Pipeline optical flow meter

The present invention provides an optical device for the measurement of flow rates of fluid through a pipe. The device broadly comprises a narrow frequency light source, an optical delivery system, a collector for light scattered from particles in the fluid, and a photo detector. In a preferred embodiment, the optical delivery system and the collector are contained within the pipe.

FIELD OF THE INVENTION
 This invention relates to an optical flow meter system for measuring the
 flow of fluid in a pipeline.
 BACKGROUND OF THE INVENTION
 One of the requirements for the successful operation of any pipeline is the
 capability to accurately measure flow rates at many locations within the
 system. A number of different flow meters are currently commercially
 available for this purpose, each having its own advantages and
 limitations. Existing meters can be classified into three main types,
 namely obstruction meters, kinematic meters and non-intrusive meters.
 Obstruction meters determine flow rate in an indirect fashion by
 introducing a physical obstruction directly into the flow and measuring
 the influence of the obstruction. For example the pressure drop across a
 flow restriction is often measured and correlated with the flow rate.
 Examples of this approach include orifice meters, venturi meters and
 critical flow nozzles [ref. Experimental Methods For Engineers, Fourth
 Edition, McGraw-Hill Book Company, J. P. Holman and W. J. Gajda, Jr.,
 Chapter Seven]. Another example of an obstruction meter is the vortex
 meter, in which the obstruction causes vortex shedding. The shedding
 frequency is determined by means of strain sensors, thermal sensors, or
 pressure sensors. The shedding frequency increases with flow rate.
 Obstruction meters extract energy from the flow and are therefore
 inherently inefficient because additional pumping capacity is required to
 overcome the induced pressure drop. The physical obstruction also prevents
 the use of pipeline-pigs for maintenance and diagnostics. Component wear
 can cause a shift in the discharge coefficient or shedding frequency, and
 therefore regular maintenance of these devices is required. Pressure or
 load transducers are normally mounted next to the obstruction, and
 therefore local power is required. For pipelines transporting flammable or
 explosive fluids, appropriate explosion-proof enclosures are required for
 these transducers. Finally, these meters have a limited turndown ratio
 owing to the nonlinear relationship between flow rate and pressure drop.
 Kinematic type meters determine the flow rate by directly sensing the
 actual velocity using a turbine blade assembly that rotates kinematically
 with the flow. The rotational speed of the turbine is measured using a
 frequency pickup and is empirically related to the flow rate using an
 experimentally determined coefficient. These meters provide an output that
 is approximately proportional to volumetric flow rate and substantially
 independent of density. The primary disadvantages of this class of meter
 are the presence of moving parts, the obstruction to flow, the need for
 calibration, the requirement of electrical power and the physical size.
 The third class of meter relies on non-intrusive methods to determine flow
 rate. The ultrasonic flow meter is the only meter in this category that
 has been commercially developed for use in high pressure natural gas
 pipelines. The operating principle is to compare the upstream and
 downstream times of flight of an acoustic pulse from one transducer to
 another, which are located near the inside surface of the pipe. The flow
 is unrestricted and therefore these devices do not produce any significant
 pressure drop. However, these devices require a relatively long
 installation length, are limited to larger pipe sizes, can suffer from
 acoustic noise and are sensitive to swirl in the flow.
 Each of the meters described above has deficiencies in one or more of the
 following areas:
 Size: The device should be small enough to permit installation in limited
 space.
 Low Maintenance: Moving parts should be avoided to reduce maintenance
 requirements.
 Power Supply: The device should not require electrical power at the meter.
 High Turn-Down: The device should provide accurate measurement of flow
 rates over a 50:1 turndown ratio.
 Optical flow measurement offers the potential to address all of the
 above-noted deficiencies of "prior art" meters.
 For example, flow rate may be determined by measuring the velocity of
 micron-sized particles suspended in a flow field. This is accomplished by
 determining the time-of-flight of these particles as they move between two
 discrete regions illuminated by laser light. This basic concept was proved
 by D. H. Thomson ["A Tracer Particle Fluid Velocity Meter Incorporating a
 Laser", Jour. of Sci. Inst. (J. Phys. E.) Series 2, Vol. 1, 929-932
 (1968)] using a large gas laser, Kosters prism, two convex lenses, an
 imaging lens and a photomultiplier.
 The time-of-flight concept has been applied to a device to measure the air
 speed of an aircraft (as described in U.S. Pat. Nos. ("USP") 4,887,213;
 5,046,840 and 5,313,263). Three pairs of laser sheets are projected into
 free air through a window located in the side of the aircraft fuselage.
