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Flow Measurement and Instrumentation 13 (2002) 125–142

www.elsevier.com/locate/flowmeasinst

Guidelines for the use of ultrasonic non-invasive meteringtechniques

M.L. Sanderson ∗, H. Yeung

Department of Process and Systems Engineering, School of Engineering, Cranfield University, Cranfield, Bedford MK43 0AL, UK

Received 1 July 2002; received in revised form 7 July 2002; accepted 17 July 2002

Abstract

This paper provides a comprehensive set of Guidelines for the application of clamp-on transit time ultrasonic flowmeters to awide range of industrial flows. These Guidelines have been drawn up in conjunction with users and manufacturers and sponsoredby the United Kingdom’s Department of Trade and Industry. They represent the best practice to be used for the application of thistechnology to liquid metering. The Guidelines identify the range of possible non-invasive technologies which can be employed forthe measurement of pipe flows and installation, pipework, fluid and operational effects on clamp-on transit time ultrasonic flowmet-ers, together with effects which may be specific to particular manufacturers. The paper concludes with the identification of furtherwork which needs to be undertaken to strengthen the Guidelines.

2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

1.1. Non-invasive ultrasonic metering technologies

One of the significant advantages in employing ultra-sonic techniques is that when they are applied to themetering of liquid, they cannot only be used non-intrus-ively, in that the sensors may be in contact with theliquid but do not intrude into the flow path, but can alsobe used non-invasively, that is with the transducersmounted onto the outside of the pipework. Such methodsare commonly referred to as clamp-on ultrasonic tech-niques, since the transducers are clamped or held ontothe outside of the pipework. These methods enablemeasurements to be made without breaking into thepipework and, therefore, measurements can be madewhere for reasons of safety, hygiene, continuity of sup-ply or cost it is not possible to break into the pipework.The method also provides a basis for checking existingmeters. Non-invasive ultrasonic metering techniques canthus be used as a temporary, semi-permanent or perma-nent method of measurement.

∗ Corresponding author. Tel.: +44-1234-754696; fax: +44-1234-

750728.

E-mail address: [email protected] (M.L. Sanderson).

0955-5986/02/$ - see front matter. 2002 Elsevier Science Ltd. All rights reserved.

PII: S0 9 5 5 - 5 9 8 6 ( 0 2 ) 0 0 0 4 3 - 2

1.2. The origin of the guidelines

The ‘Guidelines For The Use Of Ultrasonic Non-invasive Metering Techniques’ were originally writtenby Prof. Mike Sanderson and Dr. Hoi Yeung in theDepartment of Process and Systems Engineering in theSchool of Engineering at Cranfield University under acontract issued by the Department of Trade and Industryfor the National Measurement System Policy Unit’s1999–2002 Flow Programme. The intention in drawingup these Guidelines was, in the light of increasing useof this technology, to identify and promulgate best prac-tice in its application and also provide material to informnational and international standards. The Guidelineshave been drawn up under the guidance of a SteeringGroup representing users and manufacturers of non-invasive ultrasonic flowmetering techniques. Theyattempt to identify the full range of factors includinginstallation effects, pipework, fluid effects, operationaleffects and manufacturer specific effects which are likelyto affect the performance of ultrasonic non-invasive met-ers. They quantify the likely effect of these factors;identify best operational practice for such meters andprovide estimates of the overall uncertainty which canbe achieved when employing best practice. The Guide-lines are based on our best current understanding of thetechnology identified from the experience of users and

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126 M.L. Sanderson, H. Yeung / Flow Measurement and Instrumentation 13 (2002) 125 –142

manufacturers and on data currently available in the

open literature. The papers identified in the Bibliography

have provided the supporting data for the guidelines.

1.3. Scope and content of guidelines

This section provides a brief overview of the completecontents of the Guidelines.

