SENTOR 101 Introduction

SENTOR 101 monitors the back-scattered signal using: the photon counting method to record intensity and the arrival time to identify position Distributed Temperature Sensors work on very similar principles to RADAR and SONAR. In RADAR, a radio frequency electromagnetic probing pulse is launched into the air. In SONAR sound waves are used. When the probing pulse encounters a reflective object, some energy is returned to a receiver. The period between the launch of the probing pulse and the detection of the reflected signal allow the objects distance to be deduced. The amplitude of the returned signal is partially due to the distance the object is away from the source and detector and partially due to reflective properties of the object, including its size.

In distributed temperature sensors such as the SENTOR 101 the probing pulse is a very short pulse of laser light (10 ns). This pulse is launched into an optical fibre that is in contact with the object to be measured. After leaving the laser the light travels through an optical coupler and onto the sensing fibre. As the light pulse travels along the sensing fibre, a small fraction of the incident pulse is absorbed by the fibre atoms and the pulse intensity is attenuated. (One way to envisage this attenuation is to imagine the fibre as a glass window that can be up to 4 km thick.) The attenuation process involves the probing pulse relatively weakly interacting with glass fibre molecules. The interaction causes light energy to be scattered from the molecules - some of this scattered light travelling back towards the launch end of the fibre as Raleigh and Raman components. Thus, while most light energy is transmitted in the forward direction along the fibre, a very small fraction of it is sent backwards, towards the coupler where it is diverted to a light detector, the photomultiplier tube PMT. It is the intensity of the returned light together with its time of arrival that allows the system to calculate the temperature profile along the sensing fibre.

Performance

SENTOR 101 has several measurement dimensions so that temperature resolution involves interplay between:

• Space resolution
• Sensing fibre length
• Acquisition time
• Temperature
  

Fibres

Fibres have predominantly been manufactured for the telecommunications industry. Recently, the manufacture of special sensing fibre has gathered momentum. Fibres are now being produced with coatings that enable them to function at extreme temperatures and pressures.

The SENTOR uses telecommunications grade 62.5/125/250 or 50/125/250 GRIN multimode fibres. The 62.5 and 50 refer to the glass core diameter in microns and the 125 refers to the outer glass diameter in microns. This glass component of the fibre is extremely fragile and is protected by and acrylate coating of 250 micron diameter.

Our standard sensing fibre for applications where a high degree of protection is afforded by external systems is 50/125/250/900 type fibre where the 900 refers to a further protective coating.

Where natural protection is not high, such as in underground power cable monitoring applications, Tyree Optech recommends a system in which the basic fibre or fibres are heavily protected. The figure below shows a cross-section of our standard underground fibre system. A bundle of four fibres is enclosed in a silicon jelly filled tube that is protected by a fibrous poly-aramid strength member and finally encased in a thick layer of Polyethylene.