 Small particles in the atmosphere passing through the laser sheets produce
 scattered light that is collected from each pair of laser sheets. This
 light is imaged onto photodiodes and the resulting signal is processed to
 determine the velocity vector. However, this prior device is designed for
 the aviation environment and cannot be used in pipeline applications.
 Optical techniques have also been employed by numerous investigators to
 make measurements in laboratory environments, particularly in wind-tunnels
 and turbo machinery, notably R. Scholdl ["A Laser-Two-Focus (L2F)
 Velocimeter for Automatic Flow Vector Measurement in Rotating Components
 of Turbomachines", Transaction of the ASME, Vol. 102, p 412, December,
 1980]. Additionally, UK Patent 2,295,670 describes such a configuration in
 which laser light from an argon ion laser is split by a Rochon prism, made
 parallel by a lens and then focused into two focused spots. Scattered
 light produced by particles passing through the two spots is imaged onto
 two photoelectric converters. Velocity is determined on the basis of the
 transit time of the particles passing between the two spots. U.S. Pat. No.
 4,125,778 claims a similar device except that the relative position of the
 two spots could be rotated using an optical component.
 A variation on the time of flight principle using laser diode arrays (i.e.
 multiple lasers in a monolithic device) was applied by M. Azzazy ["GRI
 Report 89/0201 Development of an Optical Volumetric Flow Meter" (1989)] to
 measure the velocity profile inside a high pressure natural gas pipe
 through a glass window. In this case, the image of the diode array
 produced a series of spots in space and the light scattered by small
 particles was collected and converted to an electrical signal. The
 frequency content of the signal and the spacing of the spots of light were
 then used to determine the flow velocity. Measurements were obtained at a
 series of locations by mechanically translating the measurement system
 which was located outside the pipe. The bulky size of the system, in
 combination with the large optical window, rendered it impractical for
 broad commercial applications.
 Improvements to this system (i.e. using laser diode arrays) are described
 in U.S. Pat. No. 5,701,172 issued Dec. 23, 1997 assigned to Gas Research
 institute. The patent also describes the system used in combination with a
 hologram and a window in a pipe to produce multiple measurement locations
 along one pipe diameter within a pipeline. All of the illustrations and
 examples of the patent are limited to the case where the optical source
 and lens are external to the pipeline.
 BRIEF DESCRIPTION OF THE FIGURES
 FIG. 1 is a diagram of a commercial orifice meter fitting.
 FIG. 2 is a schematic diagram of an optical meter according to this
 invention.
 FIG. 3 illustrates the steps in processing the data from the device.
 FIG. 4 is a schematic optical layout of a preferred embodiment of this
 invention with specific components.
 FIG. 5 is a schematic diagram of a preferred form of a collection lens.
 FIG. 6 is a diagram of a rigid plate which fits into an orifice meter and
 houses the optical system.
 FIG. 7 is a plot of experimental results of the device.

SUMMARY OF THE INVENTION
 The present invention provides an optical device for generating a signal
 which contains information which may be used to describe the fluid flow on
 the basis of the motion of suspended particles contained in a fluid
 flowing through a pipeline, said device comprising: a narrow frequency
 light source; one or more optical delivery systems; one or more collectors
 which collect light scattered by particles in the flow; and one or more
 photo detectors. In a preferred embodiment, the optical delivery system(s)
 and collector(s) are contained within the pipeline.
 That is, the device of the present invention generates signals based on the
 light scattered by particles flowing through a pipeline.
 The present invention further provides an optical device for generating
 information to determine the flow rate of a fluid within a pipe comprising
 in cooperating arrangement an orifice fitting having mounted therein a
 rigid plate. The rigid plate holds at least one optical delivery system
 providing at least two parallel beams of light and a collector receiving
 scattered light from particles in the fluid moving through the pipeline.
 The optical device of this invention contains two optical sub-systems,
 namely a "delivery" system and a "collection" system. The delivery system
 is designed to provide a parallel pair of light beams (preferably laser
 beams), separated by a known distance, through the center of a pipeline.
 The optical delivery system may comprise one or more of: a collimator, a
 light splitter and a focussing lens. The two beams are preferably
 perpendicular to the axis of the pipe, with one light (laser) beam located
 upstream of the other. The light beams are conditioned (focused) such that
 they are most intense at the location within the pipe that the measurement
 is to be made. Small particles suspended in the natural gas flow pass
 through the two beams, producing brief pulses of scattered light. These
 pulses of light are received by the second optical sub-system (the
 "collection" system) which collects scattered light from a small region of
 interest. The optical sub-systems are aligned such that the optical
 collection region is coincident with the most intense regions of the light
 beams, which defines a localized measurement volume.
