Conventional Temperature Monitoring Systems Engineering Essay

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Conventional temperature monitoring systems are usually based on the use of point measurements where data is gathered from individual sensors and gauges that measure single values at specific locations. This limits the speed, accuracy and resolution of the monitoring system in many applications. Distributed sensing is a technology that relies on analysis of light pulses reflected through optical fibres hence it offers a better and more efficient way to monitor changes in temperature and pressure. By using an optical fibre as a sensor, distributed sensing makes it possible to take real-time readings of temperature every meter, along the fibre which is up to 60km long.

Distributed Temperature Sensing (DTS) makes continuous measurements over the full length of the optical fibre instead of making measurements at discrete, pre-determined points. Distributed sensing is also real time - to get continuous monitoring at all points along the cable at all times.

3.2 Measuring principles

Distributed Temperature Sensing (DTS) system works on the following basic fundamental and principles.

3.2.1 Optical Frequency Domain Reflectometry (OFDR)

Some Distributed Temperature Sensing evaluation units deploy the method of Optical Frequency Domain Reflectometry (OFDR). The main principles of OFDR technology are the narrow-band detection of the optical back scatter signal and the quasi continuous wave mode employed by the laser beam. When the backscatter signal detected during measurement OFDR system provides information on the local characteristic only. The entire measurement time is considered as a function of frequency in a complex fashion, and then subjected to Fourier transformation, which is a linearity requirement for the electronic components. Because of this complex signal processing OTDR technology is more popular in practice.

3.2.2 Optical Time Domain Reflectometry (OTDR)

OTDR works on simple method which is very similar to the time of flight measurement used for radar which was developed more than 30 years ago. A short pulse of light is transmitted along the fibre and at the launch end of the fibre, backscattered Rayleigh signal and Raman signal are detected, which is shown in the Figure 3.1. The spatial information is determined by time interval between sending the pulse and detection of the backscattered signal. The fibre attenuation is obtained by the intensity of the backscattered signal.

Figure 3.1: OTDR Principle (Sentinel DTS manual, 2006)

The reason OTDR has become prominent both for telecom equipments and within distributed temperature sensing is that OTDR offers a number of technical benefits.

Reliability in Measurement

Distributed Temperature Sensing (DTS) measurements using OTDR are reliable, robust and are less affected by potential anomalies within the fibre and accessories (e.g. bends, connectors, reflections).

Wide Range

The Distributed Temperature Sensing (DTS) technology based on OTDR currently measures along sensing cables of distances greater than 60 km.

Temperature Resolution

OTDR based Distributed Temperature Sensors are able to resolve temperature changes with an accuracy of 0.01°C.

Measurement Parameters

OTDR is a more versatile technology for distributed sensing and it is possible to measure both distributed temperature and distributed strain.

Laser Safety

The majority of the OTDR DTS sensing systems are classified as 1M units. (The laser safety classification of all DTS units must be clearly marked on the outside). This is safe under all reasonable working conditions and the majority of OTDR DTS systems are safe for use in hazardous zone ratings and comply with EU regulations EUR16011EN (1994). (Sentinel DTS User guide, 2006)

3.2.3 Raman Effect

In 1928, Raman observed the inelastic light scattering connected with a frequency shift of the scattered light. The nature of this effect could be explained by the classical theory of electrodynamics. Hurtig et al., (1994) have explained this phenomenon and its relation to temperature variation, as described below:

When an electromagnetic wave with frequency 0 is incident on a molecule, the energy is given as

E = E0 sin (2 0 t) (1)

The electromagnetic field stimulates oscillation of the molecular electron. The oscillating dipole moment M is

M = E0 sin (2 0 t) (2)

With polarizability  = 0 = constant. In the real situation of molecular vibration with frequency k the polarizability is an oscillating expression, given as :

 = 0+ 1 sin (2k t) (3)

Thus the dipole moment is given as

M = { = 0+ 1 sin (2k t)} x E0 sin (2 0 t) (4)

M =0 E0 sin (2 0 t) + ½ 1 E0 cos{ 2 (0 - k) t} - ½ 1 E0 cos (2 (0 + k) t}

(5)

The first term describes the elastic part of the light scattering and gives Rayleigh scattering. The second and third term describes the elastic part of the scattering with frequency shifts of

(0 + k) are called Raman scattering with the components

(0 + k) Strokes frequency and

(0 - k) Anti - Strokes frequency.

