Analysis of 8 path meter diagnostics

1
ANALYSIS OF DIAGNOSTIC DATA FROM AN 8-PATH ULTRASONIC METER
Dr Gregor J Brown, Director of Application Engineering, Cameron
William R Freund, Principal Engineer, Cameron
1 INTRODUCTION
Multipath ultrasonic meters were first developed for gas custody transfer applications in the mid to late
1980’s. The technology offered significant benefits over traditional orifice metering in terms of increased
rangeability and reductions in pressure loss, upstream straight length requirements, and routine
maintenance. It was also hoped that based on measurement on meter geometry and correction for non-
fluid timing errors in the signals, determined during factory bench testing, it would be possible to use
ultrasonic meters without flow calibration, in the same way that orifice meters are still used today.
In practice, technology and design limitations, coupled with a drive by the industry towards lower
measurement uncertainties, have resulted in a situation where not all of the potential benefits of ultrasonic
technology have yet been harnessed.
In particular the elimination of the need for long upstream straight lengths has generally been achieved by
use of flow conditioning devices, typically of the perforated plate design. This in turn negates a large part
of the reduction in pressure loss, and also introduces a maintenance requirement, as either the plate can
become blocked with debris, or a filter is required upstream to protect the plate. In the latter case the
filter then introduces additional pressure loss and maintenance requirements.
Other issues that have been reported are the failure of transducers, particularly those made with epoxy
parts exposed to the gas, and concern over the effects of corrosion and/or deposition or fouling on the
interior of the meter body.
At the 2013 AGA Operations conference a paper [1] was presented showing test results obtained as part
of the process of certifying an 8-path gas ultrasonic meter to the requirements of AGA9, ISO17089 and
OIML R137. Of these standards, in terms of installation effects the requirements of OIML R137
Accuracy Class 0.5 are the most stringent. On the basis of those tests the 8-path meter has been certified
as meeting the requirements of OIML Accuracy Class 0.5 when installed only 5 diameters downstream of
bends, tees and reducers, including single bends and out-of-plane combinations, without use of a flow
conditioner.
There is an argument that says that flow conditioning is useful for two purposes: firstly to reduce the
influence of upstream conditions on measurement uncertainty; and secondly to provide a known
‘baseline’ for velocity diagnostic analysis, so that changes can used to identify potential problems. The 8-
path meter configuration without a flow conditioner has been shown to outperform 4-path (and other)
meter configurations even when latter are used with a flow conditioner. This challenges the first point of
the argument above, i.e. it demonstrates clearly that there are ways to reduce installation effects by virtue
of employing a first-principles approach to meter design. In other words, a good meter design can, both
in principle and practice, achieve better results without flow conditioning than a poorer design that is
reliant on a flow conditioner.
In terms of addressing the second point of the argument in favour of flow conditioning, we could assert
that with the improved meter design monitoring for apparent velocity profile changes becomes of much

2
lower importance. However, even if we accept the argument that diminishing the influence of upstream
hydraulics reduces the need for monitoring, it is still worthwhile considering if something is lost in terms
of velocity profile diagnostic monitoring capability when an 8-path meter is employed and the
requirement for the flow conditioning is removed.
This paper aims to explore that issue through detailed examination of the velocity diagnostic data
obtained during the testing that was described in the 2013 AGA paper. Relative to the 2013 paper there is
some repetition and summarisation here in order that this paper can be read without requiring access to
the 2013 paper. However, for more details of the testing the reader should refer also to that paper [1].
2 LIMITATIONS AND ADVANCES IN MULTIPATH ULTRASONIC DESIGN
Multipath ultrasonic meters have been in continuous development since the 1960’s. In early publications
and patents, it was noted how multipath meters that employ numerical integration methods could
significantly reduce the sensitivity to distortions in the axial velocity profile caused by upstream hydraulic
disturbances. Studies of the accuracy of the numerical integration methods have shown that chordal
meters with four chords spaced according to the rules of Gaussian integration could typically be expected
perform with errors of less than one or two tenths of a percent.
In the earliest implementations of chordal integration schemes, it was common to place only one
measurement path at each of the prescribed chord locations. In the patents and papers of Westinghouse
published in the 1970’s [2, 3], the paths of their Leading Edge Flow Meters (LEFM) were shown as
residing a single plane, typically angled at 45° to the pipe axis, as illustrated in Figure 1 below.
Figure 1 Illustrations of the Westinghouse multipath meter patent
An individual path at an angle of 45° is sensitive not only to the axial flow velocity but is equally
sensitive to any non-axial component of flow such as that generated by pipe bends. The result is that in
disturbed flow conditions where swirl or non-axial flow exists, the inputs to the integration method are in
error, and this in turn results in poorer flow rate measurement accuracy than can be achieved in a non-
swirling flow. In some special cases, such as a single-vortex flow that is centred between the two inside
paths of the Westinghouse arrangement the errors cancel, but in general they do not.

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In the mid 1980’s British Gas (BG) began development of a chordal multipath ultrasonic flow meter
intended for custody transfer measurement of natural gas. This design was based on a similar
arrangement of four horizontal chords to that used by Westinghouse, but with the paths criss-crossed such
that the first and third paths were at +45° to the pipe axis and the second and fourth paths were at -45°, as
illustrated in Figure 2. This design variation has been justified by technical arguments regarding
sensitivity to cross-flow, but the fact that the 1976 patent of Westinghouse [3] was still in force in 1986
when BG filed for their patent [4], suggests that patent considerations may also have come into play.
Figure 2 Illustrations of the British Gas multipath meter patent
One particular form of disturbance which it has been claimed the BG arrangement is insensitive to, is a
form of cross-flow where the relative magnitude and direction of the cross-flow is equal at each of the
chord locations in the cross-section [5]. With a Westinghouse arrangement of all chords at the same angle
relative to the pipe axis this would result in a systematic over or under reading, whereas it is shown that
for the criss-crossing arrangement of paths in the BG design this cancels. However, this is a hypothetical
form of non-axial flow, which is unlikely to occur in practice in closed pipes, as in reality any disturbance
that creates a cross-flow in one part of the cross-section likely to create a counter-rotation in another part.
A more realistic form of cross-flow is that produced downstream of a single bend, where there is a strong
cross-flow in the plane of the bend in the form of two counter rotating vortices. The BG design differs
from the Westinghouse design in its response to this situation in that the BG design would in principle be
insensitive to the presence of these two counter rotating vortices if those vortices were symmetrical about
the line that is centred between paths B and C. However, in practice, owing to a combination of factors
including effects from components further upstream, small asymmetries in bend geometry and the fact
that the flow wants to recover to a fully developed condition, it is virtually impossible to create two
symmetrical counter-rotating vortices. This is borne out in the results presented in the 2013 AGA paper
mentioned in the introduction. In the case of the single bend, with both the bend and the paths of the
meter aligned horizontally, the resulting errors for the BG 4-path arrangement were significant, and much
larger than for the Westinghouse 4-path arrangement [1].
Single-vortex swirl is another basic ‘test case’ for the path layout in an ultrasonic meter. In the
Westinghouse 4-path arrangement, if the single-vortex swirl is symmetrical about the centre of the pipe,
then the effect on path 1 would exactly cancel with that on path 4 and likewise the effect on path 3 would
cancel with that on path 3. This is because the magnitude of the swirl would be the same in the top and
bottom of the pipe but the swirl direction would be opposite relative to the path angle. For the BG design
the effect of single-vortex bulk swirl cancelation relies upon a mathematical quirk of the design, whereby
if a solid-body rotation of the flow is assumed, the combined effect on the outside paths (paths A and D)
cancels with the effect on the inside paths (paths B and C). The two inside paths in the BG design see the
swirl from the same direction and the two outside paths see the swirl from the opposite direction.
However, true cancellation does not result in the case of single-vortex swirl, even when that properly
centred and symmetrical as BG design relies on the magnitude of the swirl effect on the inside paths

