TDLAS

Tunable diode laser absorption spectroscopy (TDLAS) has been used to measure oxides of nitrogen during flight (71). By tuning the laser to specific infrared absorption bands, the technique can selectively measure each compound. Detection limits are higher (25-100 pptrv for a 3-min response time) than the best chemiluminescent methods, and the instrumentation is less amenable to aircraft operations than the chemiluminescence techniques because of weight and size. 

Carbon Monoxide. Methods for determining carbon monoxide include detection by conversion to mercury vapor, gas filter correlation spectrometry, TDLAS, and grab sampling followed by gas chromatograph (GC) analysis. The quantitative liberation of mercury vapor from mercury oxide by CO has been used to measure CO (73). The mercury vapor concentration is then measured by flameless atomic absorption spectrometry. A detection limit of 0.1 ppbv was reported for a 30-s response time. Accuracy was reported to be 3% at tropospheric mixing ratios. A commercial instrument providing similar performance is available. 

TDLAS measures the absorption of monochromatic light by CO in a multipass flow cell (77). The time response is controlled by the flow of air through the cell. In theory, the instrument is fast enough for application to aircraft measurement of CO flux by eddy correlation. A reported precision of 1 ppbv or 1% is superior to other techniques. The instrument is not commercially available. 

FTIR, TDLAS, and LIF are in situ techniques, whereas DOAS is a long-path method that gives only a path-integrated result. NO, N02, NO,3, and HONO have been successfully measured with DOAS even in rural and remote regions. PAN, HN03, and NH3 have been measured with FTIR in urban areas, but its sensitivity at present is not adequate for levels below a few parts per billion by volume. NO, N02, PAN, HN03, and NH3 have been measured with TDLAS down to sub-ppbv levels. A review with references for applications of these three methods is available (13). The LIF method has been more recently developed, has been applied to the measurement of NO, N02, NH3, and HONO (see reference 14 for an example), and offers sensitivity down to the parts per trillion by volume level. 

As an example, a recent intercomparison (37) included three N02 measurement techniques aTDLAS-based system and two chemical-based systems— the photolysis-ozone chemiluminescence system diagramed in Figure 7 and an instrument based on N02 plus luminol chemiluminescence. Above 2 ppbv the three instruments gave similar results, but at sub-ppbv the results from the three techniques became dissimilar. Tests on the prepared mixtures showed that the luminol results were affected by expected interferences from 03 and PAN. No interferences were found in the TDLAS system, but near the detection limit the data analysis procedures calculated levels of N02 that were too high. The outcome of this intercomparison was close to the ideal the sensitivity, specificity, accuracy, and precision of each instrument were objectively analyzed previous data sets taken by different systems can now be reliably evaluated and each investigator was able to perceive areas in which the technique could be improved. 

In-situ measurement technique of water vapor concentration in gas flow channels in PEMFCs using tunable diode laser absorption spectroscopy (TDLAS)31-36 is also shown with fundamental descriptions on its measuring principle and validity of a practical system. Localized current density and through-plane water-back transport index are obtained with variation of vapor concentration along the gas channel taken into account. Demonstrative results showing that effect of the micro porous layer (MPL) on variation of through-plane water-back transport index is shown in an operating PEMFC. 

TUNABLE DIODE LASER ABSORPTION SPECTROSCOPY (TDLAS) AS A DIAGNOSTIC TOOL FOR IN-SITU DETECTION OF WATER VAPOR CONCENTRATION IN PEMFCs... 

Localized remote sensing for chemical compositions in operating PEMFC can be performed by tunable diode laser absorption spectroscopy (TDLAS). Measurement of water vapor and carbon dioxide has been reported by using TDLAS.31-36 In TDLAS measurement, the absorption of the laser beam passing through the gas sample is expressed with the partial pressure of the absorbing species, Pa (atm), by Beer s law,... 

TDLAS System Hardware for Water Vapor Measurement... 

Figure 7. Schematic of optical sensor head of TDLAS.

Figure 8. Schematic of an experimental setup with TDLAS for in-situ measurement of water vapor in an operating PEMFC.

TDLAS Signal Calibration for Measurement of Water Vapor Concentration... 

Figure 9. Schematic of signal processing with WMS method in TDLAS.

To obtain water vapor concentration, output signal intensity by TDLAS is calibrated under a well-controlled environment in variation of relative humidity and surrounding temperature. Figure 10 shows a relationship between water vapor concentration and output signal intensity detected by TDLAS. It is shown that the... 

Figure 10. Calibration curve for measurement of water vapor concentration by TDLAS.

In Situ Measurement of Water Vapor in PEMFC Using TDLAS... 

Figure 11. Variation of water vapor concentration in the cathode and the anode channel of the PEMFC with the MPL under the counter-flow mode at a setting of 0.5 A/cm2 measured using the TDLAS system.

As shown above, in-situ TDLAS measurements show its unique potential to investigate localized water transport in operating PEMFCs, leading to fundamental understanding on water management. It is also capable of monitoring localized phenomena with good temporal resolution. 

Chapters 4-6 address specific diagnostic methods in PEFCs. Martin et al. provide a detailed review of methods for distributed diagnostics of species, temperature, and current in PEFCs in Chapter 4. In Chapter 5, Hussey and Jacobson describe the operational principles of neutron radiography for in-situ visualization of liquid water distribution, and also outline issues related to temporal and spatial resolution. Tsushima and Hirai describe both magnetic resonance imaging (MRI) technique for visualization of water in PEFCs and tunable diode laser absorption spectroscopy (TDLAS) for measurement of water vapor concentration in Chapter 6. 

OP-TDLAS has been applied commercially only in air monitoring. Instruments based on these lasers are capable of making very sensitive mea-... 

Some Representative Gases and Approximate PIC Detection Limits for OP-TDLAS, Assuming Ability to Measure Absorbance to 1 Part in 10s... 

The detection of molecular 02 by the NIR spectroscopic technique involves a measurement that is often described as a forbidden transition. Many molecules can absorb radiation (energy), enter a temporary excited or radiative state, and release energy (reradiation), which is called fluorescence. In the case of molecular Oz, there is such a transition that can be monitored in the NIR region of the spectrum near 760 nm. Therefore, 02 concentration can be measured in this "forbidden region" by NIR methods, such as using the TDLAS. 

In the TDLAS detector, a diode laser is scanned across a chosen absorption line. The selection and application of the 02 spectral line is proprietary to most vendors, but it is usually one of the 02 lines around 760 nm (Figure 3.37). The laser source wavelength is modulated as the absorption line is scanned, which makes it possible to use the spectroscopic oxygen technique, which previously was not considered possible. 

The advantages of Tunable Diode Laser Absorption Spectrometry (TDLAS) for measuring trace atmospheric gases are universality positive identification good sensitivity and rapid response time. An instrument is described which can measure two gases simultaneously under automatic computer control with detection limits better than 100 parts per trillion and with response times better than 5 minutes. Procedures have been established for the measurement of NO N0a, HNOa NH3 HbQb and HCHO. These species have been measured under a variety of conditions in smog chambers and in ambient air from mobile laboratories and from aircraft. 

Qn these occasions the luminol method gave values some 35% higher than the TDLAS method indicating that the luminol method was also responding to some other species present under these conditions at night. 

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