1.3.1. Possible non-invasive ultrasonic technologies

In employing non-invasive ultrasonic technologies

there are three potential technologies which can be

employed. These are the Transit Time Ultrasonic Flow-

meter (TTUF), the Doppler Ultrasonic Flowmeter (DUF)

and the Cross Correlation Ultrasonic Flowmeter

(CCUF). The scope of this Guideline is restricted to theuse of TTUFs, since such flowmeters are likely to give

rise to a metering technique with the widest range of

application and the highest accuracies and lowest uncer-

tainties. Section 1.4 provides a description of the non-

invasive TTUF, typical application areas, the perform-

ance measures which should be used in assessing the

meter performance and the likely performance in terms

of error, repeatability and reproducibility when used

under ideal flow, pipe, fluid and operational conditions.Because of the need to distinguish between the different

ultrasonic technologies and their applications, Sections

1.5 and 1.6 provide a comparative overview of Doppler

Ultrasonic Flowmeters and Cross Correlation Ultra-

sonic Flowmeters.

1.3.2. Installation effectsTTUFs clamped on to the outside of the pipework are

effectively single diametrical beam TTUFs and as a

consequence suffer the velocity profile problems and

errors associated with similar spool piece designs with

non-intrusive sensors. These problems and errors arise

as a consequence of variations in the velocity profile of

a fully developed flow as Reynolds Number is varied

and also as a consequence of upstream pipework con-

figurations such as bends and valves. The effects of these

are considered in Section 2. Section 2.1 provides a guideas to suitable distances downstream of disturbances

which should ideally be employed to minimize the level

of additional error introduced into the measurement.

Estimates of the likely effects on the accuracy of the

meter of not achieving these distances are provided in

Section 2.2. A method of further reducing the errors of disturbed flows by measuring in more than one diametri-

cal plane is identified in Section 2.3.

1.3.3. Pipework effects

In addition to the effects of upstream disturbances onclamp-on ultrasonic flowmeters, additional uncertainties

arise as a consequence of their use with a wide variety

of pipe sizes, pipe materials, wall thickness and lining

materials. Section 3.1 provides guidelines as to the range

of pipe sizes and types with which the technology can

be applied. Measurements are often made on pipes

which have been in service for sometime and in some

cases can have suffered corrosion and erosion. Section3.2 deals with the effect of pipe wall roughness on the

measurement performance of the meter. Section 3.3identifies the ultrasonic properties of the pipework which

are required to be supplied by the user.

1.3.4. Fluid effects

Clamp-on TTUFs can also be used with a wide variety

of liquids with varying viscosity and density. The effects

of these are considered in Section 4. For clean fluids

there are two effects. The fluid dynamic effects caused

by the variation of Reynolds Number with density andviscosity, and the effect of varying speed of sound on

the performance of the meter. These are considered in

Sections 4.1 and 4.2. Although TTUFs are meant for

operation with clean fluids, it is possible that such fluids

may have small amounts of entrained air bubbles or sol-

ids associated with the flow. The effects of these are

identified in Sections 4.3. Section 4.4 deals with limi-

tations caused by highly attenuative liquids.

1.3.5. Operational effects

The overall performance of the meter is likely to

depend significantly on the operational conditions under

which the meter is employed. Section 5.1 identifies the

need for the equipment to be used by trained users if

accurate measurements are to be achieved. The trans-

ducers can be mounted in such a way so that a single,double or quadruple traverse of the pipe by the ultrasonic

beam is used to undertake the measurement. The relative

merits and drawbacks of each of these is considered in

Section 5.2. The location of the transducers with respectto the pipe methods for obtaining the correct positioning

of the transducers and methods of clamping the trans-

ducers to maintain their positions are considered in Sec-

tions 5.3, 5.4 and 5.5. Operational methods to reduce

the effect of flow profile disturbance are identified and

discussed in Section 5.6. The ultrasound produced by thetransducers has to be well coupled to the pipework. The

preparation of the pipework and the use of an appropriate

couplant is important to obtaining an accurate measure-

ment. A range of couplants is available depending on

the temperature of the application and whether the instal-

lation is temporary or semi-permanent. Guidelines for

the preparation of the pipework and the application of

the couplant are provided in Section 5.7. The process

temperature has an impact on the choice of couplant and

also transducer selection. These are discussed in Section

5.8. Section 5.9 provides guidelines as to the selectionof the frequency of operation of the ultrasonic trans-

ducers with differing pipe size. Clamp-on TTUFs require

a knowledge of the inner diameter of the pipe. This is

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obtained by requiring the user to input the pipe outer

diameter and the pipe wall thickness. Methods available

for obtaining the pipe outside diameter and the pipe wall

thickness, including the use of ultrasonic thickness

gauges are identified in Section 5.10. The likely effectof errors in either of these measurements is included in

this Section.

