Ametek Process Instruments - THERMOX

New Process Gas Analyzer for the Measurement of Water Vapor


TDLAS, WMS, Laser Spectroscopy, Water Vapor Measurement, Gas Analyzer

A new analyzer for the measurement of water vapor concentration based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) was developed. The water-vapor line at 1854-nm was selected for the measurement, and the performance of the system was evaluated on different background eases to confirm that the measurement was free of interferences. An original feature of the instrument design is the use of an all-digital protocol for the modulation of the laser drive signal and the demodulation of the detector response. This innovation provides a simple means of varying the type of modulation protocol and all associated instrumental parameters. A reference cell is used in parallel with the sample cell to continuously validate the performance of the system. A limit of detection (LOD) of 5 ppmv and a range of 5 to 2500 ppmv were demonstrated.

The measurement of water vapor in process streams has long been of key importance in a number of different industries.  One of the most important applications is the measurement of water vapor in natural gas. Most of the technologies employed for this application are based on chemical sensors, which have several limitations. The most severe limitation is foulins of the sensor, which can lead to inaccurate measurements and hish maintenance costs associated with periodic replacement of the sensor. Spectroscopic measurements are relatively insensitive to the effects of sample fouling, and can help to reduce the long-term cost-of-ownership for moisture analyzers in many applications.
Tunable diode laser absorption spectroscopy (TDLAS) has been used for a number of industrial applications, measuring a variety of different analytes (1-2). Emission bandwidths for the tunable diode lasers are on the order of 10'4 - 10'5 cm'1, giving the TDLAS technique extremely high spectral resolution, which results in the ability to isolate a single rotational-vibrational transition line of the analyte species. TDLAS offers extremely high specificity for the analyte, even in a complex sample matrix. A second advantage of TDLAS is the ability to rapidly tune the lasers, so techniques like wavelength modulation spectroscopy (WMS), which yield dramatic sensitivity enhancements over a direct absorption approach, are easily implemented (3). Because TDLAS is an optical technique, it also offers a very fast response speed. The high specificity, sensitivity, and response speed of TDLAS make it very suitable for the measurement of water in natural gas.

The principal objective of the work reported here is to characterize a new TDLAS-based instrument- In its initial configuration, the instrument has been set up to perform wavelength modulation spectroscopy (WMS). Using a simple WMS technique for this first study provides a better means of evaluating the system hardware. At the same time the instrument can be easily configured to implement other protocols, and the associated detection signal-processing techniques. A key feature of this instrument is the use of a sealed reference cell, which contains a known amount of water vapor, for referencing the emission wavelength of the laser. Measurements of the reference cell are performed in parallel with measurements of the sample stream, providing a continuous validation of the system performance. Because the instrument is designed to measure a high-resolution spectrum of the absorption line, it has the capability to identify the presence of, and compensate for, minor background interferences that may be present in a complex sample matrix. Methane has minor background interference at the wavelength used for measuring water vapor, and as such, provides a good test for the system capabilities.

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