fibre-optic-linear-temperature-sensing.htm

 PR-615 Distributed Linear Heat Sensing
 

• Up to 8kms dual fibre connection
• Single or Double ended fibre connections
• Multiple Alarm levels / “zone”
• Broad operating temperature range
• Inherently safe in operation
• Low power consumption
• Operating Lifetime < 20 years
• ISO9001 Manufacturing Certification
• Conforms with appropriate International Standards

Introduction

Proline PR-615 fibre optic linear temperature sensing system provides a low maintenance means of monitoring, detecting and reporting the presence of a fire condition at any and all points along a multiple kilometres length of standard telecommunications grade dual fibre optic cable. Additional information outputs include the fire location, size and spread.
PR-615 system operation is NOT affected by adverse environments (dirty, dusty, damp, corrosive etc) or by electrically noisy (RFI , EMC) conditions.

Monitoring capabilities can be programmed for the following three (3) conditions:-
 

 
 
System Configuration            

 “Example” of PR-615 system configuration

The Distributed Temperature Sensing (DTS) unit which is used in conjunction with the specified metallic or non metallic fibre cable utilises Optical Time Domain Reflectometry (OTDR) technology as the basis for its operation. With connections between the DTS unit and fibre being single or double ended... the latter allowing continued detection with a break in the fibre.
Communication options available for integration with a customer central control system include:

• Relays
• TCP/IP

• Modbus 

Operation

A pulsed laser is coupled to an optical fibre through a directional coupler. Light is then back scattered through the fibre due to changes in density, composition, molecular and bulk vibrations.  Because the velocity of light propagation in the fibre is well known, the distance can be determined from the time associated with the returning back scattered light.
RAYLEIGH, BRILLOUIN, RAMAN peaks and bands are different spectral components within the back scattered light.

RAYLEIGH by density and composition fluctuations is the strongest of the three (3) spectral components and is not sensitive to temperature. It has the same wavelength as the primary laser pulse and may be used to identify breaks in the fibre.
 
BRILLOUIN back scattering components are caused by vibrations from the propagating light pulse. Because these peaks are so close to the original laser pulse it is difficult to separate these from the Rayleigh signal.

RAMAN back scattering is generated by thermally influenced molecular vibrations from the light pulse. Their intensity therefore depends on temperature.  RAMAN  itself has  two components that lay symmetric to the Rayleigh peak................STOKES (weakly related to temperature) and ANTI-STOKES (strongly related to temperature). Calculation of the ratio between the two levels of signal intensity allows an accurate temperature measurement to be obtained. Combining this with the velocity of light propagation in the fibre, the DTS unit provides temperature measurement incrementally along the total length of the fibre.

Operating Specifications

Installation and Application

 The PR-615 allows configuration of different alarm temperatures along sections (“zones”) of a single fibre optic linear temperature sensor cable installation. Compensating for differences in ambient temperatures that may be present within a risk area without loss of sensitivity.
PR-615 systems are equally as effective when  installed at the exact point of risk as when used for more area protection risks. Monitoring at the exact location of a potential overheat/ fire condition removes any requirement for favourable airflows to carry the heated air up to a roof or ceiling height. Care must always be taken during system design stage with installation location recommendations and should always consider the potential obstruction that any incorrectly located fibre optic sensor cable could present to site engineers who may have to regularly access the area/s protected by the linear detection system for routine maintenance, repair or replacement of plant /process machinery.           
                                    
For Protection of more “open” area risks ( road /rail /cable tunnels, car parks, bulk warehouses etc) the PR-615 provides superior response to point type devices as it monitors for temperature changes at ANY and ALL points along its length.  Mechanical flexibility also allows vertical runs of the sensor cable to be integrated within higher level horizontal runs – bringing the “continuous detector” closer to the origin of the temperature rise. Care should be taken to ensure sensor cable fixing locations do not expose the system to excessive levels of vibration and that any bend that may be introduced by an installation contractor in the cable is within manufacturers recommendations tolerances.  

      
 
                                                 
The above sketch represents fibre optic linear temperature sensor (MAXIMUM 8000metres length/s) fixed at 1m intervals to the underside of the tunnel roof using Proline PR-001 LSZH Fixings.                                  
 
PR-001 “T” clip    (Not to Scale)          
 

 
 
                   
 See Proline “Accessories” sheet for full details of sensor fixing range
 
DTS Installation location (end or mid tunnel for road tunnel application example given) will be dependant upon whether fibre configuration is single or double ended  and/or position of any tunnel control /security office in which it is to be installed.