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Introduction to the Principle of Distributed Fiber Optic Temperature Sensing System

Distributed Temperature Sensing System (DTS) is a fiber optic optoelectronic instrument that measures the temperature along the length of the fiber optic sensing cable. The unique feature of a distributed temperature sensing system is that it provides a continuous (or distributed) temperature distribution along the length of the sensing cable, rather than discrete sensing points that must be predetermined.

How fiber optic temperature sensors work

The DTS system includes a pulse laser that sends approximately 1m pulses (equivalent to 10ns of time) to the fiber. When the pulse propagates along the length of the optical fiber, it interacts with the glass. Due to small defects in the glass, a small amount of raw laser pulses are reflected back into the DTS sensing system. By analyzing the reflected light, DTS can calculate the temperature of the event (by analyzing the power of the reflected light) and the location of the event (by measuring the time it takes for the backscattered light to return), typically in instruments.

Distributed temperature sensing cable

Usually, DTS technology uses standard telecommunications fiber optic cables, and specialized cables or sensing points are only required for measurements when temperatures exceed 100 ° C. Sensing fibers are usually based on multimode fibers, suitable for short distances (up to 40km) and single-mode fibers, and suitable for long distances (40-100km).

Fiber optic temperature measurement specification for DTS

Distributed temperature sensing systems can typically locate temperatures within a distance of 1 meter (referred to as spatial resolution), with accuracy within the range of ± 1 ° C and sensing resolution as low as 0.01 ° C. However, there is an inverse relationship between measurement resolution, range, and sampling time, where temperature resolution decreases with range and prolongs the time required to obtain specific measurement data.

Distributed temperature sensing Raman measurement principle

Fiber optic is made of doped quartz glass, and when laser is transmitted in the fiber, interactions occur between optical particles (photons) and molecular electrons. At specific frequencies in the electromagnetic spectrum (known as the Stokes and anti Stokes bands), light scattering (also known as Raman scattering) occurs in optical fibers. The intensity of the so-called anti Stokes band is temperature dependent, while the so-called Stokes band is actually temperature independent. The local temperature of optical fibers comes from the ratio of anti Stokes and Stokes light intensity.

Measurement Principles – OTDR and OFDR Technologies

There are two basic measurement principles for distributed sensing technology: optical time-domain reflectometer (OTDR) and optical frequency-domain reflectometer (OFDR).

OTDR was developed over 20 years ago and has become an industry standard for telecommunications loss measurement. The principle of OTDR is very simple, similar to the flight time measurement used for radar. Basically, narrow laser pulses generated by semiconductors or solid-state lasers are sent into fibers and analyzed for backscattered light. Starting from the time when the backscattered light returns to the detection unit, the location of the temperature event can be located.

A method to replace DTS evaluation of unit signed frequency domain reflectometer (OFDR). The OFDR system only provides information about local characteristics and then performs Fourier transform when the backscattered signal detected throughout the measurement time is measured in a complex manner as a function of frequency.

The vast majority of currently available distributed temperature sensing systems are based on OTDR technology.

The advantages of DTS system

Some unique features of distributed temperature sensing systems include:

Excellent economies of scale. System designers/integrators do not have to worry about the precise location of each sensing point, so the cost of designing and installing sensing systems based on distributed fiber optic sensors is greatly reduced compared to traditional sensors.
Low maintenance and operational costs. The sensing cable has no moving parts and is designed with a lifespan of over 30 years. The maintenance and operating costs are much lower than traditional sensors.
DTS sensing cables are not affected by electromagnetic interference or vibration
These sensors can be safely used in hazardous areas (laser power below the level that can cause ignition), making them an ideal choice for industrial sensing applications.
Design of fiber optic sensing cables

Fiber optic cables are essentially passive and do not have separate sensing points, so they can be manufactured based on standard telecommunications fibers and in many cases, packaged using standard telecommunications fiber optic cables.

In some cases, specialized optical fibers are required, and similarly, specialized cable encapsulation is required. Some considerations when designing distributed temperature sensing cables include:

Temperature: The working temperature of standard telecommunications fiber optic and cable materials can reach up to 100 ° C. On top of this, you will need specialized glass and cable materials. For example, oil wells typically exceed 200 ° C
Mechanical protection: Depending on the specific monitoring environment, there may be high vibrations or possible crushing forces, which will require additional cable layers to provide protection for sensing optical fibers
Hydrogen protection: In certain environments, there may be high levels of hydrogen gas, which can cause fiber optic degradation (or darkening). Some protection can be provided by using hydrogen to remove the gel – but for a longer duration, the special fiber itself with special properties (dopants) in the core and cladding of the fiber must be used.
Laser safety and system operation

When operating optical measurement based systems (such as optical DTS), it is necessary to consider laser safety requirements for permanent installation. Many systems use low-power laser designs, such as those classified as laser safety level 1M, which can be applied by anyone without the need for approved laser safety personnel. Some systems are based on high-power lasers rated at 3B, and although approved laser safety personnel can safely use them, they may not be suitable for permanent installation.

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