1.3.6. Manufacturer speci fic effects

The overall accuracy achievable in addition to show-ing a dependency on the factors identified above will

also depend on items which are manufacturer specific.

These include the method the manufacturer employs to

measure the transit time difference, the compensation, if

any, provided for Reynolds number and the speed of

sound in the liquid. Guidelines as to the best practice inthese are identified in Sections 6.1–6.3. Confidence in

the measurement can be enhanced by the provision of a

series of diagnostic tools. The range of possible diagnos-

tic tools is discussed and compared in Section 6.4.

1.4. Clamp-on transit time ultrasonic fl owmeters

(TTUF)

Fig. 1 shows the elements of a typical clamp-onTTUF. The meter consists of three elements—the trans-

ducers, the clamping arrangement and the signal pro-

cessing and user interface electronic package. The trans-

ducers are piezoelectric devices which generate

ultrasound which penetrates through the pipewall, and

which is then transmitted along the paths shown within

the fluid. The clamping arrangements enable the trans-ducers to be in good sonic contact with the pipe walland at the correct distance for the particular pipe and

liquid application. The signal processing electronics

Fig. 1. A typical clamp-on TTUF. (Panametrics used with

permission.)

measures the difference in time between ultrasonic

beams passed upstream and downstream, as a conse-

quence of the flow of the liquid, and converts this to a

volumetric flowrate which is displayed on the user inter-

face typically either in numeric or graphical format ona user interface, which also provides the means of entry

of application specific data by the user.

1.4.1. Operation of TTUFs

Fig. 2 shows the principle of operation of the clamp-on TTUF measuring the flowrate of a liquid whose speed

of sound is cl in a pipe of internal diameter D.

Although different manufacturers undertake the

measurement in a variety of ways and employ different

algorithms to compute the flowrate from the measure-

ments they make, the following gives, in generic formthe basis of the measurement. The flowrate is measured

by using the transit time difference between ultrasound

travelling from transducer T1 to transducer T2 and when

the ultrasound is travelling from transducer T2 to trans-

ducer T1. If the ultrasonic beam in the fluid is at an

angle q to the pipe axis then the volumetric flowrateQv is related to the time difference T T12T21 by:-

Qv k .p . D.c2

l . T

16.cotq(1)

where cl is the speed of sound in the liquid, D is the

internal diameter of the pipe and where k , the correction

factor, is given by:-

k v̄ pipe

v̄ beam

(2)

where v̄ pipe is the actual average velocity in the pipe, and

v̄ beam is the average velocity measured along the beam.If the wedge on which the transducers are mounted is

made of a material for which the speed of sound is cw

and having an angle a to the pipe axis, then Eq. (1) can

be re-written as:-

Qv

k .p .cl.cw. D.1cl.sina

cw2

.T

16.sina (3)

By measurement of the transit time difference, T ,

together with knowledge of factors specific to the

measurement, namely the internal diameter of the pipe

and the speed of sound of the liquid, input by the user

(or measured on line) and internally held variables suchas the k factor, the wedge angle and the speed of sound

in the wedge it is possible to provide a measurement of

the volumetric flowrate of the liquid.

In order to obtain an accurate measurement it is neces-

sary to obtain an accurate measurement of the requiredseparation of the transducer T1 and T2. From simple

geometric considerations this separation, s, in the case

of a single path meter to be:-

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Fig. 2. Schematic of a clamp-on transit time ultrasonic flowmeter.

s 2.sw 2.sp 2.sl (4)

where sw is the separation through the wedge, s p is the

separation through the pipe and sl is the separation

through the liquid. This can be shown to be given by:-

s 2.h.tana 2.t

cp.sina

cw

1cpsina

cw2

(5)

2. D.

cl.sina

cw

1cl.sina

cw

2

The correct separation of the transducers can be obtainedby providing the flowmeter with the pipe wall thickness,

t , and internal diameter, D, and the speed of sound in

the pipe wall, cp, material and the speed of sound in the

liquid, cl. This can be used with internally stored values

to indicate to the user the required separation.