4
versus the outside paths being in inverse proportion to the weighting factors. Owing to mathematics of
circular geometry that assumption holds true if the swirl is a solid-body rotation of the fluid. However as
Zanker has pointed out, in practice that particular case is unrealistic as the swirl must have its own
boundary layer and go to zero velocity at the pipe wall [6].
In the 1990’s a gas ultrasonic meter with five chords was jointly developed by Statoil and Fluenta (then a
subsidiary of Christian Michelsen Research). The original design had a criss-crossing arrangement of
paths, with paths 1, 3 and 5 in the same plane and paths 2 and 4 in the opposite plane. However, a few
years later the meter design was altered to a 4-chord, 6-path design in order to account for the adverse
effects of symmetrical double-vortex swirl. The new Fluenta/FMC design placed two crossed paths in
each of the chord locations in the top half of the pipe, and one path in each of the chord locations in the
bottom half of the pipe. This configuration has the benefit of tackling both a single-vortex swirl and the
cross-flow caused by symmetrical double-vortex swirl, but similar to some of the 4-path cases discussed
above it is truly insensitive only if the vortex pattern is symmetrical about the diametric plane that is
parallel with the chord arrangement.
Throughout the 1990s and into the 2000s numerous laboratory tests were carried out on ultrasonic meters
for the natural gas industry. Particularly notable are the programmes of the Gas Research Institute in the
USA [7] and GERG in Europe [8]. These tests exposed the weakness of 4, 5 and 6-path configurations in
some installation configurations and demonstrated that for these particular designs, using either direct or
reflected paths, a flow conditioner is generally needed if the requirements of today’s standards are to be
met.
Despite the clear recognition in the natural gas industry of the importance of installation effects on
ultrasonic meters, it appears that developments in other industries either went unnoticed by the gas meter
manufacturers, or if developments were noted by some, they chose not act to improve their meter designs
owing to other considerations. The use of flow conditioners has therefore become a de facto standard in
many parts of the industry today despite the stated aim in the BG patent to have a solution that “causes no
blockage to the flow and generates no pressure loss”. Moves towards including the ‘end treatments’ of
the metering package in the calibration in addition to the meter run and flow conditioner represent a
further departure from the original promise of ultrasonic technology.
As mentioned in the start of this section, the advantage of the Gaussian integration method, if a sufficient
number of chords are used, is that it is relatively insensitive to distortions of the axial velocity profile. It
was also stated that the main problem that prevents the method from achieving its potential is the
influence of non-axial flow or swirl on the individual paths that are used to provide the axial velocity
estimate to the integration method. This problem was recognised early on by Westinghouse and
ORE/Accusonic who were deploying their ultrasonic meters for large-scale measurements in rivers,
hydroelectric and nuclear plants.
A description of the solution can be found as far back as the 1977 publication by Lowell [9] where the
author highlighted the influence of non-axial flow and stated that the resulting error “can be reduced by
the addition of one or more acoustic paths, at the same elevations as the original ones but installed at the
opposite angle. Exact cancelation of errors can be accomplished on the crossed paths and an estimated
of the cross-flow component used to adjust the readings on the non-crossed paths.” The significance of
this statement is that it encourages pairs of crossed paths at each elevation used in the integration method.
It also highlights that for paths that are not crossed in the same elevation the cross-flow can only be
estimated by making some assumptions.
The way that swirl or cross-flow interferes with the measurement of axial velocity and how a pair of
crossed paths solve the problem can be described quite simply. Swirl or cross-flow introduces an

5
unwanted non-axial component of velocity to measurement path. This unwanted component of velocity
can be additive or subtractive. If the non-axial flow velocity is going in the same direction as the
ultrasound when it travels from the upstream transducer to the downstream transducer then the effect will
be to increase the measured velocity, as illustrated in Figure 3 below. If the non-axial velocity is opposite
in direction to the downstream travel of the ultrasound then the effect will be to decrease the measured
velocity.
Figure 3 The influence of non-axial flow on an ultrasonic measurement path
As a result, a crossed pair of paths located on the same chord allows the true axial velocity data to be
recovered, as illustrated in Figure 4 below.
Figure 4 An illustration of how crossed paths cancel the effects of swirl
With this understanding of the fundamentals of how these meters work, it is relatively simple to examine
different non-axial flow scenarios or swirl patterns and evaluate whether or not the interfering non-axial
flow components would cancel partly or fully. This exercise has been performed for a variety of direct
path chordal meter designs and the results are shown in Table 1 below. From this table it can be observed
that meters with only single paths in each chordal plane, whether all in the same angled plane with respect
to the pipe axis, or in a non-planar criss-crossing arrangement, only cope properly with one particular
form of symmetrical swirl. With the addition of a second crossing path at each of the top two chordal
planes, the 6-path arrangement is able to cope with both forms of symmetrical swirl but still has problems
with asymmetric swirl patterns. However, it is only when a second crossing path is added to each of the
Actual velocity
Upstream transducer Downstream transducer
Axial component (wanted)
Transverse
component
(unwanted)
Measured velocity
1 up
5 down
1 down
5 up
Actual velocity
Axial component (wanted)
Transverse component (unwanted)
Measured velocity
Path 1 Path 5
Path 1 + Path 5
Path 1 + Path 5
2
Key:
=

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chordal planes and every crossed pair works together to cancel the effects of swirl that the meter design is
able to cope with swirl of any form.
Table 1 Ability of chordal path configurations to correct for different forms of swirl
The ability of the 8-path configuration to cope with a wide variety of disturbed installation conditions has
been evaluated in numerous analytical, computational and laboratory studies by the meter manufacturers
and third parties. In circular pipes both ORE/Accusonic and Westinghouse deployed 8-path meters with
pairs of crossed paths in each of four chordal planes from around 1980. These meters were designed
inherently insensitive to the swirl and cross-flow that exists in applications where flow conditioning was
not practical. Caldon, as successor to Westinghouse having acquired the LEFM technology from
Westinghouse in 1989, then went on to use the 8-path concept in high accuracy liquid meters first in
nuclear applications and later for liquid hydrocarbon custody transfer. As a result of this heritage there is
a wealth of data validating this design in a wide range of hydraulic configurations, including almost 100
meters for nuclear plants that have been calibrated in a grand total of more than 400 installation
configurations.
In Caldon 8-path meters, a crossed pair of paths located in each of four chordal planes, those chordal
planes being located in accordance with the Gaussian integration methods described in the original
Westinghouse patents. The four chord selection made by Westinghouse was based on extensive research
and although further gains could be made by adding more chords, others have also concluded that a four-
chords integration is sufficient to obtain an appropriately small error in integration of the axial velocity
profile.
A recent paper by Zanker and Mooney [10] re-examined the choice of the number of chords from the
perspective of velocity profile integration in fully developed and asymmetric flows. The analysis is
broadly in line with work carried out by Westinghouse and others, and the conclusion the authors appear
to reach is that increasing the number of chords beyond four is of questionable valve when it comes to
obtaining a representative average of the axial velocity profile. However, although the Zanker and
Mooney paper discusses fully developed, distorted asymmetric and symmetric axial flow profiles and
4 paths, 4 chords,
planar
4 paths, 4 chords,
non-planar
5 paths, 5 chords,
non-planar
6 paths, 4 chords,
two crossed chords
8 paths, 4 chords,
four crossed chords
    
