This guide is intended to help the new user planning for a fiber-optic measurement system. See Measurement System Definitions for more basic information to help with the planning of a fiber optic sensing system.
Fiber optic sensing technology offers a number of advantages for measurement of critical physical process parameters such as static pressure, dynamic pressure, temperature, acceleration, position, load, and density.
Davidson fiber optic transducers are tolerant to high temperatures, intrinsically safe, and immune to electromagnetic interference.
Because the sensors can tolerate up to 1000°F, they can be placed into the process flow and make direct measurements providing the highest accuracy and fidelity.
Davidson has designed its systems to be safe for use in industrial applications. The standard Davidson sensing system is eye-safe and intrinsically-safe.
The Davidson instrumentation uses visible and near IR LED light source which operate at such a low optical power that the amount of light energy and the wavelengths of light transmitted into the optical fiber are not sufficient to damage the eye.
Furthermore, the power transmitted into the optical fiber for many systems is not sufficient to ignite vapors in hazardous locations. 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. How Does a Fiber Optic Sensing System Measure Pressure?
See the illustration of the signal processing that occurs within a Davidson fiber optic measurement system.
The illustration shows the light source and the characteristic of the light which is generally uniform across the spectrum.
Davidson’s fiber optic transducer contains an optical interferometer (sensor) at the tip of the transducer. Because the Davidson transducers are very small and designed to “take the heat”, the transducers can be positioned in high temperature locations where other sensors would quickly fail.
In the illustration, the sensor is shown as two red parallel bars. The optical sensor is actually comprised of two parallel and partially reflective surfaces. The first partially reflective surface is a fixed optical window. The second partially reflective surface is an integral part of the pressure diaphragm.
The pressure diaphragm moves in response to changes in process pressure. The distance between the fixed optical reference and the surface of the diaphragm at any instant in time is directly related to the applied pressure at that instant in time.
The characteristics of the light that is transmitted to the sensor are changed at the sensor according to the distance between the fixed reference and the sensing diaphragm. In the illustration, this modulated light is illustrated by the sinusoidal waveform. This modulation of the spectral characteristics of the reflected light is the key to measuring the process pressure.
The modulated light is reflected from the sensor and transmitted back to the electronics where the system demodulates the optical signal and precisely measures the length of the gap between a fixed optical reference and the sensing diaphragm.
This demodulation is performed instantaneously and optically using a second optical interferometer (wedge shaped optical component in the illustration). The cross-correlator converts the sinusoidal waveform into a measurement of the sensor gap at up to 80,000 times per second and outputs a voltage signal proportional to pressure as a function of time.
Schematic of the a Fiber Optic Measurement System Signal Processing Steps
4. What are the Major Components of a CDMS?
There are three basic components in the Davidson fiber optic measurement systems:
- Transducers (Sensors),
- and Signal Conditioning Electronics.
Transducers contain the sensors and are installed in the process line.
Cables and junction boxes are used to transmit the light from the signal conditioner to the transducers and back to the signal conditioner.
Signal conditioners contain light sources that transmit light through the cables to the transducer and back again where the modulated reflected light signal is detected and converted into an electronic signal that is transmitted to a data acquisition system for display, process control, and archiving.
Davidson transducers, cables, and signal conditioners are generally not interchangeable with those manufactured by others.
5. Getting Started
To get started with the task of configuring a fiber optic sensing system, some basic questions need to be answered.
5.1.1 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.
5.1.2 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?
5.1.3 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.
5.1.4 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. The Davidson fiber optic transducers can tolerate temperatures to 1000°F and can be located safely in explosion hazardous areas.
This allows the system designer 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.
Davidson can design transducers with a variety of external configurations. The sensors are almost identical. The transducer bodies can be configured quite differently.
Note the following possibilities:
- As small as 0.125 inch in diameter
- Flexible transducers for installation in difficult access locations
- Male or female NPT fittings or flange connections
5.1.5 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.
5.1.6 What other environmental factors need to be considered?
Will the transducer be subjected to high thermal gradients, mechanical strain, vibration, or severe cold?
5.2 Signal Conditioners:
5.2.1 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.
5.2.2 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.
5.2.3 What is the ideal output signal?
Davidson offers standard options including:
- 0 to 5 VDC
- RS-485 Modbus
5.2.4 What power is available?
Davidson systems work with either 110/220VAC or 24VDC.
5.2.5 Where will the signal conditioners be located?
Davidson signal conditioners are best located in a control room environment and configured as a 19" rackmount chassis 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 these explosion-proof enclosures are NEMA certified, the optics are intrinsically-safe and there is not functional need for the electronics to be packaged as such.
5.3 Cables and Junction Boxes
The number, location, and type of transducers and signal conditioners need to be specified to service the application. Then, the circuit needs to be defined including the number and length of cables and the number, size, and location of 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:
5.3.1 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.
5.3.2 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.
5.3.3 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 Cable Standard 2020.
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 1000°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 connections.