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Content archived on 2024-04-19

Remote Monitoring of Hazardous Gases and Vapours Using Mid Infra-Red Optical Fibres


The detection of chemical species by fibre optic techniques holds considerable promise for on-line process monitoring with intrinsic safety. Whereas successful commercial developments with fibre optics have been achieved at near infra-red to visible wavelengths, much greater potential exists in the mid infra-red region with its inherent analytical capability. Following the latest developments in mid-infra-red optical fibres and hollow waveguides, this project applies experimental demonstrators to assess their development potential for remote chemical sensing at mid infra-red wavelengths.

The prototype measurement techniques adopted the following main objectives:
(a) the demonstration of remote sensing of gases and vapours at mid infra-red wavelengths using optical fibres and hollow waveguides;
(b) the detection at parts per million / billion levels (ppm-ppb) with the potential for wide analyte selectivity;
(c) the validation of the methodologies with novel vapour standards of ppm concentrations.
Experimental hollow waveguide gas cells with optical path lengths ranging from 1-8 metres were tested successfully using a wavelength tunable CO2 laser. The path length to volume ratios for these cells were 2-3 orders of magnitude greater than conventional optical gas sample cells. A precision of measurement of the order of 10-4 absorbance units was achieved using waveguide gas cells of 1 metre optical path. An increase in absorbance (and thus sensitivity) was demonstrated with extended waveguide sensors of up to 8.32m in length and operating with gas sample pressures up to 10A. The minimum detection level for ethylene gas was less than 100 ppb.
Investigations on the evanescent wave sensor approach using solid core fibres confirmed the low sensitivity of this approach. Optical fibre connected open path gas cells were also developed and these provided a remote deployment capability of several metres. The cells were interfaced to a tunable diode laser spectrometer operating at the wavelengths of 8 and 12 um. Spectral analyses undertaken in derivative mode (wavelength amplitude modulation) and also harmonic mode (centre line lock amplitude modulation) gave minimum levels of detection of less than 1ppm for n-nitrosodiethylamine and 2-furaldehyde.
Breadboard mid infra-red laser diode and CO2 laser spectrometer systems were assembled and a number of different fibre optic gas cell configurations were tested. Three distinctive measurement approaches were investigated. They included a single pass variable path length solid core fibre coupled device, a double pass multiple path hollow waveguide gas cell, and a solid core fibre optic evanescent wave sensor. Energy throughput and signal processing techniques were applied to derive maximum detection and remote deployment of the sensors. Calibrated vapour mixtures of 2-furaldehyde and n-nitrosodiethylamine in the range 1-30 ppm were developed for evaluation of the sensor devices.


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University of Sussex
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Sussex House Falmer
BN1 9RH Brighton
United Kingdom

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