Questions? Answers!

Here are answers to questions we are most frequently asked. You may find your question answered here. If not, please contact us we are here to help, and it may save you precious time.

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Why consider distributed sensing when conventional sensors (e.g., strain gauges, extensometers, Pt100 temperature sensors) do a good job?

Conventional sensors:

  • Deliver information from designated points along a structure
  • The range of networked sensors is limited to hundreds of meters
  • Installation costs may be high
  • The area you want to monitor may be difficult or dangerous to access
  • Positioning and connecting the sensors over kilometers may be cost and
  • What happens between sensors remains unknown
  • The cost of maintaining each sensor can be significant.

Distributed sensing relies on optical fiber sensing cables:

  • Inherently sensitive to strain, temperature, and vibration all along their length
  • Provide uninterrupted monitoring, essential for managing risks and maintaining large
  • Light, flexible, EMI-immune, passive sensing element which can be embedded or surface
  • Measurement parameters can be set post-installation and changed.

Distributed fiber optic sensing is the only cost-effective way to monitor kilometers of concrete, steel, soil, or rock for early signs of failure. As a sensing technique, it is a powerful and highly efficient tool.

It detects and locates deformation, cracking, ground movement, and temperature events that could infer erosion, seepage or a leak. Optical fibers are truly distributed sensors, detecting and locating early signs of deformation inferred from changes in strain or temperature (or both). Electrical fields do not affect fiber optics, which withstand lightning, chemically aggressive environments, and long-term aging effects.

Distributed fiber optic sensing is cost-effective for larger or longer projects. As a rough guide, monitoring 100’000 points over 50 km works out usually at less than 10 Euros per point (including instrument and fiber optic sensing cable. Engineering cost, installation, and sundries not included). That’s economically advantageous for continuous automatic sensing, especially since the sensing cable needs no maintenance.

Summary of distributed fiber optic techniques

Technology Sensing Physical Effect Main Applications Capabilities
Distributed Strain and Temperature Sensing Brillouin DTSS Strain and temperature Brillouin
Structural health Geohazard detection Deformation monitoring Power cable condition Long-term stable measurements of both strain and temperature No re-calibration required
Distributed Temperature Sensing Raman DTS Temperature Raman
Fire detection
Power cable condition monitoring
Leakage detection
Ideal for temperature only measurement
Distributed Acoustic Sensing Rayleigh DAS Vibration/Acoustics Rayleigh
Seismic profiling
Intrusion detection
Leakage detection
Dynamic distributed acoustic data

There are systems to meet most requirements for continuous monitoring of large, long, or complex structures. Since each project is different, we can help you specify a configuration to fulfill the project’s requirements. And if fiber optic sensing is not ideal for your project, we’ll tell you that too.

Here are a few questions for you that help define the configuration:
  • What are you looking to detect? Deformation, displacement, erosion…
    settlement (inferred from strain monitoring), leaks, erosion, water ingress, insulation breakdown, change in operating temperature (inferred from temperature changes)
  • What length or area do you want to measure or monitor?
  • How far do you want to measure (distance range) – from a few tens of meters to
    hundreds of kilometers?
  • Is the project linear (pipeline, power cable, pile), or does it cover a wide area (embankment, slope, tunnel)?
  • Are both strain and temperature monitoring required?
  • On what kind of material, or in which environment will you install or embed the sensing
  • Precision required: is accuracy or speed of measurement more critical?
  • How do you want to receive the data?

Yes. Our Brillouin technology-based instruments (BOFDA, BOFDR) monitor strain and temperature or temperature compensation for strain. The sensing cable contains fiber in a loose tube to measure temperature. While for strain, a tight-buffered fiber is required. Contact us for help defining which fiber optic sensor (sensing cable) is the right one for your project.

Distance is no problem for fiber optic sensing. fibrisTerre’s system monitors from a few tens of meters to 80 km from a single interrogator. A geogrid or geosynthetic may be used as a carrier. A multi-channel switch allows multiple sensing cables to be interrogated, enabling monitoring of larger areas or cross-sections of complex structures.

Talk to us. As necessary, we will put you in touch with an integrator, specialist installer, or agents near you to make sure you benefit from local and application-specific service and support.

The sensing cable is very similar to a fiber optic telecoms cable. The fiber inside is single-mode. For strain monitoring, the fiber will be held tightly in the cable (tight-buffered) so that the cable detects the strain in the structure. For temperature measurement, the fiber is contained in a loose tube within the cable. Sensing cables are available with various ruggedized structures and materials to ensure fitness-for-purpose and longevity in different applications.

A broken fiber can be repaired or replaced. Redundancy is built into a good monitoring design. There are guidelines for handling the fiber optic cable, which is why we rely on specialist installers and agents to specify and install the sensor correctly. Our in-house specialists help with both equipment and sensing cable selection and configuration.

An application-specific sensing cable will give the best results. Sometimes, temperature monitoring can use an existing fiber optic cable. Monitoring strain usually requires a tight-buffered fiber which is unlikely to be found in an existing telecoms cable.

fibrisTerre’s fTB instrument series provides a range of interface options for data handling and instrument control. These enable seamless integration into SCADA systems, third-party data visualization software, FE modeling, GIS representation, and digital twins.

fibrisTerre’s fTB 5020 interrogator range is designed for reliability. The unit weighs only 7 kg, consumes a mere 40 W, and is sealed against dust ingress. It operates at temperatures between 0 and 45°C.

Your fibrisTerre instrument and its computer arrive in a ruggedized trolley as standard. This is ideal for surveys and interventions. You will need a power supply. For continuous monitoring, the instrument requires housing in a controlled environment. A rack-mounted option is available, with UPS, suitable for substations.

Designed for long-term, continuous monitoring, the system divides into two parts:

  • the sensing cable
  • the interrogator unit.

Fiber optic cables are robust and passive and are deployed in industrial projects demanding a life expectancy of one hundred years. For the instruments, optical switches, computers, and other electronic components, fibrisTerre offers servicing and warranty plans to ensure that your measurements are continuous.