 The optical flow meter is invested into the pipeline either via an existing
 meter fitting or a new fitting. Most preferably, this is achieved by
 encasing critical elements of the optical flow meter in a housing which is
 adapted to an existing meter fitting.
 Thus, the present invention enables the use of an optical system to measure
 the flow of fluid in a pipeline, especially the flow of natural gas in a
 high pressure pipeline.
 The present invention further provides a process for measuring the flow
 rate of a fluid having entrained suspended particles through a pipe
 comprising comparing the timing of light scattering events from at least
 one light sheet in said pipe to the light scattering events from at least
 one other light sheet in said pipe using one of the devices described
 above and comparing the events from both light sheets to generate a
 histogram (called a corrologram) which has a characteristic peak
 corresponding to the flow rate.
 The optical flow meter described herein is a robust device which may be
 used in a remote environment.
 DETAILED DESCRIPTION
 As used in this specification a narrow frequency light source means a
 system that provides essentially monochromatic light having a wavelength
 preferably within a range of 50 nanometers (nm), most preferably within a
 range of 10 nm.
 The measurement volume is the location or locations where the sheet of
 light or beam of light is focused within the interior of the pipeline to
 detect particles passing through such location(s); particles scatter light
 from the optical delivery system and scattered light is gathered by the
 collector.
 In accordance with the present invention, the input for and output from the
 optical delivery system and collector communicate externally from the
 pipeline preferably via an optical transmission path, most preferably an
 optical fiber(s). The optical fiber(s) may be continuous or they may
 include appropriate coupling devices such as those sold by AT&T under the
 trademark ST connectors. However other means for providing input and
 receiving output from the device of the present invention would occur to
 those skilled in the art.
 Preferred embodiments of the invention will now be described in detail with
 reference to the accompanying drawings.
 The flow meter of the present invention may be installed in the pipeline,
 for example between adjacent flanges. However, such an installation is not
 easily removed for maintenance or servicing. In a preferred embodiment of
 the present invention the device is installed in a removable cooperating
 plate and fitting such as an orifice plate carrier and fitting. FIG. 1
 shows a cross section of a typical commercial orifice fitting.
 The standard orifice meter in the natural gas industry such as that shown
 in FIG. 1 consists of a meter body 101 that permits the orifice plate 102
 to be inserted into or removed from a high pressure pipe 103. The orifice
 plate 102, which is a round steel plate with a hole in the center, is
 fitted with a thick rubber gasket 104 around its circumference. This
 gasket provides a seal when the plate is in use to ensure that all of the
 flow passes through the central hole 105. The gasket and orifice plate fit
 into a larger rectangular "plate carrier" 106. The plate carrier is
 inserted into the body of the meter 101 through a closable opening 107 and
 holds the orifice plate 102 in place. The housing of the meter can
 generally be of two types: those that allow the plate to be inserted and
 removed while the system is under pressure; and those that require that
 the pipe be depressurized before the orifice plate can be removed.
 FIG. 2 shows a preferred configuration of the optical flow meter. A narrow
 frequency light source 201 (preferably a laser) is located at some
 distance from the actual meter housing, and the energy emitted by the
 laser is transmitted to the meter via a single mode polarization
 maintaining optical fiber link 202 (i.e. an optical fiber). The optical
 fiber enters the body of the meter through a high-pressure fitting 203
 accommodating both the transmission fiber and multi-mode receiving optical
 fiber 209. The transmission optical fiber terminates in a collimator 204.
