How DTS Works
Distributed Temperature Sensing (DTS) utilizes standard optical fibers, typically spanning dozens of kilometers, to serve as linear temperature sensors. This approach provides a spatially well-resolved temperature profile along the entire length of the sensor cable, with a resolution down to one meter.
Light Scattering in Fiber Optics, OTDR
DTS technology is based on either Raman or Brillouin optical scattering. A laser pulse is periodically launched into the fiber, from which a small fraction is scattered back due to the interaction of the light with the glass structure. The fraction that is scattered back to the interrogator is continuously analyzed. The position of the temperature reading is derived from the arrival time of the returning light, analogous to radar echo analysis. This method is referred to as Optical Time Domain Reflectometry (OTDR). Technically slightly different, but mathematically equivalent is the Optical Frequency Domain Reflectometry (ODFR). For covering long distance with best signal to noise ratio, the time domain (OTDR) approach shows advantages due to lower shot noise.
Temperature Monitoring Based on Raman Reflectometry
In DTS systems based on Raman scattering, two different Raman components are individually analysed, the Stokes and the anti-Stokes band. The intensity ratio between both signals depends on the local temperature and is used to calculate the temperature along the optical fiber.
AP Sensing uses Raman-OTDR technology with tremendously enhanced performance by the proprietary code correlation (CC) technology. The implementation of CC-OTDR introduces several advantages: It clearly increases the signal-to-noise ratio and thus temperature resolution and distance range, and it facilitates the use of low optical pulse power, effectively mitigating concerns related to potential laser degradation. This makes AP Sensing DTS systems inherently safe in the usage as well as in deployments in EX-rated areas.
Furthermore, a patented single-receiver design provides superior stability of relative amplitudes of both signals and thus temperature. A noteworthy outcome of this approach is the exceptional system reliability achieved.
The Raman system is insensitive to strain on the sensor fiber, allowing undisturbed temperature measurement even in the presence of mechanical tension, a factor that may lead to anomalous temperature readings (e.g. from Rayleigh or Brillouin signals) if not treated in a suitable way.
Continuous Temperature Profile Measurement:
DTS systems provide continuous thermal information, thus recognizing and precisely localizing any thermal anomalies. With the high coverage of many dozens of kilometers per measurement unit and real-time data, full temperature monitoring makes it possible to pinpoint the exact locations of any hot or cold spot. This feature is crucial for early detection of hotspots, fires or temperature abnormalities indicating faults or other issues.
Additionally, optimized control of cool-down operations is easily enabled with the continuous thermal information provided by the DTS system. This attribute empowers industries to maintain a continuous temperature monitor along the fiber’s length, which aids in anomaly detection, improving operational efficiency, and precise temperature control across a variety of industrial applications.
Maximum Reliability Under Harsh or Explosive Conditions:
DTS systems are proven in maximum reliability under harsh and industrial conditions, such as heat, dirt, dust and corrosion. With optical fibers used for sensing, DTS system are non-corrosive and intrinsically safe in explosive atmospheres.
Depending on cable design and used fiber coating, DTS applications cover a wide sensing temperature range from as low as -185 °C (-300 °F) up to +750 °C (+1400 °F) and more.
Immunity to Electromagnetic Interference:
The use of optical, naturally non-conductive fiber makes the sensor immune to electromagnetic interference (EMI or RFI). This immunity stems from the use of light in glass for sensing and signal transmission instead of electrical voltage and current signals that can be disrupted by electromagnetic fields or radio frequency interference. This ensures undisturbed temperature measurements even in environments with high levels of electromagnetic noise.
High Sensitivity and Accuracy:
DTS systems provide accurate and reliable temperature measurement for precise analysis. They offer high sensitivity to detect even minor temperature changes, which is also a benefit to predictive maintenance. Thus, they are ideal for applications where small temperature variations are critical, such as in industrial processes and research or in monitoring of changes in the burial depth of a submarine power cable.
Cost-Effectiveness and Low Maintenance Requirements:
A DTS system is truly distributed, meaning it responds to heat at any point along the cable equally. The result is a continuous temperature profile across the entire extension of the fiber optic cable, which makes it a unique benefit over conventional sensors for a wide range of applications. Utilizing a single passive fiber optic cable over long distances leads to cost savings in equipment and installation, where usually multiple traditional sensors are required. Low maintenance requirements enhance operational efficiency and long-term cost-effectiveness.