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Part 6: Measuring Water Vapor and Hydrocarbon Dew Point in Natural Gas - Tunable Diode Laser Absorption Spectroscopy

This is part six of a seven-part series about measuring water vapor and hydrocarbon dew point in natural gas. Click to read parts onetwothreefour, and five.

Tunable Diode Laser Absorption Spectroscopy (TDLAS)

This optical or spectroscopic technique utilizes laser light for the measurement of water vapor in natural gas and has received very favorable response from industry. A diode laser emits light, which is tuned over an extremely narrow bandwidth at a wavelength where water molecules absorb the light energy. The light is transmitted into a measurement cell which contains a mirror at one end and the laser and detector at the other end. The sample gas passes through the measurement cell. A certain amount of the energy is absorbed by the water within the light path, and the detector measures the amount of light energy lost which is related to the moisture concentration. 

This “non-contact” approach significantly reduces maintenance requirements of the instruments and reduces overall operating expense. TDLAS analyzers virtually eliminate measurement interferences from glycol or methanol that can occur with more traditional contact-based sensor technologies, such as aluminum oxide. The technology provides fast response to both wet-to-dry and dry-to-wet changes in the moisture content of the gas and provides excellent long-term stability. Some TDLAS analyzers feature line-locking and continuous verification using reference cells.

TDLAS Narrow Emission Bandwidth vs. Conventional IR Techniques

TDLAS analyzers are sensitive to the background gas and should be calibrated in a gas with a similar composition to the process gas to ensure no interferences from other components in the gas. TDLAS analyzer are also sensitive to temperature and pressure, and typically have internal temperature and pressure compensation to ensure reliable performance. The junction temperature of the laser diode is critical to the measurement; any small change in the temperature of the diode will shift the center wavelength of the emission and can result in erroneous measurements or alarm conditions in the analyzer.

Another disadvantage is the range and accuracy capability of the TDLAS device. While it is acceptable to use these instruments in traditional pipeline natural gas applications, their use for low-level detection of water vapor is limited due to the detection capability in a methane-based background. Specialized techniques such as differential spectroscopy add maintenance, cost, and potential errors to the measurement systems.

For more detailed information about this application, refer to our White Paper, Analytical Devices for the Measurement of Water Vapor and Hydrocarbon Dew-Point in Natural Gas.”


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