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  Guide to Configuring a Fiber Optic Sensing System


 

Guide to Configuring a Fiber Optic Sensing System  (PDF)

This guide is intended to help the new user with planning for a DavidsonSensors™ fiber optic sensing system.  See Fiber Optic Sensing Basics and Fiber Optic Sensing System Definitions for more basic information to help with the planning of a fiber optic sensing system. 

1.  Introduction

Fiber optic sensing technology offers a number of advantages for measurement in harsh industrial environments.   Fiber optic transducers are tolerant to high temperatures, intrinsically safe, and immune to electromagnetic interference.  Since many fiber optic transducers can be multiplexed with a single signal conditioner, significant cost savings can be achieved.   To realize the full potential of this technology, the sensing system must be configured to balance cost, accuracy, update rate, frequency response, transmission distance, etc. 

2.  Safety

Davidson has designed its systems for industrial applications.  The systems are eye-safe and intrinsically-safe.  DavidsonSensors™ use broadband white light from tungsten lamps and narrow-band LEDs as the light sources.  The amount of light energy transmitted into an optical fiber is not sufficient to cause damage to the eye and is not sufficient for ignition.  The maximum energy transmitted in a fiber is below the standards set by ANSI/ISA-TR12.21.01-2004, Use of Fiber Optic Systems in Class I Hazardous (Classified) Locations. 

3.  Components of a Fiber Optic Sensing System

There are three basic components in a fiber optic sensing system – Transducers, Cables, and Signal Conditioners.  Fiber optic transducers are passive devices that require a fiber optic signal conditioner to convert the light signal into an electronic signal in the appropriate engineering units.  Cables are used to transmit the light from the signal conditioner to the transducer.  Signal conditioners are devices that transmit light to the transducer and convert the reflected light signal into an electronic signal that is transmitted to a control system. 

It is important to understand that fiber optic transducers must be designed to interface with specific fiber optic signal conditioners.  At the present time, transducers and signal conditioners from one manufacturer are not interchangeable with those manufactured by others.  Fortunately, fiber optic cable is manufactured to standards that enable interchangeability between all fiber optic cable suppliers.

 

Schematic of Fiber Optic Sensing System

4.   Getting Started

To get started with the task of configuring a fiber optic sensing system, some basic questions need to be answered.

4.1    Transducers:   

4.1.1    What do you need to measure, i.e. temperature, pressure, level, flow, density, acceleration?

            Temperature sensors can be inserted into pressure transducers to make two measurements with a single point of penetration.

4.1.2    What accuracy is required?

            Since all transducers have some thermal sensitivity, the highest accuracy is obtained when temperature correction is applied.  This is especially true when the temperature range is uncertain or for transducers subjected to very high temperatures.  Review your application and the product specifications carefully to determine if temperature correction is warranted for your application.  Is the process stable or cyclic? 

4.1.3    How many sensors of each type need to be included in the design of your sensing system?

            The standard multiplexing package is an eight-channel signal conditioner but other options are available.  Multiplexing eight channels is a good compromise between cost and update rate.  For measurements where redundancy is critical, a dedicated signal conditioner may be a better trade-off.

4.1.4    What is the ideal physical interface and location for the transducers?

            Fiber optic transducers can handle higher temperatures and more corrosive environments than electronic transducers.  Many of Davidson fiber optic transducers can tolerate temperatures to 1000°F and can be located safely in explosion hazardous areas.  This allows you to locate the transducers directly in very harsh operating environments and eliminates the need for purging systems, capillary tubes, and impulse lines, and all of the associated weatherization issues.  Huge cost savings can be accrued through the elimination of such systems.   Further, Davidson can design transducers with a variety of external packaging.  The sensors are identical, only the transducer body is different.  Note the following possibilities:

  • As small as 0.100 inch in diameter
  • Flexible transducers for installation in difficult access locations
  • Male or female NPT fittings or flange connections

4.1.5    What is the process media?

           Davidson transducers can operate in a verity of process media. Unless otherwise specified, Davidson transducers use Inconel-718 for wetted parts. If your process media is not tolerant to Inconel-718, call Davidson application engineers to discuss other available materials.

4.1.6    What other environmental factors need to be considered?

            Will the transducer be subjected to high thermal gradients, mechanical strain, vibration, or severe cold?

4.2    Signal Conditioners:

4.2.1    Does the application require an absolute or a dynamic measurement?

            Davidson offers two families of signal conditioners, one for absolute high-resolution measurements and another for dynamic measurements requiring high frequency response.

4.2.2    What is the required update rate/frequency response?

            Absolute systems offer greater accuracy with lower frequency response (update rate).  Absolute systems can resolve better than 0.01% of full scale and provide an updated output signal several times per second.  Dynamic systems offer reduced accuracy but much higher frequency response than absolute systems.  Dynamic systems can resolve better than 0.5% of full scale and provide frequency response exceeding 5kHz.

