Measurement distances of greater than 30 km can be monitored and some specialised systems can provide even tighter spatial resolutions.
Thermal changes along the optical fibre cause a local variation in the refractive index, which in turn leads to the inelastic scattering of the light propagating through it.
Alternative DTS evaluation units deploy the method of Optical Frequency Domain Reflectometry (OFDR).
The essential principles of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical backscatter signal.
This is offset by the technically difficult measurement of the Raman scattered light and rather complex signal processing, due to the FFT calculation with higher linearity requirements for the electronic components.
Thus a light source with lower peak power compared to OTDR technology can be used, e.g. long life compact semiconductor lasers.
The temperature measuring system consists of a controller (laser source, pulse generator for OTDR or code generator for Code Correlation or modulator and HF mixer for OFDR, optical module, receiver and micro-processor unit) and a quartz glass fibre as line-shaped temperature sensor.
Additionally, because the sensing cable has no moving parts and design lives of >30 years, the maintenance and operation costs are also considerably less than for conventional sensors.
In the oil and gas industry an XML based file standard (WITSML) has been developed for transfer of data from DTS instruments.
Some DTS systems on the market use a special low power design and are inherently safe in explosive environments, e.g. certified to ATEX directive Zone 0.
Temperature distributions can be used to develop models based on the Proper Orthogonal Decomposition Method or principal component analysis.