    
    
    
1 up 1 down
2 down
3 down
4 down
2 up
3 up
4 up
1 up
5 down
1 down
5 up
2 down
6 up
3 down
4 down
2 up
6 down
3 up
4 up

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factors such as the effect of steep velocity gradients, transducer cavity effects, it neglects to examine the
effects of swirl or transverse flow and gives these only a passing mention. The paper opens with a
discussion involving a 32-path meter design and states later that that eliminating the need for a flow
conditioner would be an advantage. In the absence of a discussion of non-axial flow there is a risk that
the reader could assume that the authors have concluded that increasing the number of paths brings little
benefit. Adding paths arbitrarily does not necessarily bring a benefit but doing it in a particular way to
address a problem using a first-principles approach is different. As the purpose of the additional paths in
the 8-path design is to cancel the unwanted effects of non-axial flow and allow the numerical integration
method to properly do its work of evaluating the mean velocity, the Zanker and Mooney paper is in fact
supportive of the 4-chord integration method employed in the 8-path meter.
3 THE 8-PATH ULTRASONIC GAS METER
The 8-path ultrasonic gas flow meter used for the tests we describe here was a Caldon LEFM 380Ci. The
Caldon brand covers a family of ultrasonic meters manufactured by Cameron with heritage directly from
the Westinghouse multipath Leading Edge Flow Meters first developed in the late 1960’s.
The arrangement of paths adopted for the Caldon LEFM 380Ci ultrasonic gas custody transfer meter is
similar to that used in Caldon 8-path liquid meters, with the exception that a steeper path angle is used to
allow for the effects of high Mach numbers. As illustrated in Figure 5 below, the meter employs 16
transducers to form eight measurement paths which are grouped in crossed pairs of paths at each of the
chordal locations associated with the 4-chord Gaussian integration method.
Figure 5 An illustration of the path layout in the 8-path Caldon LEFM 380Ci

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When introducing the LEFM 380Ci product for gas custody transfer, three steps were taken in an effort to
advance the technology in some of the areas where it had previously been lacking in gas meters.
First, the adoption of the 8-path configuration previously described was seen as a necessary step to enable
the meter to perform with high accuracy without the need for a flow conditioner. Eliminating the flow
conditioner would not only reduce energy losses, but would also allow metering stations to be reduced in
size, and remove the requirement for maintenance of the flow conditioner and the frequently reported
problem of partial blockage.
Secondly, the meter body and transducers were designed such that each transducer capsule is placed in a
metal alloy housing that is integrated into the meter body and fully isolates the transducer from the
process fluid and pressure. This not only results in a very robust transducer by eliminating failure modes
associated with aggressive chemical components or rapid depressurisation, it also means that if necessary
transducer can be easily removed and replaced without requiring depressurisation of the line. Each metal
alloy transducer housing is fully pressure retaining and all work required to replace the transducer is done
on the low pressure side. There is no breach of the pressure boundary and therefore no special extractor
tools are required; transducer replacement can be performed quickly and safely.
A third enhancement is provided in the form of a proprietary coating that is applied to the inside of the
meter to inhibit corrosion and reduce contamination build up inside the meter body. The coating is
applied to the bore of the meter and to the wetted surfaces of the transducer housings. The obvious aim
here is to minimise changes to the interior of the meter would otherwise result in changes to its calibration
over time.
4 PERFORMANCE TESTING REQUIRED BY THE STANDARDS
In order to be accepted for use in custody transfer applications, it is necessary that ultrasonic gas meters
comply with the requirements of the relevant standards. In this case the relevant standards under
consideration are AGA9 (2007) [11], ISO 17089-1 (2010) [12] and OIML R137 - 1&2 (2012) [13].
The above standards describe the performance expectations that have been set for gas ultrasonic meters
for custody transfer applications. In terms of installation effects, AGA9 requires that the “manufacturer
shall ... recommend at least one upstream and downstream piping configuration without a flow
conditioner or one configuration with a flow conditioner, as directed by the designer/operator, that will
not create an additional flow rate measurement error of the meter of more than 0.3% due to the
installation configuration. This error limit should apply for any gas flow rate between qmin and qmax. This
recommendation shall be supported by test data.”
ISO 17089-1 prescribes a series of disturbance tests that are intended to cover a range representative of
the type of conditions that may be encountered in practice. These include a single bend, out-of-plane
bends, contractions, expansions and steps. The manufacturer is allowed to specify the length between the
meter and the disturbance at which the meter will be tested, and then the meter should be tested at that
distance and at a second distance that is ten pipe diameters further away. The requirement in ISO 17089-
1 is that above qt, all calculated mean additional errors shall be within 0.3 %. For ISO 17089-1, the tests
have to be performed at one flowrate below qt and two flowrates above qt. In addition to the installation
tests, ISO 17089-1 requires that tests should be performed to evaluate repeatability, reproducibility, the
effect of transducer change out and simulated transducer failure. The general performance requirements
in ISO 17089-1 are very similar to those required by AGA9.

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A new edition of OIML R137 was issued in 2012. Although the 2012 edition has been partly harmonised
with ISO 17089, some differences remain, not only in terms of the tests required, but also in the
evaluation criteria by which the flow meter is deemed to pass or fail. Unlike the other standards, OIML
R137 allows classification of the meter performance to different levels, the most demanding being
Accuracy Class 0.5. In terms of the installation effect testing, the test configurations have a large degree
of overlap with those in ISO 17089-1, but for OIML the requirement is that “the shift of the error due to
these disturbances shall not exceed one third of the maximum permissible error”; which means in the
case of Accuracy Class 0.5 the shift of the error should be within +/- 0.167 %, which is approximately
half that allowed by AGA and ISO.
In addition to the general requirements of these standards, and the flow tests mentioned above, the
standards also require a series of ‘environmental’ tests be performed to ensure the that metrological
characteristics of the meter are immune to factors such as radio frequency interference, damp heat,
vibration and surges on electrical supply lines.
5 PERFORMANCE TEST RESULTS
A comprehensive test programme jointly prepared by Cameron and NMi, the weights and measures
authority of the Netherlands was performed to cover all the requirements of the AGA, ISO and OIML
standards, with minimum duplication.
The majority of the flow testing was performed at the CEESI high pressure natural gas calibration facility
in Iowa, USA. All tests were witnessed by NMi as a notified body (issuing authority) for the type
approval of gas meters according to the requirements of OIML and the European Measurement
Instruments Directive (MID).
The results of the flow tests were described in detail at the 2013 AGA Operations Conference and will
only be selectively summarised here.
The tests were performed with three different upstream pipe arrangements between the prescribed
disturbance and the meter: 5D of straight pipe with no flow conditioning, 15D of straight pipe with no
flow conditioning, and an arrament where the disturbance was followed by 5D then a CPA 50E perforated
plate flow conditioner then a further 10D before the meter, as illustrated in Figure 6 below.
Figure 6 An illustration of the 5D, 15D and 5D-CPA-10D upstream pipe configurations
5D
15D
CPA