The schematic representation of the spectrum of back scattered light is shown in Figure 3.2. The theory of photon-photon interaction in solid-state physics gives a statement required for Raman Effect in sensorics. The occurrence of Raman scattering is based on the assumption of several energy levels in the vicinity of energetic ground state of the molecules.

Figure 3.2: Backscattering spectrum (Sentinel DTS manual, 2006)

The probability Wse of spontaneous emission of a stroke or an Anti-strokes photon is given by:

Wse (hs) = D q0 n0 (6)

Wse (has) = D q0 n0 exp ( - hk / k T) (7)

These probability expressions indicate that the intensity of Anti-stroke line is lower than Strokes line and the intensity of Anti-strokes line is dependent on temperature. The efficiency of this effect can be estimated after calculating the Raman scattering flux. The flux of Raman scattering is 10-30 and when the efficiency of spontaneous Raman Effect is about 10-30. The efficiency can be improve to 10-9 by using laser light and induced nonlinear Raman Effect. (Hurtig et al., 1994)

3.3 Fundamentals of DTS technique

The fibre optic DTS technique using the Raman-effect was developed at the beginning of the 1980s at Southampton University in England, UK (Hartog et al., 1991). DTS is based on Optical Time Domain Reflectometry (OTDR). A pulsed laser is coupled to the optical fibre which is the sensing element. The light is backscattered as the pulse propagates through the fibre owing to changes in density and composition as well due to molecular and bulk vibrations.

The distance on fibre optic is determined by the time-of-flight of the returning backscattered light because the velocity of light propagation in the optical fibre is known. The backscattered light consists of different spectral components due to different interaction mechanisms between the propagating light pulse and the optical fibre. A portion of the backscattered light is guided back to the source and is split off by the directional coupler to the receiver as shown in the Figure 3.3.

The backscattered light consists of Rayleigh, Brillouin and Raman backscattering light. The Rayleigh backscattering component is the strongest due to density and composition fluctuations and has the same wavelength as the primary laser pulse. The Rayleigh component controls the main slope of the intensity decay curve and may be used to identify the breaks and heterogeneities along the fibre. The Rayleigh component is less sensitive to temperature variations.

Figure 3.3: Principle of DTS technology (modified from Sentinel DTS manual, 2006)

The Brillouin backscattering components are caused by lattice vibrations from the propagating light pulse. It is difficult to separate the Brillouin components from the Rayleigh signal because its peaks are spectrally so close to the primary laser pulse.

The Raman backscattering light is caused by thermally influenced molecular vibrations and is used to obtain information about the temperature distribution along the fibre. The Raman backscattering light has two components: the Stokes and the Anti-Stokes components. The Stokes component is only weakly dependent on temperature, while the Anti-Stokes component shows a strong relation to temperature as shown in the Figure 3.4.

Figure 3.4: Variation of Intensities (Vogel et al., 2001)

The basic principle of fibre optic temperature measurements consists of filtering the Stokes and the Anti-Stokes components out of the backscattering light. The ratio of the intensities of both components is calculated and transferred in temperature values using both, the internal reference temperature of the equipment and an externally determined calibration function for the particular fibre type.

The space coordinate is obtained from the travel time of the propagating light pulse. The temperature is determined as an integral value for a short section of the optical fibre. Therefore, it is possible to measure the temperature simultaneously along the entire length of the fibre. The space resolution at present is 1 m (optional 0.5 m or 0.25 m). The specific material properties of the optical fibre (geometry and chemical composition) and their temperature dependence are used to determine the absolute temperature using a calibration function.

The calibration function must be determined for the individual optical fibres before the measurements are performed because these properties are different for different optical fibres. The accuracy of the temperature measurements is 0.3 K, and a resolution up to 0.05 K can be reached. The available resolution and precision depend on the specific material properties of the used optical fibre, whereas the accuracy of measurement is controlled by the accuracy of the fibre-specific calibration function.

The measured backscattering intensity defines the integral temperature of given fibre section because Raman backscattering intensity is integrated for fibre section (1 m, 0.5 m or 0.25 m, respectively), whereas standard temperature sensor give the local temperature at the position of sensor. Backscattering of light is a stochastic process, therefore, it is necessary to integrate the backscattering intensity for a given time interval. Taking a geometrical resolution of 1 m an integration time of 1 min is sufficient to minimize the stochastic noise. The integration time must be increased by a factor of 2 or 4 if the space resolution is increased to 0.5 m or 0.25 m, respectively.

