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Raman scattering in optical fibres provides a means of measuring continuous temperature distributions over extended distances. However, in many applications it has been found that the background transmission properties of the optical fibre either change after deployment of the fibre or vary over time due to ageing effects. Changes in the fibre transmission can be distinguished from thermal effects by measuring the Rayleigh backscatter, which is relatively insensitive to temperature. The combined use of Raman and Rayleigh data is discussed here in the context of a particularly simple and cost-effective sensor design, where a single, fixed optical filter and a single light source are used for both measurement modes. The Rayleigh backscatter measurement allows the accurate correction of background transmission changes in a probe arrangement that includes several splices between different fibres with different attenuations. In this way, the temperature in a 290 °C test region is accurately recovered from the anti-Stokes Raman signal. However, preliminary measurements of the fibre attenuation as a function of temperature and exposure to an accelerated ageing environment show that these two effects will be difficult to separate in practice. Therefore it remains challenging to perform accurate, unambiguous background corrections in situations where high-temperature ageing occurs.

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A spectrophotometric investigation has been made of the 1 : 1 complexes formed by sulphur dioxide with 8 olefines and several aromatic hydrocarbons. Equilibrium constants have been derived over a range of temperature (10 to 40°C) using n-hexane as solvent. Values at 25°C range from 0.037 1. mole-1 for cyclopentene to 0.19 for 2 : 3-dimethyl but-2-ene and 0.30 for mesitylene. Derived values of -AH range from 0.7 kcal mole-1 for cyclopentene to 3.55 kcal mole-1 for 2 : 3-dimethyl but-2-ene. The values of -AS' show a correlation with AH and fall in the range 7 to 15 cal deg.-l mole-*.The absorption spectra of the complexes sometimes show two maxima, sometimes a point of inflection in the region of 290 mp and sometimes a single band. These have been interpreted in terms of two distinct transitions : (i) charge-transfer absorption by the complex ; this gives rise to the band whose position shows a correlation with the ionization potential of the hydrocarbon; (ii) an enhanced absorption of the sulphur dioxide transition at 290 mp ; this gives rise to the band whose position remains approximately constant at 296 mp.Subsidiary measurements were made on the infra-red absorption and vapour pressures in these systems. The system 2 : 3-dimethyl but-2-ene + SO2 gives a phase diagram with a broad maximum at the 1 : 1 composition, m.p. -36°C.