Sandip Pal

A computational study of tomographic measurement of carbon monoxide at minor concentrations

In order to develop a chemical species tomography system, this paper carries out various computational steps to address the problem of measuring minor species concentration using single-pass, short path-length absorption techniques in the mid-infrared. It focuses on the imaging of carbon monoxide (CO) in combustion exhaust as a case study, with an average concentration of 10 ppm over a 50 mm diameter cross-section, taking account of the presence of other absorbing species. CO absorption transitions R6, R7, R9 and R10 are identified as possible measurement targets. The joint effects of spectral absorption linewidth and laser source linewidth are considered at length, resulting in recommendations for laser linewidth to achieve appropriate levels of CO absorption signal purity. Measurement strategies are considered for achievement of the necessary sensitivity, noise and bandwidth performance. A feasible beam arrangement for tomographic imaging is discussed, providing 48 measurements of path concentration integral. Representative phantom reconstructions are presented, with encouraging results for application to such dynamic gaseous subjects.

Auto-digital gain balancing: a new detection scheme for high-speed chemical species tomography of minor constituents

In many dynamic gas-phase reaction processes, there is great interest to measure the distribution of minor constituents, i.e. <10?3 by volume (1000 ppm). One such case is the after-treatment of automotive gasoline engine exhaust by catalytic conversion, where a characteristic challenge is to image the distribution of 10 ppm (average) of carbon monoxide (CO) at 1000 frames per second across a 50 mm diameter exhaust pipe; this particular problem has been pursued as a case study. In this paper, we present a novel electronic scheme that achieves the required measurement of around 10?3 absorption with 10?4 precision at kHz bandwidth. This was not previously achievable with any known technology. We call the new scheme Auto-Digital Gain Balancing. It is amenable to replication for many simultaneous measurement channels, and it permits simultaneous measurement of multiple species, in some circumstances. Experimental demonstrations are presented in the near-infrared. In single scans of a tunable diode laser, measurements of both CO and CO2 have been made with 20 dB signal-to-noise ratio at peak absorption. This work paves the way for chemical species tomography of minor constituents in many dynamic gas-phase systems.

A spectroscopic study for detection of carbon-monoxide using mid-infrared techniques for single-pass measurement

Tomographic imaging of minor species in combustion exhaust is an important requirement for combustion engineering. Only one group has tried carbon monoxide imaging in laminar flames in the mid-infrared region. A number of attempts have been made by different groups for single-channel multi-pass or long path-length measurement of CO. Single-pass measurement is being assessed in our work for tomographic measurement of the distribution of minor species. We report here the spectroscopic study of CO in different absorption lines of the strong fundamental bands in the mid-IR range. The criteria for selection of a light source in this context are also considered. Although strong spectral lines are available for CO in the mid-infrared region, strong water vapour and carbon dioxide interferences are also present. This addresses the choice of such useful spectral lines under interference by major species. The HITRAN 2004 database is used to predict the spectral lines of H2O and CO2 near the fundamental spectrum of CO in the mid-IR range. The exhaust condition in respect of temperature and pressure is considered in order to study the broadening of the spectral lines. The percentages of absorption for different species for different types of light sources are reported on the basis of computations. The concept can be readily extended to further species of interest in the presence of other interfering species. Different laser sources and spectroscopic methods are considered to design a strategy suitable to this application.

Digital gain balancing technique for sensitive detection of minor gas concentrations

Ultra-sensitive detection of minor species is important in many fields. The requirement of direct online measurement of gas species in process engineering motivates fast measurements over short pathlengths. The resulting high bandwidth and low absorptions reduce the measurement Signal-to-Noise Ratio (SNR). Our target is to detect CO in engine exhaust with sensitivity of the order of 0.05 ppm-m at several kHz. In this paper, a novel electronic scheme is discussed and presented with simulation results. A generic approach is followed, to use it with either a pulsed or a continuous wave (CW) laser source. A detailed noise analysis is presented for improved understanding of the SNR of the overall scheme.

Speech alarm and cycle helium gas monitoring of helium refrigerator

Two helium liquefiers are running round-the-clock in parallel. One of them is always connected to the Superconducting Cyclotron in refrigeration mode. EPICS based alarm system is running with alert sound in computer speaker. But the fault cannot be located without opening the fault screen. An audible English speech alarm system was envisaged and implemented in EPICS platform. Voice message are now being played in relation to faults or alarms. In continuously operating refrigeration system pure helium gas inventory is an important parameter to log. We need to calculate total pure equivalent helium cycle inventory. Now, we have put these calculations together in EPICS controller. This continuously calculates displays and archives the pure equivalent cycle helium gas. If any leak develops in the process, this trend gives a quick indication.