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Distributed Temperature Sensor

Paul Stoddart, Alex Mazzolini, David Booth, Peter Cadusch, Romesh Nagarajah, Dragan Vukovic

As the name implies, a distributed temperature sensor (DTS) is able to measure temperature as a continuous function of position over an extended distance. The capability for long-range distributed sensing is unique to optical fibres.

The basic principle of a DTS is analogous to that of radar. An optical pulse is transmitted along an optical fibre light guide and a small part of the signal is scattered back along the fibre through interactions with the glass. Knowing the speed of light in the optical fibre, it is possible to correlate the time of arrival of the backscattered light with the position along the fibre at which the scattering occurred (see figure).

In the case of temperature sensing, the phenomenon of Raman scattering (where light is scattered through an interaction with molecular vibrations within the glass) provides a convenient, temperature-dependent signal. The early workers in the field used the well-known temperature dependence of the ratio between the powers in the upper and lower sidebands of the Raman signal as the sensing mechanism. Later on, the instrumentation was simplified by using only the anti-Stokes (high-frequency) signal, as this is the more sensitive of the two sidebands.

However, with this approach, it is much more difficult to apply corrections for errors associated with changes in the optical fibre, transmitter and receiver characteristics. The proprietary Sentor® DTS technology solves this problem by using a novel system characterisation procedure to periodically recalibrate the range fibre. This provides all of the advantages of instrumental simplicity without any loss of measurement accuracy - thereby reducing the overall cost.

The Raman scattering mechanism lends itself to applications where a precision of ±1°C is required. Temperatures can be measured in a range from about -100 to 600°C, depending on the type of fibre cladding that is used. However, within this broad specification, different applications may require different combinations of range and range resolution (i.e. the shortest distance over which changes in temperature can be resolved). The DTS research programme at Swinburne University of Technology (SUT) aims to address these various issues, while assisting with the commercialisation of the Sentor 101® system.

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