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As explained previously the 8-path meter comprises two planar sets of 4 paths with the paths set at the
same chordal heights as in a 4-path design. By making a selection of only some of these paths it is
therefore possible to use the 8-path meter to replicate other path arrangements such as a single-plane 4-
path arrangement (Westinghouse) or a 4-path criss-crossing arrangements (BG). Figure 7 shows the path
arrangements that were evaluated; Plane A and Plane B being of the Westinghouse type, BG1 and BG2
being of the British Gas type. In all these evaluations, the abscissa (path chordal heights/locations) and
weighting factors, were the same as prescribed by the 1976 Westinghouse patent [3] and later adopted by
BG [4] and others.
Figure 7 4 and 8-path configurations selected for evaluation
Arguably the most important of the tests prescribed by ISO17089-1 and OIML R137 are those
downstream of single and double bends as they are broadly represented of a range of typical piping
configurations. The results of the installation effect tests downstream of bends were summarised in the
2013 AGA paper in terms of the shift in the flow weighted mean error (FWME) relative to the straight
pipe baseline calibration of the same meter configuration. That method of summarising the results is the
same as was used for the data from the GRI and GERG projects referred to in the introduction and
enables comparison of different installation/meter combinations on the basis of a single number.
The FWME summary of the data obtained with the Caldon meter in both 4-path and 8-path format is
reproduced in Table 2 below. For each meter type and upstream meter run arrangement (i.e. 5D, 15D,
CPA), the outer extremes of error shift have been highlighted. This table clearly shows that the flow
weighted mean error shifts are lowest for the 8-path meter at 0.08% or less and are typically around one
third of the 4-path planar arrangement. The 4-path non-planar arrangement produces the largest flow
weighted mean error shifts, typically around 4 or 5 times greater than the 8-path meter, but larger still in
the 5D configuration. In terms of the flow weighted mean errors, the benefit for the 4-path meters when
moving from 5D to 15D and then including the CPA flow conditioning plate is fairly clear, but the
improvement for the 8-path meter is not very significant, showing the extremes of +/- 0.06 at 5D reducing
to a range of -0.04 to +0.06 % in the 5D-CPA-10D case.
8-path
PlaneA Plane B BG 1 BG 2

11
Table 2 Bend summary data in terms of flow weighted mean error shift for 4 and 8-path meters
Perhaps the most important finding when looking at the data in Table 2 is that even when a flow
conditioner is used the 4-path meters show FWME shifts that are larger than the results obtained with the
8-path meter at 5D with no flow conditioner, as illustrated graphically in Figure 8 below.
Figure 8 FWME performance comparison for the 8-path meter at 5D with no flow conditioner
versus the 4-path meters with 15D inclusive of flow conditioning
Disturbance Upstream Path orientation A B 1 2
Horizontal 0.06% -0.08% 0.21% 1.02% -0.90%
Vertical 0.03% 0.00% 0.07% -0.86% 0.93%
Horizontal 0.02% -0.10% 0.15% 1.17% -1.12%
Vertical -0.06% -0.26% 0.14% 0.45% -0.57%
Horizontal -0.08% -0.04% -0.13% 0.30% -0.46%
Vertical -0.05% -0.02% -0.08% -0.61% 0.51%
Horizontal -0.05% -0.24% 0.13% 0.09% -0.20%
Vertical -0.08% -0.06% -0.11% -0.12% -0.05%
Horizontal -0.02% -0.06% 0.02% -0.12% 0.07%
Vertical -0.04% -0.01% -0.07% -0.14% 0.06%
Horizontal 0.03% -0.05% 0.11% -0.11% 0.17%
Vertical 0.06% -0.08% 0.20% 0.12% 0.00%
Planar 4-path
(Westinghouse)
Non-planar 4-path
(British Gas)
5D - CPA - 10D
Single Bend
Double Bends
8-path
meter
Flow Weighted Mean Error Shift
5D
15D
Single Bend
Double Bends
Single Bend
Double Bends

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6 COMPARISION WITH PUBLIC DOMAIN PERFORMANCE TEST RESULTS
Given the fact that ultrasonic meters are commonly used today with flow conditioners, and that this is
often put forward as ‘best practice’, the results shown in Figure 8 may challenge some preconceptions
about using meters with or without flow conditioners. It is mainly practical experience that has brought
about the common usage of flow conditioners, and that experience is valid, but it is valid only for the
meter designs on which that experience is based.
The fact of the matter is that while flow conditioners do reduce non-axial flow velocities, they do not
completely eliminate them. What the data shown in Figure 8 shows is that as the 8-path meter is designed
to do a first-principles cancellation of non-axial flow, it fares better than a meter design that is adversely
affected by non-axial flow, even when the latter is used with a flow conditioner.
Rather than relying solely on the 4 and 8-path data obtained with the Caldon meter, this can be validated
by comparing the 8-path results with the data from the GRI and GERG tests that were carried out under
similar conditions.
Both the GRI and GERG projects conducted tests on multipath ultrasonic meters from the same three
manufacturers and included single bend and double-bend out-of-plane configurations in their tests. The
meters were a 4-path chordal design, a 6-path chordal design and a meter with reflected paths which was
a 5-path version of the meter for the GRI tests and a 4-path version for the GERG tests. The GRI tests
were conducted on 12-inch meters at SwRI whereas the GERG tests were conducted on 8-inch meters at
the Advantica (now DNV GL) facility in the UK. The results were summarised in terms of the flow
weighted mean error (FWME) shift relative to the calibration baseline, in the same way as was done to
produce the data in Table 2.
The shortest length of upstream pipe without flow conditioning was 10D in the GRI tests and 12D in the
GERG tests. Figure 9 below compares the FWME results from the GRI and GERG projects with the 8-
path data, all without flow conditioning. It can be observed that for 10 and 12 D without a flow
conditioner the GRI and GERG results are typically in the range of +/- 0.5 to 1 % whereas for the 8-path
meter the results are less than +/- 0.06 % for 5D and no flow conditioner.
Figure 9 Comparison of 8-path meter at 5D vs GRI and GERG results at 10 and 12 D

