The technique works by sending a laser pulse of light down an optical fibre and measuring changes in its properties when it is reflected back. Variations in the reflected light translate into temperature readings, while the time between injecting the pulse and receiving the reflected signal pinpoints the location of the temperature reading.
Traditional temperature sensing relies on discrete sensors, such as thermocouples, which provide information only from their own location. They must be linked to a data acquisition unit, which often causes complex wiring problems.
When there is a need for multiple-point or shifting monitoring, discrete sensors are inflexible and expensive. Optical fibre sensors can discriminate between temperature readings only a metre apart for distances of up to 40km - long enough to reach across the English Channel.
Users can monitor thousands of points without needing to decide where to take the measurements.
Adopting optical fibre to monitor temperatures is the work of York Sensors, a Southampton company with close links to Southampton University, which has pioneered many developments in optical fibres. Peter Orrell, sales and marketing manager of York Sensors, says it might cost around pounds 10 per point to install discrete sensors, whereas an optical fibre system costing pounds 50,000 could measure up to 10,000 points.
'Not only can an optical fibre system provide information from thousands of points, it can respond to temperature changes in less than a second and continues to measure even if the fibre is broken,' Mr Orrell says.
This makes the technique very powerful in fire detection. It also has an advantage over infra-red fire detection systems, which cannot distinguish between smoke and fire, in being able to pinpoint the seat of a fire.
York Sensors is working with companies to develop new applications. In the chemical industry, the method is being used for surface monitoring of vessels that operate under high temperature and pressure. Such vessels have refractory linings that can fail. By winding the optical fibre on to the outside of the vessel, any hot spots, indicating lining failure, can be detected.
Mr Orrell says companies using the technology do not want to be named, as they believe the ability it gives them to operate safely with fewer maintenance shutdowns provides an advantage over competitors.
Optical fibre is flexible and easy to install, and is now so cheap (a few pence per metre) that it can economically be used to monitor long lengths of pipeline.
The fibre can be sheathed in different coatings, allowing it to operate at temperatures between minus 190C and 460C.
One low-temperature application, being tested by Gaz de France, is monitoring gas pipelines. Gas is moved as a liquid at low temperatures. Any leak - which could cause an explosion - is registered as a cold spot.
Oil pipelines could also be monitored to ensure that the water in unrefined oil did not freeze, blocking the pipeline.
'Because the fibre does not depend on electrical current to transmit its measurements, it is an ideal monitoring technique for oil pipelines and refineries, where electric sparks can cause explosions,' Mr Orrell explains.
The National Grid, in collaboration with Electricite de France, is currently assessing the sensors for monitoring underground cables, which must operate below a certain temperature to avoid burn-out.
National Grid has 600 route kilometres of underground cable and its safe capacity varies, depending on the time of the year and ambient temperature.
The difficulty of monitoring underground cables has led electricity companies to specify a higher capacity than they need (at a significantly higher cost). According to Mr Orrell, York Sensors has now given electricity companies the confidence to specify lower-capacity cables.Reuse content