 The beam exiting the collimator then passes through a beam splitting prism
 205 to generate two beams, followed by a focusing lens 206 which produces
 parallel light beams 207 focused within the pipe typically at or adjacent
 to the centerline of the pipe for a single point measurement using two
 sheets of light. As shown in FIG. 2 the parallel light beams are focused,
 that is they narrow to a waist at or within the measurement volume (and
 they diverge on each side of the measurement volume). This results in an
 intensely illuminated region at the light beam waist preferably a high
 intensity sheet of light (but it could also be a spot of light). Note that
 for any velocity measurement at a specific location at least two closely
 spaced sheets of light are required. For measurements at more than one
 location the measurement volumes may be located at different positions
 within the cross-section of the pipe. Light scattered by small particles
 (micron and sub-micron sized particles) passing through the beam in the
 measurement volume, is collected by a collection lens 208 (which
 collection lens 208 is preferably a refractive doublet or
 diffractive/refractive doublet) that focuses the collected light onto the
 end of a receiving optical fiber 209. The receiving optical fiber 209
 transmits the scattered light pulses back to a photo detector 210
 (avalanche photodiode(s) or photo multiplier tube(s)) located near the
 laser. The electrical output from the photo-detector is analyzed in a
 signal processor unit 211 that correlates the optical pulses and
 determines flow rate at the measurement volume. It will be appreciated by
 those skilled in the art that the small particles referred to above are
 typically found in fluid flows (if an ultra clean fluid is being measured,
 it may be necessary to add such particles).
 Each particle that passes through the pair of light beams (or sheets) emits
 two pulses of scattered light, separated by an amount of time .DELTA.t. By
 measuring .DELTA.t, and knowing the physical spacing between the two beams
 (S), it is possible to determine the average velocity from the
 relationship U.sub.avg =S/.DELTA.t. Correlation techniques are used to
 analyze streams of pulses produced by numerous particles passing through
 the beams over a prescribed period of time. This technique eliminates the
 ambiguity that results from overlapping pulse pairs, plus it minimizes the
 influence of single pulse events which occur when a particle passes
 through only one of the two beams. One method of correlating high data
 rates is outlined in U.S. Pat. No. 4,887,213.
 FIG. 3 shows a preferred method of calculating the velocity of a fluid. The
 figure consists of three components: a plot of output signal versus time
 (a); the process of digitizing an individual pulse (output) (b); and a
 histogram (corrologram) of transit velocities (c). The relationship
 between these three aspects of the velocity calculation are described
 below.
 The signals, such as those shown in FIG. 3(a), are generated by the device,
 with a characteristic lag between events (detection of scattered light) on
 the upstream and downstream channels (e.g. photo detectors). Each of these
 pulses is digitized when a certain threshold 301 (e.g. exceeding the
 background noise) is exceeded as shown in (b). The time that the pulse
 occurred has to be identified for each pulse. The time of occurrence
 (temporal centroid) is determined, either by midpoint between the time
 when the signal exceeds the threshold 302 and when the signal falls below
 the threshold 303, or by a weighted average of the digitized pulse shape,
 as shown in FIG. 3(b). The time and amplitude of the pulses are stored for
 later reference. Each pulse that occurred on the downstream channel (e.g.
 305) is compared to previous pulses that occurred on the upstream channel
 306 (limited by a time which is equal to or greater than the transit time
 for the minimum detectable velocity) to generate a series of potential
 transit times .DELTA.t for each down stream pulse. Each transit time is
 converted into a possible transit velocity by dividing the beam spacing by
 the transit time. This series of potential transit velocities are
 accumulated and stored in a histogram called a corrologram, such as that
 shown in FIG. 3(c). The corrologram shows the number of possible transit
 velocities that lie in a narrow range called a "bin". The number of events
 in each bin is represented by the bars in FIG. 3(c). The average transit
 velocity can be determined by the average of the velocities in the most
 populated bin (and the appropriate number of neighboring bins).
 Schemes that weight the importance of the transit velocity based on the
 similarity of the amplitudes of the upstream and downstream pulses may be
 used to increase the accuracy. The premise in such schemes is that pairs
 of pulses caused by the same particle should be of similar amplitude, and
 that since the objective is to determine which pairs correspond to one
 particle passing through both sheets, the pairs with similar amplitude
 should be weighted more heavily.
 This method can be generalized to the case where both pulses are generated
 by the same detector. In this case all pulses are considered as possible
 upstream and downstream pulses (since the upstream and downstream
 information is indistinguishable), and the corrologram still shows a
 characteristic transit velocity.
 Broadly the present invention seeks to provide a process for determining
 the velocity of a fluid moving through a pipe comprising converting the
 electrical signals generated by a device according to the invention
 comprising:
 (a) digitizing each pulse of the detector output signal caused by collected
 light scattered from a particle passing through a light sheet exceeding a
 threshold above the ambient noise and determining its temporal centroid;
 (b) comparing the temporal centroid of each pulse that could have
 originated on the downstream sheet of light to the time of the pulses that
 could have occurred on the upstream beam in the recent past to obtain a
 possible transit time of a particle from one sheet of light to another;
 (c) converting each transit time to a transit velocity, by dividing the
 distance between the sheets by the transit time and recording the transit
 velocities in a corrologram;
 (d) identifying the peak in the corrologram generated over a finite time
 period by the large number of transit velocities clustered at a specific
 velocity;
 (e) averaging transit velocities within that cluster in the corrologram.