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Both the GRI and GERG projects also included results where they tested the meters first in straight pipe
with a CPA flow conditioner at a distance of 10D from the meter, and then downstream of the disturbance
with the 10D position of the conditioner relative to the meter unaltered. Figure 9 below compares the
FWME results from the GRI and GERG projects with the 8-path data. It can be observed that although
the magnitude of error the GRI and GERG results is reduced with the CPA plate, they are typically in the
range of +/- 0.3 to 0.6 %, still much larger than for the 8-path meter with 5D and no conditioner at +/-
0.06 %.
Figure 10 Comparison of 8-path meter at 5D vs GRI and GERG results with CPA conditioner
7 FLOW CONDITIONING CONSIDERATIONS
The data presented in sections 5 and 6 shows that the improvements in performance achieved by the 8-
path design outweigh the improvements obtained when a 4, 5 or 6-path meter is coupled with a flow
conditioner. That in itself should be sufficient to challenge any notion that all ultrasonic meters must be
used with flow conditioners. However, the following additional considerations add further strength to the
assertion that improving the meter performance with respect to upstream effects has advantages relative to
employing flow conditioning.
Flow conditioners create pressure loss. While this is not always a larger concern, in some cases, for
example when summed over many measurement points, it can have a significant operational cost
implication.
The principles of chordal integration used either explicitly or implicitly in all multipath ultrasonic meters
favour a relatively smooth velocity profile. The job that the multipath design is doing (once non-axial
velocity effects are accounted for) is akin to attempting to curve fit a function with only a limited number
of points on the curve. If the velocity profile has lumps and bumps, then it will be difficult to account for
these. In that respect the way that flow conditioners divide the flow into a number of discrete jets is
contrary to the desired velocity profile characteristics according to the principles of the design. This is the
reason it is always advisable to have some distance between the conditioner and the meter to allow the
profile to recover to a smoother form. It also means that when a flow conditioner is to be use it is

14
advisable for the meter and conditioner to be calibrated together and maintained that way as reflected, for
example, in the advice given in ISO 17089-1:
“Installing a flow conditioner at any position in the meter run upstream of the USM will cause a change
of the meter’s indicated flowrate. This change depends on many factors (e.g. flow conditioner type, meter
type, position relative to the USM, flow perturbation upstream of the flow conditioner, etc.)” . . . “To
avoid this additional uncertainty, the best option is that the USM is calibrated with the actual flow
conditioner and meter tube as one package (USMP).”
The practical implications are that the meter and conditioner must now be calibrated (and recalibrated) as
one package with associated logistical challenges and costs. It also means that operationally, any partial
blockage of the conditioner will have an immediate, sustained and serious effect on the accuracy of the
measurement. While it is of course possible to protect a conditioner with a filter or even another
conditioner upstream, the alternative approach of improving meter performance and eliminating the
conditioner should be more attractive than placing further burdens on system design.
As mentioned in the introduction, it is often argued that flow conditioning is required to provide a
baseline for flow profile diagnostics during calibration and in service. For a meter that is sensitive to non-
axial flow, such as the 4-path Westinghouse and BG type designs that makes some sense, but it is worth
re-evaluating in light of the benefits of the 8-path design.
First and foremost, the question to ask is this: With a meter design that uses a first-principles approach to
reduce the effects of swirl and cross-flow, is monitoring of the flow profile still as important as it is for 4-
path meters? Recent presentations CEESI workshops have shown that different ‘end treatments’ can have
significant effects on some meter designs [14], and these might be detected by means of velocity profile
monitoring [15]. These effects are clearly similar to those that appear in the GRI and GERG testing, in
that the flow conditioner is not eliminating all of the non-axial flow and profile distortion. This supports
the conclusion that reducing the performance deficiency also reduces the need for monitoring. Secondly,
aside from upstream effects that the conditioner does not completely eliminate, what is it that velocity
profile monitoring is being used for? It would appear from many of the presentations and papers on this
topic that flow profile monitoring is primarily being used to detect flow conditioner blockage. It is
therefore easy to conclude that if the conditioner can be eliminated with no detrimental effect on
performance with varying upstream conditions, the primary reasons for monitoring velocity profile are
eliminated at the same time.
Flow conditioners can of course be used with 8-path meters, and although the flow weighted mean error
analysis of Table 2 does shows limited additional benefit when the conditioner is used, a slight
improvement could be seen in terms of the reduction of error shifts at different flowrates on a point by
point basis. So for the user that insists on a flow conditioner, the impact on the 8-path meter performance
itself is only marginal but the performance benefit of the 8-path meter over the other meter designs
considered in sections 5 and 6 is still significant.
8 USM DIAGNOSTIC CONSIDERATIONS
If the data is reviewed and the arguments made earlier in this paper are accepted, it seems there is indeed
much less need for velocity profile monitoring when an 8-path meter is used than is the case of some
other meter designs. However, as we are advocating elimination of the flow conditioner, it is still worth
examining if anything is lost in terms of velocity profile diagnostic monitoring capability when an 8-path
meter is employed and requirement for the flow conditioning is removed.

15
Velocity profile diagnostics are of course only one aspect of a suite of ultrasonic meter parameters that
can be evaluated as part of a condition monitoring or condition based maintenance system.
Similar to other multipath ultrasonic meters, an 8-path meter can provide a variety of ‘path level
diagnostics’, some associated with signal detection (such as gain, SNR and performance performance)
and others that relate to the process and flow conditions such as velocity of sound and ‘turbulence’. At a
path level the majority of these diagnostics are relatively insensitive to flow conditioning. The relative
standard deviation of the path velocity measurement, often called ‘turbulence’, will display sensitivity to
flow conditions and in the absence of a flow conditioner has its greatest use in monitoring conditions
relative to an installation baseline rather than against laboratory conditions.
Some parameters such as gain and velocity of sound per path are sensitive to process conditions and are
best used in a comparison with the other paths in the meter. In that respect the 8-path meter has the
advantage that it has a larger population of paths which can be inter-compared: four long ‘inside’ paths
and four shorter ‘outside’ paths.
At the ‘meter level’ most summary diagnostics are concerned with charactering the velocity profile and/or
the presence of non-axial flow. The exception is the average sound velocity which finds its greatest use
in a comparison with a ‘theoretical’ sound velocity value determined from composition, temperature and
pressure by means of an appropriate equation of state.
The meter level diagnostics used to characterise velocity profile and/or non-axial flow often have similar
names but can be calculated differently and will respond in different ways to the same flow conditions.
Diagnostic terminology and how different path configurations react will now be discussed.
8.1 Profile Factor/Flatness
The term ‘profile factor’ (PF) is commonly used in the gas industry describe the ratio of the inside path
velocities over the outside path velocities and is therefore a measure of how flat or ‘peaked’ the flow
profile is. For Caldon LEFM flow meters the term ‘flatness ratio’ has been used for this purpose for
many years, the difference being that the convention was to take the outside path over the inside paths, i.e.
1/PF. In this respect, having both the terms ‘flatness’ and ‘ratio’ in the term, and having the value start at
less than 1 in fully developed flow at low Reynolds numbers and increase towards 1 as the profile flattens
with increasing Reynolds number would seem to be preferable. However, as ‘profile factor’ is well
established in the gas industry we will use that terminology with the slight modification to call it ‘profile
flatness’ in order to avoid potential confusion with a velocity profile related correction factor.
The definition of profile flatness (PF) for 4-path single-plane (Westinghouse), 4-path criss-crossed (BG)
and 8-path meters is shown below in Figure 11 where the numeral represents the velocity measured on
that path.

