Optical flow meter integrally mounted to a rigid plate with direct optical access to the interior of a pipe

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 United States Patents ("USP") U.S. 
Pat. Nos. 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, Dec. 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.

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 embodiment 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 SDL SDL-5421-G1 
(830 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.OGR-40-3-3A- 
equalize pressure 0.6-SP 
410 Wollaston prism 
Karl WQ6.35-05-V830 or 
(0.5.degree. or .75.degree.) 
Lambrecht WQ6.35-075-V830 
V coated for 830 nm 
Corp. 
411 Cylindrical Lens 
Melles Griot 
01 LCP 001 or 
cut down to 01 LCP 005 
6.35 mm sq. 
413 Cemented doublet 
NOVA 
15 mm diameter 
designed, 
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) 
Opto- 
with transimpedance 
electronics 
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. Kinghorn 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: 
______________________________________ 
Center-point 
Criteria factor, a.sub.v 
______________________________________ 
a.sub.u &gt; 0.86 0.8941 
0.86 &gt; a.sub.u &gt; 0.83 
0.8526 
0.83 &gt; a.sub.u &gt; 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", 3.sup.rd 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.