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02 March 2001

Seeing the light

by K Dutton, K Evans, R Green, Sallehuddin Ibrahim, Abdul Rahim

Tomographic imaging measures solids and particles in linear fashion.

Believe it. Lensed optical fiber sensors can monitor flowing materials, and their measurements are highly linear when measuring solids flow rates.

Designing sensors with very high bandwidth is easier when using optical fibers. This, in turn, permits the measurement of high-speed flowing particles or droplets. The light extinction method operates whereby the detector is aligned with the light source.

The method measures the particle shadow size as it crosses the light path. It can measure particles or droplets that are 100 millimeters or larger. Optical sensors are viable where the conveyed component ratio is 10% or less of the flow volume.

Radiation without nuking

An optical fiber sensor is a hard field sensor where the field distribution is independent of the medium between the source and the detector. One can assume that the material in the flow varies intensity of the received signal.

Using conventional (nonradiation) sensing methods, the operational environment may affect the transducer's performance and result in an incorrect measurement. In the case of optical-fiber-based sensor systems, these effects are reduced because fibers have well-documented stability and immunity to external interference.

The utilization of optical fiber sensors also enables the electronic part of the flowmeter to be apart from the measurement head, and thus problems of electrical/flameproof safety do not arise.

The use of optical fibers also allows more sensors to be placed around the process than with other systems and so a higher resolution results. Optical fiber sensors provide a cross-sectional image of material distribution.

Such an image forms by the reconstruction of data obtained from an array of sensors. These sensors monitor the emitted radiation (light) for fluctuations caused by particles or droplets interfering with the beam when passing through it.

Collimating the radiated beam from a light source and passing it through a flow regime ensures that the intensity of radiation (light) detected on the opposite side relates to the distribution and absorption coefficients of the different phases in the path of the beam.

As different phases in the flow usually have different absorption coefficients, so the intensity of the transmitted radiation is associated with the proportion of different phases in the beam path and hence to the flow regime. Arranging several such beams can generate information about a flow field across an area, such as a cross-section diameter of pipe.

The main advantages of this optical-fiber-based system are that each light beam averages the flow information within its cross section, and the sensors are mounted outside the flow pipe so they do not invade the flow. This provides a noncontacting method of measurement.

The system's performance is independent of temperature, pressure, and viscosity of the fluid.

Interrogate a finite section

The lensed optical fiber system employs two orthogonal projections, with each consisting of 16 sensors. Each sensor interrogates a finite section of the measurement section and provides a view.

The fibers mount into holes machined into the measurement section in a nonintrusive manner. Their presence does not affect or interfere with flow. Optical energy projects into the measurement volume as a well-collimated beam from two powerful projectors.

The receiver fiber couples to a photodiode for measuring the received light level. The sensor operates in photoconductive mode and produces a voltage proportional to the intensity of light incident on it.

As an object passes through the light beam, the level of light energy falling on the receiver fiber is less. The diode output is also less. The conditioned voltage, when there is no flow and there is maximum light intensity at the receiver, results in a zero voltage reading.

With high particle flow rate, a maximum output of 5 volts is generated. The main effect is the particles' attenuating optical energy. They cut through the light path. The effects of diffraction are negligible.

Results obtained from the 32 measurements transfer into the computer, and a linear back-projection algorithm estimates the solids distribution and provides measurements.

This calculation uses sensitivity maps combined with the measured data from the sensors' readings to obtain an image of the particle concentration distribution inside the conduit. IT

Figures and Graphics

• 3-D data interpretation
• Fiber side view
• The aluminum block and polishing rotor
• The tomographic system

Sidebars

• Further reading
• Optical fiber preparation
• Silica sand testing zone
• Terminology
• Transmit light without divergence

Author Information

R. G. Green, K. Dutton, and K. Evans are lecturers at Sheffeld Hallam University in England at its School of Engineering. Sallehuddin Ibrahim and R. Abdul Rahim lecture at the Universiti Teknologi Malaysia and are on the electrical engineering faculty.