16
Figure 11 Definitions of profile factor/flatness
Figure 12 shows how profile flatness would be expected to vary versus Reynolds number for four and 8-
path meters typical of the Westinghouse and BG design using the Gauss-Jacobi path spacing.
Figure 12 Profile flatness versus Reynolds number
In a fully developed flow or even a distorted profile free of non-axial velocity components, we would
expect the two 4-path designs and the 8-path design to produce the same (or in practice, very similar)
values of profile flatness. However, when various forms of non-axial flow are present, this can have an
adverse effect on the profile flatness values registered by the 4-path meters. For example, in the case of a
single-vortex swirl, this would cause either the inside or the outside paths of the BG design to read high
or low and the other pair to do the opposite. Thus swirl can fool the 4-path meter and result in a change in
PF when in fact the axial velocity profile may be unchanged. The single-plane Westinghouse 4-path
arrangement is not fooled in the same way, but for that design two counter rotating vortices, one in the top
of the pipe and the other in the bottom, would produce a similar effect whereby the indicated value of
flatness would change.

17
Zanker summed up this issue nicely in a paper at the NEL America’s Workshop in 2009 when he said:
“In general four paths are not sufficient to resolve any arbitrary 3-dimensional flow field containing
asymmetry, swirl, peaked or flat profile and cross flow.”
In a way that mirrors the discussions about performance earlier in this paper, the accuracy of the
‘diagnosis’ of the velocity profile is also adversely affected by the fact non-axial flow interferes with the
single paths at each chord location. For 4-path meter designs the solution to this problem is to install a
flow conditioning plate to try to reduce the number of degrees of freedom by one by eliminating non-axial
flow. The 8-path meter addresses this same concern by cancelling the non-axial flow by combining pairs
of paths on each chord, thus removing the interfering effect of non-axial flow from the determination of
the profile flatness.
Practical examples showing this are presented in section 9 of this paper.
8.2 Asymmetry/Symmetry Ratio
The term asymmetry or symmetry is commonly used to describe the ratio of the velocities in one half of
the pipe over the other half. The definition of asymmetry ratio (AR) for 4-path single-plane
(Westinghouse), 4-path criss-crossed (BG) and 8-path meters is shown below in Figure 13 where the
numeral represents the velocity measured on that path.
Figure 13 Definitions of asymmetry ratio
As suggested by the name the intention of the asymmetry ratio is to register changes in the symmetry of
the distribution/profile of axial velocity. In a manner similar to the discussion in section 8.1, in the case
of 4-path meters, this parameter can be fooled when non-axial flow is present. This time if we consider a
single-vortex swirl and the single-plane Westinghouse arrangement, then it is clear that if the direction of
swirl were such that paths 1 and 2 were to read high, then paths 3 and 4 would read low at the same time.
This would then affect the asymmetry ratio, making it impossible to separate effects due to profile
asymmetry from those due to swirl. Similarly, for the BG design if the swirl was in the form of two
counter-rotating vortices, one in the top of the pipe and one in the bottom, the asymmetry ratio would
register a spurious change. In the case of the BG design the clockwise rotation causing an over-reading
on paths A and B would be accompanied by an under-reading on paths C and D due to the accompanying
anti-clockwise vortex.
Yet again the swirl cancelling nature of the 8-path configuration allows a change in asymmetry to be
registered correctly without having to resort to flow conditioning to reduce the effects of non-axial flow.

18
8.3 Cross-flow and plane balance
Terms that attempt to quantify cross-flow and swirl are arguably of less value in terms of diagnosing
meter performance issues than profile flatness and asymmetry. The reason for making such an assertion
is that in the case of 4-path meters it is clear that non-axial flow adversely influences any attempt to
characterise the axial flow profile and therefore a clear separation of axial profile and non-axial flow
effects is not possible. For 8-path meters, the aim is to cancel swirl effects by design and thereafter a
measure of swirl is of relatively importance though it can be used to make second-order corrections
related to cavity and boundary layer effects.
In the case of the BG design, a cross-flow term is defined by dividing the sum of the paths that reside in
one angled plane by the sum of the paths in the other. As suggested above, this cross-flow term is
potentially susceptible to being fooled by asymmetry in the flow profile. For the Westinghouse single-
plane arrangement, clearly a cross-flow calculation of this type is not possible. For the 8-path meter a
cross-flow or plane balance term can be defined by taking the ratio of all the paths in one angle plane (1 –
4) to the paths the second plane (1 - 8). For those familiar with the ‘4 + 4’ concept of two 4-path meters
in a single body, the 8-path plane balance diagnostic gives an equivalent measure of the difference
between two 4-path results. Relative to the BG design the 8-path plane balance changes only with cross-
flow and is unaffected by changes in axial profile symmetry.
Figure 14 Definitions of cross-flow or plane balance
8.4 Transverse velocity per chordal plane
As discussed in the preceding subjections, for 4-path meters it is not possible to properly separate non-
axial flow effects/swirl from changes in axial flow profile. The simple reason for this is that when we
only have single paths in each plane it does not permit separation of the axial and non-axial velocity
contributions to the measurement of velocity in that plane. The beauty of the pairs of cross paths in the 8-
path meter is that they allow exactly that as the transverse velocity can be calculated from the difference
in single-path velocities at each height multiplied by a simple geometric factor. As illustrated in Figure
15, this gives the 8-path meter a capability that is not available in either of the 4-path arrangements.
As an aside, it should be noted that while use of single-bounce paths where each leg of the path traverse
the flow in the same chordal plane will result in some in-built cancellation of the effects of axial velocity
it does not have the same diagnostic capability as a pair of crossed paths. Simply put it cannot supply a
measure of transverse flow as the single bounce path measurement does not provide information on what
happens in each leg of the path.

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Figure 15 Transverse flow velocity calculation
9 PROFILE DIAGNOSTICS DOWNSTREAM OF BENDS
In this section of the paper we present diagnostic data for a selection of installation configurations to
illustrate the issues that were discussed in section 8 above.
9.1 Baseline Straight Pipe
Figure 16 shows a photograph of a baseline straight pipe set up.
Figure 16 Straight pipe test set up
Figure 17 shows the velocity profile diagnostics for the two 4-path meter configurations and the 8-path
meter. Figure 17 (a) and 17 (b) show the profiles for 4-path Westinghouse and BG arrangements
respectively. Figure 17 (c) shows the profiles for the 8-path meter, along with the derived non-axial flow
represented in the right hand figure showing the results at the corresponding path locations in the meter.