 The light source and the photodiode are external to the pipe; however a
 preferred embodiment of the invention requires that the optical lenses be
 located within the pipe. The advantage of placing the optical lenses
 inside the pipe is that for high pressure applications, windows are not
 required, and the optical system is confined to the inside of the pipe
 thereby reducing the safety issues associated with exposed laser light
 sources. The use of optical fiber also means that the electronics and
 processing system may be located many hundreds of meters from the actual
 measurement location. Additional advantages of the optical device of this
 invention include: (a) it does not introduce a pressure drop in the
 pipeline; (b) it may require less than five centimeters of pipe length for
 installation; and (c) it does not have moving parts that can wear over
 time. The device is fully compatible with a range of pipeline environments
 including high-pressure natural gas. Furthermore, no electrical supply is
 required at the meter location.
 The optical device described above may be constructed using a number of
 alternative components and each of these individual configurations may be
 installed in various mechanical housings. One preferred embodiment
 described below is tailored to be installed in a standard orifice plate
 carrier housing used in natural gas pipeline systems. This bodiment is
 particularly attractive to those users who have many orifice fittings
 already installed in the field.
 FIG. 4 shows the optical components used in a preferred design of a plate
 that replaces the plate carrier, and Table 1 provides a detailed
 description of each component.
 TABLE 1
 Optical Components in FIG. 4
 Item # Description Supplier Model/Part Number
 401 150 mW Laser (830 SDL SDL-5421-G1
 nm)
 402 Laser current driver Seastar LD 1000
 404 Laser/fiber coupler OZ Optics LDPC-02-830-5/125-
 P-40
 407 High pressure pass- PAVE PT-SS-150-FOSM-
 through fitting Technology Inc.
 408 FC Polarization- OZ Optics
 maintaining
 connector
 409 Collimator with vent OZ Optics LPC-04-830-5/125-P-
 hole in housing to 0.86-3.0GR-40-3-3A-
 equalize pressure 0.6-SP
 410 Wollaston prism Karl Lambrecht WQ6.35-05-V830 or
 (0.5.degree. or .75.degree.) Corp. WQ6.35-075-V830
 V coated for 830 nm
 411 Cylindrical Lens cut Melles Griot 01 LCP 001 or
 down to 6.35 mm 01 LCP 005
 sq.
 413 Cemented doublet NOVA designed,
 15 mm diameter built by
 Lumonics Optics
 414 Collection fibers Thor Labs FG-200-LCR
 (multimode) (3M product)
 NA = 0.22, low OH
 415 SMA connectors Thor Labs 10230A
 416 High pressure pass- PAVE PT-SS-150-FG200-
 through fitting Technology Inc.
 417 Silicone Avalanche EG & G C30657-010-QC-06
 Photo Diode (APD) Optoelectronics
 with transimpedance
 amplifier
 Preferred Optical Delivery System
 Light energy is provided by a 150 mW near-infrared diode laser 401 (such as
 that sold by SDL Inc. (San Jose, Calif.)), driven by a laser current
 supply (402) such as that sold by Seastar Optics Inc. (Sidney, BC) that is
 connected to diode laser 401 by an electrical cable 403. The laser diode
 is directly pigtailed to polarization-maintaining fiber (405) using laser
 to fiber coupler (404), such as that sold by OZ Optics Ltd. (Carp, ON).
 The fiber enters the high pressure housing (406) through a pressure
 fitting (407) such as those sold by PAVE Technology Co. Inc. (Dayton,
 Ohio). For convenience an FC polarization maintaining connector (408) is
 located on either side of the bulkhead such that the individual assemblies
 can be easily separated. The light energy exiting the fiber 405 is
 collimated into a uniform laser beam using a collimator 409 (which uses a
 0.25 pitch GRIN lens) such as those sold by Oz Optics Ltd. The collimator
 has a small vent hole to prevent damage to the optical fitting under
 pressure. The beam is then split into two beams using a Wollaston prism
 410 such as those sold by Karl Lambrecht Corp. (Chicago Ill.). If required
 by space restrictions, the two beams may reflect off a planar mirror
 towards a cylindrical lens 411, otherwise the two beams project directly
 to the cylindrical lens 411. The cylindrical lens refracts the beams such
 that they become parallel. The cylindrical lens also focuses each of the
 beams such that two high intensity laser sheets are produced in the
 measurement volume 412. This may be at a distance of approximately 50 mm
 from the lens.
