Chayan Mitra

Raman analyzer for sensitive natural gas composition analysis

Abstract. Raman spectroscopy is of significant importance in industrial gas analysis due to its unique capability of quantitative multigas measurement, especially diatomics (N2 and H2), with a single laser. This paper presents the development of a gas analyzer system based on high pressure Raman scattering in a multipass Raman cell and demonstrates its feasibility for real-time natural gas analysis. A 64-pass Raman cell operated at elevated pressure (5 bar) is used to create multiplicative enhancement (proportional to number of passes times pressure) of the natural gas Raman signal. A relatively low power 532-nm continuous wave laser beam (200 mW) is used as the source and the signals are measured through a cooled charge-coupled device grating spectrometer (30-s exposure). A hybrid algorithm based on background-correction and least-squares error minimization is used to estimate gas concentrations. Individual gas component concentration repeatability of the order of 0.1% is demonstrated. Further, the applicability of the technique for natural gas analysis is demonstrated through measurements on calibrated gas mixtures. Experimental details, analyzer characterization, and key measurements are presented to demonstrate the performance of the technique.

Development of Steam Quality Measurement and Monitoring Technique Using Absorption Spectroscopy With Diode Lasers

Techniques using a tunable diode laser (TDL) and multiple broadband lasers have been developed and tested for measuring the steam quality or steam wetness fraction. The steam wetness fraction was estimated using a ratiometric technique combining measured absorbance of water and water vapor overtone transitions in the near infrared spectral band. Using these techniques we were able to measure a wide variation in steam quality ranging from saturated steam condition to 80% mass fraction of steam with less than 1% error. Our sensor can be tailor-made to suit cost, sensitivity, and operating conditions as per requirements, and could be used eventually for measuring the quality of steam in the low pressure stage of steam turbines.

Diode laser-based trace detection of hydrogen-sulfide at 2646.3 nm and hydrocarbon spectral interference effects

Abstract. This paper presents the first diode laser-based measurements of trace hydrogen sulfide at the 2646.35 nm absorption line and evaluates the feasibility of this line for natural gas sensing applications. The balanced detection technique is used to achieve a detection limit of 25 parts per million-meter H2S in 150 Torr (T) balance N2. Spectral interference effects of major natural gas constituents (methane, ethane, and propane) are also presented. It is shown that methane interference can be avoided by lowering the sample pressure, while propane has no interference. However, strong interfering lines of ethane are observed around the 2646.35 nm region. To our knowledge, this is the first report where individual ethane lines in this spectral region are observed. These observations make the 2646.35 nm H2S line unfavorable for natural gas applications. This line, however, still remains attractive for H2S sensing applications with low ethane content.

Temperature dependence of the reflectance of metals at visible wavelengths

Non-contact temperature measurement is a preferred technique for rotating, hazardous and inaccessible objects. A major challenge for IR thermometry is the dependence of metal emissivity on wavelength and temperature. Optical reflectivity of metals is known to depend on metal temperature, plasma frequency of metal, angle and wavelength of the incident light. A major challenge in reflectance based temperature measurement techniques is the dependence of the reflectance on the surface roughness of the target metal. Sudden change in surface roughness (related or unrelated to temperature) can lead to spurious changes in reflectance irrespective of the temperature. To mitigate the surface roughness effect, we have investigated the speckle pattern emanating from the surface irregularities on the metal. An initial measurement on the speckle pattern also shows an enhanced sensitivity in temperature measurement of the surface that is a function of the inherent surface properties of the metal.

Diode Laser‐Based Sensors for Extreme Harsh Environment Data Acquisition

The world has witnessed several step changes in living standards, productivity, growth, and innovation. We are currently witnessing a convergence of intelligent devices, intelligent networks, and intelligent decision making. Obtaining long‐term accurate, in situ, and real time data from the machines is necessary for enabling the industrial Internet. This relies heavily on sensor systems. Development of robust sensors that can operate reliably in extreme environments will make it possible to gather data from previously inaccessible locations in the equipment. This will enable machine opera‐ tors to monitor and optimize the performance of their machines. Diode laser‐based diagnostics technology has found applications in a variety of areas and a versatile range of operating conditions. It has proven to be a strong and reliable technique for remote measurements of concentrations and temperatures in harsh environments. Some of the major challenges for implementation of these sensors in real world are machine vibrations, window clogging, cooling, etc. In this chapter, the authors discuss about the application details and specific technologies suitable for the applications. Few case studies are considered, and the theoretical approach, algorithm development, and experimental validation are also discussed.

Optical sensors for harsh environment applications

The development of a harsh environment ammonia slip sensor based on tunable diode laser absorption spectroscopy is presented. A hybrid optical sensor design, through combination of wavelength modulation spectroscopy (WMS) and alignment control, is proposed as an approach towards reliable in-situ measurements in misalignment prone harsh environments. 1531.59 nm, 1553.4 nm and 1555.56 nm are suggested as possible absorption lines for trace ammonia measurement (<1ppm at 10m path length at 500K) in gas turbine exhaust conditions. Design and performance of the alignment control system are presented in detail. Effect of misalignment related measurement degradation is investigated and significant improvement in measurement fidelity is demonstrated through the use of the hybrid optical sensor design.

An optical method for measuring Metal Surface temperature in Harsh environment conditions

Commercially available instruments for measuring and monitoring surface temperature of metal parts are very limited and often unsuitable for applications at harsh environment conditions. Another major challenge is to measure temperature of a rotating surface, as it is very difficult to take an electrical signal out from rotating parts. A novel optical reflectance based non-contact temperature measurement technique is discussed which can be used for temperature measurement on metal surfaces. The optical reflectivity of metals is known to depend on metal temperature and wavelength of the incident light. An increase in metal temperature resulted in the change (increase or decrease depending on particular metal properties) of reflected laser power from the metal surface. This also depends on the surface geometries of the metal surface being measured. We have shown that the sensitivity of the temperature measured depends on the angle of incidence, surface topology and surface properties of the object.