The fibre optic temperature sensing system operates without any electronic circuits along the fibre. External influences such as changes in light source or optical fibre are eliminated by taking the ratio of the intensities of stokes and Anti-stokes components as show below.

Ia / Is = {(x0 + xk )4 / (x0 - xk )4 } exp ( -h c xk / k t ) (7)

In order to apply induced Raman Effect in temperature sensors:

the induced Raman effect and laser light must be used.

because the Raman scattering has low efficiency it is measured in backscattered light.

the intensity ratio of Anti-stroke and stroke is used thereby eliminating all environmental and material influences except Temperature.

3.4 Equipments used

3.4.1 DTS unit

For this particular experimental study a Sentinel DTS system from Sensornet Company is utilised. The system is packaged in a standalone unit which contains both the onboard PC to record and analyse the data also sensing optoelectronics including transmitting and recording units. The system operates with a visceral user-friendly software interface (based on Windows OS), making it a simple-to-use and easily transportable system along with fibre optic cable as shown in Figure 3.5.

The system has been designed with safety in mind and has been tested to some of the industry's most rigorous standards. The system is optimised for measurements on single-ended cable installations, but with the help of multiplexing module it is easily configured to produce double-ended measurements. (Sentinel DTS manual, 2006)

Figure 3.5: DTS measuring unit. (Sentinel DTS manual, 2006)

User configurable zones and alarms functionality are also available for a wide variety of applications. This system consists of class 1M laser with a mean power output of 1mW and is classified as EN 60825-1(2001-2003) which is suitable to monitor Zone 0 Hazardous area according to European Commission. (Sentinel DTS manual, 2006)

The sensing capabilities of this system are given in the Table 3.1

Table 3.1: Sensing capabilities (Sentinel DTS manual, 2006)

Capabilities

Extent

Range

0 to 10 Km

Model

Sentinel DTS-LR

Description

Long Range

Temperature Resolution

0.01ËšC

Spatial Resolution

1m

Sampling Resolution

0.5m

The operating temperature of DTS system is + 50 C to + 400 C, with storage temperature ranging from - 150 C to + 650 C and this system could work in 5% to 95% relative humidity environment. Figure 3.6 shows the performance of DTS Long Range (10 km) system from Sentinel Sensornet.

Figure 3.6: Sentinel DTS-LR performance (1m Spatial Resolution) (Sentinel DTS manual, 2006)

It consumes about 120 W of power and has an option of running on AC or DC power supply. The equipment measures 180 X 435 X 480 mm and has a total weight of 21 kg

3.4.2 Optical fibre cable

An optical fibre cable is a cable containing one or more optical fibres, these fibres are very thin pieces of glass which act as a pipe through which light ray can pass. The light that is passed down can be turned on and off to represent digital information and it can be gradually changed in amplitude, frequency. The optical fibre elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed.

Types of optical fibre cables

Light is kept in the core of the optical fibre by total internal reflection. This causes the fibre to act as a waveguide. Fibres which support only a single mode are called single-mode HYPERLINK "http://en.wikipedia.org/wiki/Singlemode_fiber"fibres (SMF). Fibres which support many propagation paths or transverse modes are called multimode HYPERLINK "http://en.wikipedia.org/wiki/Multimode_fiber"fibres (MMF) as shown in Figure 3.7.

Figure 3.7: Types of fibre (modified from Sentinel DTS manual, 2006)

The light travelling along a fibre can be considered as rays of light that bounce off the interface between the core and the cladding of the fibre. Multimode fibre has a large diameter core which allows the rays of light to travel along several different pathways through the fibre, bouncing at different angles between the core and cladding. These different pathways are referred to as spatial modes, and hence it is described as a multimode fibre.

A fibre optic cable which has a small core and only allows one path for the rays of light to travel through it referred as spatial mode is known as a single-mode fibre.  Multimode fibre typically comes with two different core sizes: 50 micron or 62.5 micron. Single-mode cable features a 9-micron glass core.

Since multimode fibre has a larger core (and a larger numerical aperture) it is possible to launch more power into the fibre than for single mode fibre. The larger core size also results in a higher threshold for nonlinearities, which can cause errors in the measurement of temperature of the fibre.  Distributed temperature sensing measures the backscattered light along the fibre. However, the amount of backscattered light is a very tiny proportion of the initial laser pulse and so in order to get enough backscattered signal to make a temperature measurement, it is very important to have as much power as possible in the initial laser pulse. Given its larger core size and higher threshold for nonlinearities, multimode fibre provides a large improvement in backscatter signal power over single-mode fibre.