 Preferred Optical Collection System
 Scattered light, caused by particles passing through the waist regions of
 the two sheets, is collected and focused onto the ends of two 200 micron
 multi-mode optical fibers 414 by a specialized collection lens 413. This
 lens is preferably designed to prevent variations in refractive index from
 altering the focal length of the collection optics. (A detailed
 description of a novel collection lens is provided in the next section.)
 The multi-mode optical fibers pass through a second pressure fitting 416
 as described above with SMA 905 connectors 415 on either side for easy
 removal. The collected light energy is transmitted by the multi-mode
 fibers 414 to a pair of avalanche photodiodes with built-in transimpedance
 amplifiers 417 such as those manufactured by EG&G Canada Ltd.,
 Optoelectronics Division. The voltage outputs from the photodiodes are
 digitized and processed to determine particle velocity.
 Lens Operation in High Pressure Environments
 Typical imaging systems use a lens which refracts light passing through it
 to form an image of the object at the desired location. Refraction occurs
 when light rays pass from the external medium (usually air) to the lens
 material (usually glass) or vice versa. Refraction, or the amount of
 bending of the light rays depends on the index of refraction of the two
 media in contact, and is governed by Snell's Law. Snell's Law states that
 n.sub.1 sin .theta..sub.1 =n.sub.2 sin .theta..sub.2, where n is the index
 of refraction of the medium, and .theta. is the angle that the ray makes
 with a normal to the surface, and the indices 1 and 2 refer to the two
 media in contact.
 If the index of refraction of the surrounding medium can change
 significantly, then the performance of the optical system will depend on
 the external medium. For instance, the surrounding medium could be air at
 atmospheric pressure, a high pressure gas, or a liquid.
 To successfully image particles inside a high pressure natural gas
 environment such as a pipeline, the optical system must work correctly at
 the full range of pipeline pressures without adjustment. For practical
 purposes the system should also work at atmospheric conditions to
 facilitate the initial setup and testing. Any mechanical adjustment of
 optical alignment to compensate for the changes in refractive index of the
 gas in the pipe would normally be very difficult to perform inside the
 pressure-containing vessel and would have to be done continuously to
 account for changes in gas pressure and temperature and composition.
 Collection Lens
 The preferred collection lens images a particle to an aperture
 independently of the index of refraction of the surrounding medium. A
 light detector is located behind the aperture. The collection lens should
 also meet the following constraints. The object and image should be small
 relative to the lens. The object and image should be near the optical axis
 of the lens. The object (e.g. the particulate in the measurement volume)
 and its image should be at specifically defined locations. To achieve lens
 performance that is independent of the index of refraction of the
 surrounding medium, the position of the object is restricted. Preferably
 the wavelength of light is fixed.
 This performance may be achieved in a lens having concave first and last
 surfaces which are defined by spheres centered at the object and image
 locations respectively. All refraction of the light as it passes from the
 object to the image occurs internal to the lens. The lens has the
 appropriate light refraction to be consistent with the object and image
 location. If the above criteria are met the lens will ensure that all rays
 from the object enter the first surface at a perpendicular and leave the
 last surface at a perpendicular, hence there will be no refraction at the
 surface in contact with the surrounding medium. The light from the image
 is directed to the detector or an optical fiber feeding the detector.
 The internal refraction of light can be achieved by a number of methods
 including using: a material with a radial gradient in the index of
 refraction; a sealed air or gas space between the elements; a sealed
 liquid between the elements; a cemented doublet with two materials of
 differing index of refraction; a cemented lens of more than two elements;
 or a combination of the above.
 The preferred embodiment of the collection lens consists of a cemented
 doublet with two different materials with a large difference in refractive
 index and high transmission at the wavelength(s) of light of interest.
 FIG. 5 shows the preferred lens 501 comprising a low index of refraction
 material 502 and a high index of refraction material 503. The high and low
 index of refraction materials are cemented together over a continuous
 surface 505 which may or may not be spherical.
 The surface of the low index of refraction material facing the object 504
 (e.g. the particle scattering light) has a radius of curvature so that
 light scattered by particles strikes the surface facing the particles
 normally (at right angles). The external surface of the high index of
 refraction material 503 faces the detector 506 (or optical fiber leading
 to the detector) and has a radius of curvature so that point 506 is at the
 center of a sphere defined by the radius of curvature.
