the sample cell with a mirror positioned at the end opposite the laser, gas inlet and outlet connections, and temperature and pressure sensors.  
A window isolates the laser source and solid state detector components from the process gas inside the sample cell. This design allows measurements to be performed with absolutely no contact between the process gas sample (and any entrained contaminants) and critical analyzer components.
Herriott Cell
TDL Analyzer with Multi-Pass Herriott Cell
In operation, process gas from a sampling probe is introduced into the sample cell of the analyzer. A tunable diode laser emits a wavelength of near-infrared (NIR) light specific for the target analyte into the sample cell where it passes through the gas and is reflected back by a mirror at the opposite end of the cell to a solid state detector. Analyte molecules present in the gas sample absorb and reduce the intensity of laser light energy in direct proportion to their concentration according to the Lambert-Beer law. The difference in light intensity is measured by the solid state detector and this signal is processed using advanced algorithms to calculate analyte concentration in the process gas.
The pathlength of the sample cell can be increased to create a multi-pass Herriott cell in which laser light traverses the cell multiple times to increase analyzer sensitivity for trace (sub-ppm) level measurements.

Please contact us for detailed explanations of this and other technologies used in our products.
Click here to read about Trace-level Analysis by Differential Spectroscopy using Spectral Subtraction.