Single-mode fibre is used for distributed strain sensing due to the nature of the optical components required. Almost all of the components are standard telecommunications components, designed to operate around a wavelength of 1550 nm using single-mode fibre. It is very difficult to use multimode fibre as the sensing fibre of the instrument because it is based on single-mode components. More amount of light is lost while trying to squeeze the light from the large multimode core into the much smaller single-mode core. The backscatter signal used in strain sensing is more powerful than that used for the temperature sensing, hence the relative lower power of using single-mode fibre can be tolerated.

The Sentinel DTS system uses complete length of optical fibres as the sensing element for distributed temperature measurement. The following sections explain the construction ad properties of the optical fibre used.

Optical fibre construction

The Sentinel DTS system uses Multimode type of optical fibre designed for data communication purpose. This optical fibre is referred as 50/125 multimode optical fibre. The optical fibre construction is shown in the Figure 3.8.

Figure 3.8: Optical fibre construction (Sentinel DTS manual, 2006)

The core and cladding:

Optical fibres consist of a cylindrically symmetric strand of glass composed of a central core which is surrounded by an outer cladding. The core guides the right travelling down the fibre and the cladding helps to reduce any losses in the guided signal. This system is optimised to use a 50/125 optical fibre which has a core diameter of 50 microns and a cladding diameter of 125 micron.

The Primary Coating

The central core and outer cladding are surrounded by the primary coating. The primary coating allows the optical fibre to be physically handled while preventing mechanical damage to the central core and cladding. The primary coating is commonly made from acrylate and has a diameter.

Graded Index Multimode Fibre

The 50/125 optical fibre used with this DTS system is categorised as Graded Index Multimode Fibre. The term 'graded index' is used to explain the glass core which is constructed to have a parabolic refractive index gradient which increases smoothly from a uniform value in the cladding to a maximum value at the centre of the core, as show in the Figure 3.9.

Figure 3.9: Refractive index profile (Sentinel DTS manual, 2006)

The term 'multimode' is used to explain the fact that the optical fibre can guide multiple rays of light or modes down the optical fibre. Graded Index Multimode Fibre is used to focus multiple modes at periodic locations along the length of fibre as shown in Figure 3.10.

Figure 3.10: Propagating of light through fibre (Sentinel DTS manual, 2006)

Optical fibre connecters

Optical fibre connectors are designed around a common principle. The optical fibre runs through a guide ferrule within the connector which provides a more rugged connection surface than just bare fibre cladding. The cladding of the optical fibre is precisely glued within the bore of the ferrule. The fibre and the guide ferrule are then polished together. The Sentinel DTS system uses an E2000 connector as shown in Figure 3.11; it features a latch system which ensures that the connector cannot become loose due to vibration.

Figure 3.11: E2000 Connector (Sentinel DTS manual, 2006)

Specifications of the optical fibre cable used

This optical fibre was brought from General Cable Company, is used in outdoor communication circuits. This cable is water blocked and termite resistant, and suitable for duct, direct buried or aerial lashed installation.

General Characteristics of optical fibre used are shown in Table 3.2 and Figure 3.12.

Table 3.2: General characteristics (General Cables, 2008)

Standard

AS/ACIF S008

Number of Optical fibres

Up to 12 colour coded Fibres

Loose Tube

Polybutylene terephthalate

Sheath

Linear Low Density Polyethylene, Black

Jacket

Nylon 12, Blue.

Figure 3.12: Specification of optical fibre cable (General Cables, 2008)

Fibre cable specification

This optical fibre is OM3 type with Maximum Attenuation 2.5 dB/km for 850 nm with a Minimum LED Bandwidth of 1500 MHz km.

It consist of 12 fibre of 8.5 mm diameter weighing approximately 60 kg/km, the other mechanical properties are:

Crushing Resistance: 2.0 N/100mm

Maximum Tensile Load: 1500 N

Minimum Bending Radius is 170 mm during installation and 85 mm when installed.

The following are the temperature requirements for the safe operation of the fibre cable:

Storage: -200 C to +700 C ; Operation: -100 C to +600 C ; Installation: -100 C to +700 C

A DTS measuring system from Sentinel Sensornet of Long range (10 km) and Loose tub, Multimode 50/125, Twelve colour coded optical fibre core cable is used in this study to carry out different test in University of Queensland Experimental Mine (UQEM).

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