 The preferred collector lens eliminates sensitivities to the index of
 refraction of the surrounding fluid which will vary with temperature,
 pressure, and composition of the fluid.
 There are computer programs available which will perform the calculations
 to define the curvature of the doublet internal surfaces 505 based on the
 indices of refraction of each component. One such computer program is
 available under the trademark ZEMAX, sold by Focus Software of Tucson
 Ariz.
 In a further embodiment of the present invention the cylindrical lens in
 the delivery system is made insensitive to the index of refraction of the
 surrounding fluid by use of a cylindrical doublet using the principles
 described above. That is, the light strikes at normal incidence
 (90.degree.) at any lens surface where the surrounding fluid is in contact
 with the lens.
 Mechanical Design
 In a preferred embodiment, the optical device of the present invention is
 constructed such that it is mounted within a rigid plate and installed
 within a standard orifice fitting. Such a plate is illustrated in FIG. 6
 showing the location of the principal components. The plate carrier and
 orifice plate is replaced by a single plate 601 with a central hole 602
 that preferably matches the inside diameter of the pipe. The optics of
 this invention are mounted in the plate 601 itself and the plate would be
 raised and lowered in the same manner as a standard plate carrier. The
 plate therefore provides a rigid, pre-aligned assembly that can be easily
 inserted into the orifice fitting. The optical fiber which delivers light
 from the light source fits in a slot 603 in the plate. The fiber
 terminates at the collimator 604 which is held in a slot 606 by supports
 605 which are designed to be adjustably positioned in the slot 606. The
 light leaving the collimator 604 strikes the Wollaston prism 607 and is
 split into two beams. The beams of light reflect off a mirror 608 and are
 focussed and made parallel by a cylindrical lens 609 which is held in
 place by adjustable supports 605. The beams are focussed to a waist at the
 center of the pipe where the (small) measurement volume 610 is located.
 Light scattered by particles passing through the measurement volume 610 is
 collected by a refractive doublet 611 which is insensitive to the index of
 refraction of the surrounding fluid. The light is focussed to an image
 point 612 and enters one of two optical fibers leading to the detectors.
 One fiber receives the image of the upstream sheet of light and the other
 fiber receives the image of the downstream sheet of light.
 The angle between the optical delivery system and the collector may be any
 angle, including 180.degree., or direct backscatter. However, a
 particularly preferred angle is in the range of 5.degree. to 25.degree.
 from forward scatter. The orifice fitting has several threaded holes in
 the body for pressure measurement and optional crank handle locations.
 These holes provide locations for the high pressure fiber optic
 pass-through fittings to be installed, thereby providing the necessary
 optical communication between the internal and external components.
 Installation of the optical system within the orifice fitting has the
 advantages of providing highly rangeable measurement with virtually zero
 pressure drop, without the expense and time of having to modify the
 existing piping.
 Flow Measurement Based on Velocity Data
 When the velocity distribution in the pipe is well conditioned (for
 example, in fully developed flow, or downstream of a flow conditioner), a
 single point centerline measurement is generally sufficient to achieve an
 accurate measurement of volumetric flow rate. In this case the flow rate
 is determined by the product of the centerline velocity and a coefficient
 which ranges from between about 0.5 for fully laminar flow and about 0.86
 for fully turbulent flow. The value of the coefficient is a function of
 the pipe Reynolds number and pipe roughness and can be determined from an
 empirical expression [ref. White, Frank M., "Fluid Mechanics", 2nd ed.
 McGraw Hill 1986, p. 310]. When a flow conditioner is located upstream of
 the measurement system a geometry specific correlation must be used to
 determine the average velocity from the measured centerline value.
 When the approaching flow is not fully developed, or is poorly conditioned,
 it is impossible to obtain an accurate measurement of volumetric flow rate
 on the basis of the centerline velocity alone. If measurements are made at
 sufficient locations within the pipe however, an accurate estimate of the
 flow rate can be made. Techniques to select the most appropriate
 measurement locations are outlined by F. C. Kinghom and A. McHugh "An
 International Comparison of Integration Techniques for Traverse Methods in
 Flow Measurement", La Houille Blanche, No. 1, 1977 and S. Frank, C.
 Heilmann and H. E. Siekmann, "Point-velocity Methods for Flow-Rate
 Measurements in Asymmetric Pipe Flows", Flow Meas. Instrum., Vol. 7, No.
