How TDL Technology Works

How It Works

Water absorption spectra in natural gas. The graph shows several moisture spectra in natural gas. The higher the concentration of moisture or CO₂, the more absorption of light, and the stronger the corresponding absorption signal. Since the calculation is a direct, fundamental measurement, the amount of moisture or CO₂ present can be measured quickly and accurately. There are no wet-up or dry-down delays like those associated with surface based capacitance sensors.

SpectraSensors' gas analyzers utilize laser absorption spectroscopy to detect the presence of one or more gases in a mixture of other gases. This technique has been applied to gas measurements since the laser was first invented over 40 years ago. Recent advancements in semiconductor lasers have made this technology economically viable for measurement of H₂0 and in CO₂ in natural gas because of the special wavelengths that are required.

SpectraSensors owns the patent for these special lasers, and has combined them with appropriate electronics and "smart" software to create an analyzer that requires no sensor element in contact with the sample gas stream. The result is a reliable analyzer which does not suffer from contamination or drift due to impurities such as glycols, amines, hydrogen sulfide, or mercaptans.

Both H₂O and CO₂ are measured by monitoring their absorption of laser light at specific wavelengths in the near-infrared (NIR) wavelength region close to 2um. The human eye responds to light in the range from approximately 0.4um (deep violet) to 0.8um (deep red), but most molecules respond to light at longer wavelengths that are invisible to the human eye (the infrared region). By using a laser that operates precisely at a wavelength where H₂O or CO₂ (many other gases can also be measured using this technique) absorbs, it is possible to determine accurately the abundance of the gas by measuring the fraction of light that is absorbed by the molecules.

The graph at the right represents multiple spectral scans in a natural gas sample with varying amounts of moisture. The characteristic shape of the H₂O (tall peak) and CH₄ (triple peak) makes it very easy to determine that the sensor is working properly because the shape of the curve is determined by the physical properties of the molecules. The curves cannot be produced in error. Also, the water peak can be visually compared to the CH₄ peak to give a quick and dirty (yet highly reliable) estimate of water content.

Illustrative representations of the SS-Series SpectraSensors “laser Sample Cells” on the right demonstrate the relatively simple construction which is the basis of a robust and trouble-free design.

The SS-Series analyzers use long lasting and resilient tunable laser diodes that emit near-infrared light at wavelengths absorbed by moisture or CO₂ in natural gas. The robustness of Laser Diodes is proven in many consumer and commercial applications such as CD players and fiberoptic communications.

As the light passes through the gas sample, energy is absorbed, reducing the amount of light arriving at the detector. The length of the laser beam affects the sensor’s sensitivity; hence SpectraSensors offer the dual-pass optical path in most applications (some applications require a different number of passes).

Please contact SpectraSensors, Inc. for detailed explanations of this and other technologies used in our products.

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