20
(a) (b)
(c)
Figure 17 Flow diagnostics from the straight pipe test: (a) Westinghouse Plane A; (b) BG 1; (c) 8-path
Also shown in Figure 17 are the corresponding values of profile flatness (PF) and asymmetry ratio (AR)
measured by each meter configuration. It can be observed that in this case, that of a long straight pipe, as
would be expected, there is good agreement in the diagnostic indicators between the three different meter
types.
9.2 5D downstream of double bends out-of-plane
Figure 18 below shows the installation 5D downstream of the double bends out-of-plane, with no flow
conditioning and the paths in the meter orientated horizontally. Figure 19 shows the flow diagnostics for
this case in the same format as Figure 17. It can be observed that the single-plane Westinghouse
configuration interprets its velocity measurements as a strong asymmetry with AR = 1.347 whereas the
BG design interprets its measurements as a strongly inverted profile with PF = 0.794. When the 8-path
meter results are examined it can be observed that in actual fact the axial velocity profile is relatively flat
and quite symmetrical with PF = 1.035 and AR = 1.009. The cause of the inaccurate profile
representation by the 4-path meters is revealed in Figure 19 (c) as a strong, clockwise single-vortex swirl.
It can even be observed that the swirl itself is asymmetric, which contributes to the measurement errors in
the 4-path meters. This asymmetry in the non-axial flow also results in some error in the more accurate of
the two profile indicators of each 4-path meter (PF for the Westinghouse and AR for the BG design),
which can be seen to be several percent different from the more accurate 8-path result.

21
Figure 18 Double bends out-of-plane at 5D upstream with no flow conditioner
(a) (b)
(c)
Figure 19 Diagnostics from the out-of-plane bends test (a) Westinghouse Plane A; (b) BG 1; (c) 8-path

22
9.3 5D downstream of a single bend
Figure 20 below shows the installation 5D downstream of the single bend, with no flow conditioning and
the paths in the meter orientated horizontally. Figure 21 shows the flow diagnostics for this case in the
same format as Figures 17 and 19. It can be observed that this time the single-plane Westinghouse
configuration interprets the measurements as a strongly inverted profile with PF = 0.836 whereas the BG
design interprets the measurements as a strong asymmetry with AR = 1.248. When the 8-path meter
results are examined it can be observed that in actual fact the axial velocity profile is again relatively flat
and quite symmetrical with PF = 1.024 and AR = 1.012. The cause of the inaccurate profile
representation by the 4-path meters is again revealed in the plot of transverse velocities: Figure 21 (c). In
this case the single bend has produced a strong, counter-rotating double vortex swirl. It can be observed
that the double vortex pattern is asymmetric, which again means that even the more accurate of the two
profile indicators from each of the 4-path meters is in error by a few percent relative to the more accurate
8-path result. This result illustrates that even with the nominally symmetrical geometry of the upstream
single bend, the resulting swirl pattern is likely to exhibit asymmetries.
Figure 20 Single bend at 5D upstream with no flow conditioner

23
(a) (b)
(c)
Figure 21 Diagnostics from the single bend test test (a) Westinghouse Plane A; (b) BG 1; (c) 8-path
9.4 Test results with the 5D-CPA-10D arrangement upstream
Figure 22 below shows the installation of the double bends out-of-plane with the 5D-CPA-10D
arrangement upstream of the meter. Figure 23 shows the flow diagnostics. In this case only the 8-path
result is shown as it can be inferred from this graph that each of the 4-path results, irrespective of which
type, are very similar. It can be observed that the introduction of the flow conditioning plate has reduced
the swirl to a negligible level and produced a symmetrical profile similar to that seen downstream of a
long straight pipe. The profile factors in this case show close similarity being 1.158, 1.164 and 1.153 for
the 8-path, 4-path Westinghouse and 4-path BG meters respectively and the corresponding asymmetry
ratios are 0.998, 1.003 and 0.994.

24
Figure 22 Double bends out-of-plane upstream in the 5D-CPA-10D set up
Figure 23 Flow diagnostics for the double bends out-of-plane with the 5D- CPA-10D arrangement

25
10 DIAGNOSTIC AND PERFORMANCE ANALYSIS
The proposition in terms of use of velocity profile diagnostic data is that if the parameters stay with set
limits then it is a good indication that the meter is performing properly; or perhaps more correctly, that if
the parameters go outside the set limits there is a potential problem.
This proposition can now be examined using the performance and diagnostic data acquired during the
OIML and ISO certification testing of the 8-path meter and the 4-path Westinghouse and BG subsets.
This exercise is particularly relevant as it allows us to examine and compare the usefulness of 4 and 8-
path diagnostics under an identical set of installation conditions.
The comparison in this paper is performed in terms of profile factor/flatness (PF) and asymmetry ratio
(AR), as these two parameters can be calculated for all three configurations.
Diagnosis and monitoring based on velocity profile indicators can be used in various ways. It can first be
used to validate the transfer of the meter’s calibration from the laboratory to the field installation.
Thereafter profile changes can be monitored alongside other diagnostics in an effort to detect the onset
problems, such as might be indicated by a sudden, unexpected change in profile. The diagnostic limits we
will discuss below are appropriate for calibration transfer, whereas once in service, monitoring to tighter
limits may be considered, alongside monitoring of the other parameters mentioned above.
The installation effect data used for this comparison is the same data that was summarised in section 5 of
this paper, i.e. the single-bend and double bend out-of-plane data. This includes the configurations of 5D
and 15D without flow conditioning and the arrangement of 15D total length with the CPA plate included
at 10D from the meters. Data for both the horizontal and vertical orientations of the paths is also
included. In addition to the single and double bend data we have now added data obtained for the OIML
R137 (2012) severe perturbation coupled with the three meter tube configurations described above. The
OIML R137 (2012) severe perturbation comprises two out-of-plane bends with a half-moon blockage
installed between them and is a more severe disturbance than what would normally be encountered in a
custody transfer metering system.
The default values of PF were set using the baseline calibration data to 1.169 and 1.136 for the
Westinghouse and BG 4-path arrangements respectively, and the default value of AR was set to 1 for
both. The limits around these default values were set as follows by giving consideration to various
publications on this topic such as those by Zanker & Floyd [16] and Lansing et al [17]:
 PF: +/- 5%
 AR: +/- 3%

26
Figure 24 below shows the asymmetry ratio plotted versus profile flatness for the 4-path Westinghouse
arrangement (plane A). Installations without flow conditioning are shown with coloured symbols,
whereas installations inclusive of the CPA plate are shown as open symbols in black. The 5 % PF and 3
% AR limits around the baseline conditions are shown as a red ‘diagnostic box’, which in this case is
rectangular owing to the extremely wide span of asymmetry registered on the graph by the Westinghouse
4-path arrangement. Next the data points, which represent each individual test run from the CEESI data
files, the flow weighted mean error shift caused by the upstream installation change is shown.
A number of useful observations can be made by examining Figure 24. Firstly, all of the data from
conditions without a flow conditioner lie outside of the diagnostics box, the only exception being the long
straight pipe condition. It can also be observed that in general, those results that lie furthest from the
diagnostics box correspond with the largest errors, up to the maximum of 3.2 % corresponding to the
OIML severe disturbance with an extreme asymmetry ratio of greater than 2.5. While the largest errors
lie at the extremes, it can be seen that the relationships are not proportional, making it difficult in field
applications to interpret what a result that lies outside of the diagnostic box would mean in terms of error.
This is most obvious when looking at the double bend results with paths horizontal in Figure 24 at both
5D and 15D. In that case the 5D diagnostic indicators lie further from the diagnostics box than the 15D
results but with a corresponding FWME of - 0.1% that is smaller than the -0.24 % that is associated with
the 15D location.
Figure 24 Diagnostic data plot for the Westinghouse 4-path arrangement (plane A)