 34, pp. 201-209, 1996. For example, using a total of five data points it
 is possible to reduce the error for a moderately ill-conditioned profile
 to less than 1.0%-2%, and for very ill-conditioned profiles the errors are
 generally less than 4%.
 In a preferred embodiment of the invention the total flow rate through the
 pipe is determined using measurements at five points located in the pipe
 cross-section. Most preferably, these five points are located, according
 to the locations specified by Frank, Heilmann and Siekmann. One point is
 located at the center of the pipe, and the other four points are located a
 distance of 0.762R from the center, where R is the radius of the pipe.
 Further, these four points are spaced equally around the circumference of
 a circle with radius 0.762R. These locations are designated subscripts: 0
 for the center point, and 1, 2, 3 and 4 for the remaining four points. A
 ratio a.sub.u is determined according to the following formula:
 ##EQU1##
 where v is the velocity at each location designated by the subscript. The
 value of a.sub.u is used to select a center-point correction factor
 a.sub.v from the following table:

Criteria Center-point factor, a.sub.v
 a.sub.u .gtoreq. 0.86 0.8941
 0.86 &gt; a.sub.u .gtoreq. 0.83 0.8526
 0.83 &gt; a.sub.u .gtoreq. 0.80 0.8167
 0.80 &gt; a.sub.u 0.7575
 The average flow rate U (i.e. the volumetric flow rate divided by the pipe
 cross sectional area) can be determined from:
 ##EQU2##
 The Optical Flow Meter described here can easily be extended to permit the
 measurement of velocities at multiple points, and therefore flow rate
 measurements can be obtained with the proposed device in ill-conditioned
 flows.
 Multiple measurement volumes may be accomplished mechanically by multiple
 implementations of the single point techniques described above (i.e.
 multiple sets of delivery and collection systems). However, optical
 systems may also be used to generate multiple measurement volumes using a
 single optical element. This may be done for example using a prerecorded
 hologram or by using a manufactured diffractive optical element (e.g. a
 diffraction grating). Diffractive or holographic optical elements may also
 be used to collect light from the multiple measurement volumes.
 The optical flow meter of the present invention is illustrated by the
 following non-limiting example.
 Experimental Results
 The accuracy of a prototype (as described relative to FIG. 6 above) was
 tested in a high pressure (5500 kPa) natural gas facility. A calibrated
 NPS 4 orifice meter that was used in the development of the revised (1998)
 API (American Petroleum Institute) orifice standard was used as the
 reference. The accuracy of this mass flow rate measurement is considered
 to be within .+-.0.3%.
 A NOVA 50E perforated plate flow conditioner was located 42 D (pipe
 diameters) upstream of the device to ensure a known and repeatable fully
 developed flow profile. Extensive measurements taken using pitot tubes and
 hot film anemometers have shown that the peak velocity (centerline) of a
 fully developed profile under the test conditions is 1.16 times the
 average velocity. Development of the NOVA 50E flow conditioner is
 explained in reference: Karnik, U., "A compact orifice meter/flow
 conditioner package", 3rd International Symposium on Fluid Flow
 Measurement, Mar. 20-22, 1995.
 In these tests the average velocity was varied between 10 and 25 m/sec. The
 centerline velocity measurement obtained with the optical flow meter was
 compared to a predicted centerline velocity based on the orifice meter
 data, the shape of the velocity profile that is produced by the flow
 conditioner, and the temperature, pressure and gas composition
 measurements. Temperature and static pressure readings were taken at the
 measurement location to allow accurate determination of gas density, and
 also the index of refraction. The total uncertainty in the reference
 velocity was estimated at .+-.0.6%.
 In the prototype the cylindrical lens in the optical delivery system was
 not insensitive to the index of refraction of the surrounding gas.
 Therefore as the refractive index of the gas increased with pressure, the
 spacing of the two sheets of light increased slightly. An optical design
 program was used to determine the necessary correction in beam spacing
 relative to the measured spacing in atmospheric air. For these tests a
 5.8% correction was required and applied. Note that a cylindrical lens
 doublet designed using the same principle as the collection lens doublet
 would not require any such correction to the beam spacing measurement that
 is used in the velocity calculations.
 FIG. 7 shows the data obtained over a two-day period of testing. In FIG. 7
 each point is a data point for one velocity measurement. The ordinate (Y
 axis) shows the deviation of the optical measurement from the reference
 orifice meter. FIG. 7 shows good overall agreement for the velocities
 obtained using the reference system and the optical flow meter of the
 present invention.