Auto-Digital Gain Balancing: A New Detection Scheme for High- Speed Chemical Species Tomography of Minor Constituents

Sandip Pal and Hugh McCann

School of Electrical & Electronic Engineering, University of Manchester, UK

ABSTRACT

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 previously reported work (Pal et al. 2008a), we have demonstrated that the spectroscopic sensitivity to achieve this goal is available via mid-IR absorption, and that viable light sources to meet the spectroscopic requirements, and components to synthesise viable measurement geometries, are commercially available.

In this paper, we present a novel electronic scheme that overcomes the remaining obstacle, viz. sensitive electronic detection of absorptions around 10-3 with 20 dB signal-to-noise ratio (SNR) at kHz bandwidth. This was not previously achievable with any known technology. The electronic scheme is called Auto-Digital Gain Balancing (Auto-DGB), and is amenable to replication for many simultaneous measurement channels. Moreover, Auto-DGB permits simultaneous measurement of multiple species, in some circumstances. Experimental demonstrations of the scheme have been carried out in the near-IR with higher CO concentrations so that absorptions are equivalent to that for 10 ppm CO in the mid-IR region. In single scans of a single tunable diode laser, measurements of both CO and CO2 have been achieved with 20 dB SNR measurement of the peak absorptions. This work paves the way for chemical species tomography (CST) of minor species in many dynamic gas-phase systems.

Keywords Chemical, infra-red, absorption, low-noise