27
Figure 25 shows the asymmetry ratio plotted versus profile flatness for the 4-path British Gas
arrangement (BG 1), with data in the same format as Figure 24. Similar to the other 4-path meter, the
majority of the data from conditions without a flow conditioner lie outside of the diagnostics box, an
additional exception this time being the single bend at 15D with paths horizontal, which for the BG
design falls inside the box. Like for the Westinghouse design, those results that lie furthest from the
diagnostics box correspond with the largest errors, in this case the maximum of 2.7 % corresponding to
the OIML severe disturbance, with extreme profile factors registered at less than 0.6. Again it can be
seen that although the largest errors lie at the extremes, the relationships are not proportional, making it
difficult in field applications to interpret what a result that lies outside of the diagnostic box would mean
in terms of error. Again focusing on the double bend results with paths horizontal it can be observed that
in terms of the diagnostic indicators both lie about the same distance outside the diagnostics box, but that
the FWME is 1.2 % for the 5D location and only 0.09 % for the 15D location.
Figure 25 Diagnostic data plot for the British Gas 4-path arrangement (BG 1)

28
Figure 26 shows a zoom the asymmetry ratio plotted versus profile flatness for the 4-path British Gas
arrangement (BG 1) highlighting some of the FWME values inside or close to limits of the diagnostics
box. What this figure illustrates is that it is possible to be inside the box, or outside but close to the limits
of the diagnostics box, and have FWME error values of the order of 0.3 %.
Figure 26 Zoom in on the data in and around the diagnostics box for the British Gas 4-path arrangement

29
Figure 27 below shows the asymmetry ratio plotted versus profile flatness for the 8-path meter with the
data in the same format as in Figures 24 to 26. In this case the baseline asymmetry ratio is again set to 1
but now the baseline profile factor is set to 1.096 to reflect the fact that the 8-path meter is intended to be
used without a flow conditioner, and hence is expected to see the flatter profiles that are typically
produced by downstream of bends, tees and headers etc.
The diagnostic box shown in red on Figure 27 is plotted with limits of +/- 10 % for profile factor and +/-
6 % for asymmetry ratio. The use of wider limits for the 8-path meter compared to the 4-path meter can
be justified on several grounds:
 The 8-path meter is intended to be used without a flow conditioner and hence is expected to see a
wider variety of conditions
 The 8-path meter does a better job of accurately quantifying profile flatness and asymmetry
 With the influence of non-axial flow greatly diminished, the 8-path meter performance is
relatively insensitive to the range of profile flatness and asymmetry changes that are observed
Comparing Figure 27 with Figures 24 and 25 we can make a number of informative observations. First
we can see that with only one exception, that one being the OIML R137 severe disturbance at 5D, all of
the results both with and without flow conditioning lie inside the 8-path diagnostic box. Second, all of the
FWME values are less than 0.08 % for the conditions inside the diagnostics box. Given that for the 4-
path meters FWME values of 0.2 % to 0.3% are typical of performance within the 4-path diagnostic box,
this comparison favours the 8-path meter, as staying inside the 8-path diagnostic box is associated with
tighter performance limits. This is also confirmed by observing the one result that lies outside the box.
The OIML R137 severe disturbance at 5D produces a FWME shift of 0.21 %. Although this takes the 8-
path meter outside of the limits associated with its diagnostic box, the 0.21 % FWME result compares
very favourably with the 2.7 % and 3.2 % errors associated with this condition for the 4-path meters, and
is on par with the 4-path FWME results that fall inside their corresponding diagnostic box.
Figure 27 Diagnostic data plot for the 8-path meter

30
11 DISCUSSION AND CONCLUSIONS
The 8-path meter design discussed in this paper addresses weaknesses of previous multipath meter
designs by employing a first-principles method of non-axial flow cancellation. Results have been
obtained showing that the 8-path meter meets the ISO 17089-1, AGA 9 and OIML R137 Class 0.5
performance requirements downstream of bends at 5D with no flow conditioner.
Comparing like-for-like installation conditions, the installation effects for the 8-path meter are typically
between 3 and 5 times lower than that for 4-path meters. Futhermore, at 5D with no flow conditioner, the
maximum errors and flow weighted mean error shifts for the 8-path meter are less than those for the 4-
path meters with the 5D – CPA – 10D upstream package, confirming that custody transfer accuracy can
be achieved by the 8-path meter without having to resort to the use of a flow conditioner.
Diagnostic principles have been discussed and data analysed. It has been shown that 4-path meters
cannot accurately quantify both flatness and asymmetry changes downstream of disturbances in the
absence of a flow conditioner, owing to the interfering effects of non-axial flow. The 8-path meter on the
other hand can accurately quantify flatness and asymmetry without the need for a flow conditioner. The
8-path meter can also quantify and display information regarding non-axial flow in a way that is not
possible for the 4-path chordal meter designs.
A combined analysis of diagnostic and performance data shows that in order to stay within the 5 % limit
for profile factor and 3 % for asymmetry ratio normally set for 4-path meters, flow conditioning is a
necessity, and then the FWME shifts range up to between 0.2 and 0.3 % in magnitude. For the 8-path
meter it has been shown that even with more generous limits in terms of asymmetry and flatness,
operation within the 8-path diagnostic box confines the magnitude of FWME shifts to less than 0.08 %.
Clearly, in principle, it would be preferable to use ultrasonic meters without flow conditioning; for a
number of reasons including pressure loss, blockage and other maintenance concerns, and the logistics of
having the conditioner installed for calibration. This is of course only acceptable on condition that it does
not expose the user to additional measurement uncertainty or risk. While the data in this paper confirms
the need for flow conditioning with the 4, 5 and 6-path meter designs considered, the combined
performance and diagnostic analysis shows that the 8-path meter can overcome these limitations. This
allows us to conclude that the 8-path configuration can be used to achieve reduced measurement
uncertainty and that this reduced uncertainty can backed up by meaningful velocity profile diagnostics, all
without having to resort to use of flow conditioning.

31
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[11] AGA9 (2007) Measurement of Gas by Multipath Ultrasonic Meters
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Part 1: Meters for custody transfer and allocation measurement
[13] OIML R137 - 1&2 (2012) Gas meters - Part 1: Metrological and technical requirements - Part
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[14] Hanks, E and Miller, R (2013) “Installation Testing NAFFMC Research”, CEESI Ultrasonic
Meter Users Workshop, Denver, 2013
[15] Hackett, D (2012) “Specifying upstream meter tube lengths for gas ultrasonic meters”, CEESI
Ultrasonic Meter Users Workshop, Colorado Springs, 2012
[16] Zanker, KJ and Floyd, A (2010) “Trending diagnostics from SMART ultrasonic meters”, NEL
America’s Workshop, Houston, 2010
[17] Kneisley, G, Lansing, J and Dietz, T (2019) “Ultrasonic meter condition based monitoring – a
fully automated solution” North Sea Flow Measurement Workshop, Norway, October 2009

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