4.2.3    What is the ideal output signal?

            Davidson offers standard options including:

  • 0 to 5 VDC
  • RS-485 Modbus

4.2.4    What power is available?

            Davidson systems work with either 110/220VAC or 24VDC.

4.2.5    Where will the signal conditioners be located?

            Davidson signal conditioners are best located in a control room environment and configured as 19" rackmount or in NEMA enclosures.  For those applications which require form-fit-function replacements of existing transducers and transmitters, Davidson offers a line of explosion-proof signal conditioners. Although not intrinsically safe, Davidson's NEMA enclosures can tolerate temperature and humidity extremes.

 5.   Cables and Junction Boxes – Once you have defined the number, location, and type of transducers and signal conditioners, you need to complete the optical circuit with a cable system consisting of cables and junction boxes. It is good work practice to create a schematic of the fiber optic circuit when designing a fiber optic sensing system. We'll start with a few definitions to help with the planning process: 

5.1    Tactical Cables – The optical cables that connect a transducer to a junction box or signal conditioner are called tactical cables.  These tactical cables can be configured with or without stainless steel armor and are cut to length and terminated at the factory. 

5.2    Home Run Cables – The optical cables that run from junction boxes in the field to the signal conditioners in an environmentally controlled area consists of many optical fibers and is called a home run cable.  The home run cables can be configured with or without stainless steel armor and are cut to length and terminated at the factory.    

5.3    Junction Boxes – Junction boxes are typically NEMA style enclosures that have an array of bulkhead connectors for making fiber optic connections between the tactical cables and the home run cables.

6.   Details of the Cable System

6.1    Selection of Fiber Optic Cable – Although fiber optic cables from different manufacturers may be interchangeable, the specifications for the optical fiber must match those used in the transducers and signal conditioners.  If the optical fiber specifications do not match, severe degradation in the system performance may occur.  For more detail on this subject, see Davidson's Fiber Optic Cable and Transmission Standard.

6.2    Cable Temperature Rating – Davidson recommends standard commercial cable rated for 185°F be used when the cable is exposed to operating temperatures ranging from –40°F to 125°F.   Temperature tolerant cable rated for 550°F should be used for higher temperature applications.  Special cables can be manufactured for exposure to temperatures up to 1200°F.

6.3    Mechanical Protection of the Cable – Davidson recommends the use of stainless steel armor for cables that may be subject to mechanical damage. 

6.4    Fiber Optic Cable Runs – When the length of the cable run is uncertain, it is best to order a cable long enough to assure a good connection in the field. For optimal system performance in process control applications, the total transmission distance (length of the cable run) from signal conditioner to transducer should be limited to 1000 feet although the system can work at ranges greater than 1000 feet with some degradation of signal quality.  For more detail on this subject, see Davidson's Fiber Optic Cable and Transmission Standard

6.5    Multiplexing of Transducers – Davidson's discrete fiber optic sensors require a dedicated optical fiber for each sensor. 

6.6    Location of Terminations – For optimal system performance, it is best to minimize the number of terminations in the fiber optic circuit and to use angle polished connectors (APC).  Junction boxes should be located in areas convenient for technicians to make the necessary conections. 

 

7.   Generating the Bill of Materials 

When the design of the fiber optic system is complete and the schematic of the system has been generated, a preliminary bill of  materials should be generated for the project. The preliminary bill of materials should provide a listing of the item number, component, location, quantity for each type of transducer, cable, junction box, and signal conditioner.   

 Sample Preliminary Bill of Materials

Component

Item

Qnty

Location

Cable Length

Transducers

 

 

 

 

Absolute 2000 psi

001

2

ECM120

 

Absolute 1000 psi

002

2

ECM100

 

Gage 3000 psi

003

1

ECM060

 

Gage 1000 psi

004

1

NHM118

 

Gage 500 psi

005

1

NHM106

 

Gage 100 psi

006

1

ECM112

 

 

 

 

 

 

Signal Conditioner

 

 

 

 

19” High Resolution

007

1

Control Room

N/A

 

 

 

 

 

Junction Boxes

 

 

 

 

16 Channel

008

1

NHM

N/A

2 Channel

009

8

ECM/NHM

N/A

 

 

 

 

 

Home Run Cable

 

 

 

 

18 Fiber

010

1

 

200’

 

 

 

 

 

Tactical Cables

 

 

 

 

Short

011

2

 

50’

Intermediate

012

4

 

100’

Long

013

2

 

200’

 

 

 
 

DAVIDSON INSTRUMENTS - THE WOODLANDS, TEXAS 281-362-4900