Distributed Sensors

13.1 Introduction Distributed optical fiber sensors are those sensors which monitor

a single measurand at a large number of locations or continuously over

the path of the optical fiber. The measurand can be a physical or a

chemical quantity. The distributed optical fiber sensor generally consists

of a common light source, optical fiber to carry light to the sensing

probes and a return fiber to carry modulated light to a photodetector.

These sensors can utilize any kind of light modulations described earlier.

The advantages associated with distributed sensing are reduced number

of components and hence the cost, reduced weight and cabling and

electrical passivity. The applications include the measurements of

temperature, pressure, strain, current and chemical concentration. If

the measurand is monitored continuously along the fiber length then

the sensor is called intrinsic distributed sensor. If it is measured at a finite

number of locations then the sensor is called quasi-distributed sensor.

The quasi distributed sensor has a number of advantages over intrinsic

distributed sensor.In quasi-distributed sensor, spatial resolution is not as

important as in intrinsic distributed sensor provided that individual

sensors can be clearly resolved. Further, in quasi-distributed sensing, the

sensor elements can be tailored in terms of responsivity and range to

meet the specific needs of the application. In this chapter we shall

describe various kinds of distributed sensing approaches and few optical

fiber distributed sensors.

13.3 Intrinsic distributed temperature sensor As defined earlier, in intrinsic distributed sensors the measurand is

determined continuously along the fiber path. In this section we shall

describe intrinsic distributed sensors for the measurement of temperature.

Optical fiber distributed temperature sensors have a wide range of

applications in power supply industry, oil and gas industries, process

industries, buildings, tunnels and transportations as mentioned below:

A. Power supply industry

1. Fiber optic distributed temperature sensor can monitor the

temperature of the power transformers windings. Thus hot spots,

if present, in the windings can be detected and the transformer can

be saved from burning.

2. Distributed optical fiber temperature sensor can be used to monitor

the temperature of cooling pipes of a large power generators. If the

cooling pipes are blocked the temperature will increase and it may

damage the generator.

3. The capacity of high voltage transmission lines depends on their

temperature. If a sensing fiber is installed inside one of the

conductors of the transmission lines their temperature can be

monitored.

4. In thermal power stations high pressure steam pipes are used. The

distributed optical fiber temperature sensor can be used to detect

leakage from these pipes.

For all these applications, the fiber must have correct dielectric

properties, be nonconductive, non-tracking, non-contaminating and

avoid the creation of locally enhanced electric fields. Suitable coatings

are required to achieve these objectives and at the same time protect

the fiber and withstand the temperature and the chemical attacks they

will be subjected to (e.g. transformer oil)

B. Petrochemicals (Oil and gas industries)

In petrochemical industries distributed temperature sensor can be

used to monitor pipeline temperature. It can also be used to detect gas

leakage because gas leakage causes change in temperature.

C. Process industry

The distributed optical fiber temperature sensor can be used to

monitor long thermal curing or, drying process industry. It can also

monitor overheating in machines ( e.g. in bearings) before damage to

the equipment has occurred.

D. Buildings, tunnels, transportation

Fires inside the buildings, tunnels ships and planes occur to fault in

cables. A distributed optical fiber temperature sensor installed in these

can detect the outbreak of a fire very quickly wherever it occurs.

E. Construction

Distributed temperature sensors can be used as a temperature

measuring device or as a fire alarm in heating and air conditioning

units in buildings. They can also monitor the exothermic curing of

concrete in dams. This is important because heat weakens the structure.

For this the fiber is buried within the structure. For this fiber is buried

within the structure.