1.4.2. Application areas

Clamp-on TTUFs are designed to be used with clean

liquids, although they can operate in the presence of lim-ited amounts of either particulate matter or gas bubbles

(Guidance on operating in the presence of particulate

matter or gas bubbles is to be found in Section 4.3.).

Table 1 drawn from manufacturers’ application data

identifies typical application areas to which clamp-on

TTUFs are being applied.The largest single sector where these meters are cur-

rently being applied is in the water industry, although

this does not represent the majority of applications. This

is not meant to be an exhaustive list, and for specific

areas of application outside those identified in Table 1,potential users should consult suppliers. Over the years,

the technology has also found applications in ‘dirty

liquids’.

The application areas for clamp-on TTUFs where they

show significant advantages over techniques are those

where the pipe cannot be cut for reasons of safety orhygiene or possible process contamination; where the

cost of breaking into the pipework is too high; where

reasons of continuity of supply installation, maintenance

or service need to be carried out without a shut down inthe process and on larger pipes where the cost benefit

over other in-line meters is greatest.

1.4.3. Performance measures for TTUFs

TTUFs generally provide outputs indicative of volu-

metric flowrate or totalised volume flow. In assessing the

performance of the meter, three measures are of concern.

These are the percentage error of the indicated volu-metric flowrate or totalised volume flow, the repeat-

ability of the measurement and the reproducibility of themeasurement. In employing these terms it is important

that they should correspond to standard usage. The fol-

lowing provides standard definitions of the three terms.

The percentage error of the volumetric flowrate

measurement is given by:-

E V indV std

V std × 100% (6)

where the V ind is the volume of liquid passed as indicated

by the meter under test during the duration of the test

and V std is the recorded reference volume passed during

the same time. It is usually expressed as a function of

flow velocity through the meter.

The repeatability of the measurement is a measure of the ability to provide repeatable measurements under the

same set of conditions. It is a measure of the random

uncertainty in the measurement, R, which is defined as:-

R 2.83.m.sE and sE n

i 1

( E i E ¯ )2

n1

(7)

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Table 1

Application areas of clamp-on TTUFs

Sector Application

Water industry Temporary/semi permanent installation

Checking of installed meters; leakage determination; network analysis

Permanent installationApplications where hygiene, operation (eg. Supply interruption), maintenance and cost are

of major consideration

Oil industry Permanent installations

Used for measurement of crude oil after first stage separator; of floading of crude oil; water

injection; refined oil

Food industry Permanent installations

Blending and batching of food and drink where issues of hygiene and contamination are of

major concern

Semiconductor industry Permanent installations

Process applications of high purity and aggressive liquids where measurements can be

made with a meter which can be serviced without having to shut down the process

Chemical industry Temporary installations

Employed as a tool for checking, servicing and maintaining existing flowmeters

Permanent installations

Process monitoring and control of liquids where application and maintenance can beachieved without breaking into the pipework and where meter process compatibility can be

achieved by using the existing pipework; pump protection

Miscellaneous applications Temporary/semi permanent/permanent systems

Measurement of cold and hot water flows in large building complexes such as hospitals,

schools, factories and of fices to monitor and control usage

Heat metering and air conditioning applications

where E i is the error of the ith run within the test series; E ¯ is the average error of all the runs in the test series

and n is the number of runs in the test series. m is ascaling factor which enables comparisons of repeat-

ability to be made between test series which are differentin length by normalising all the repeatabilities to a stan-

dard length of test run.

m T test set

T std

(8)

where T test set is the average length of the test run in a

particular set of runs and T std is the time to which the

uncertainty is to be normalised.

The reproducibility of the measurement is concernedwith the degree to which the measurement can be repro-

duced if the conditions of measurement are changed. Of

particular importance in clamp-on TTUFs is the degree

to which the readings of the meter are reproducible under

unclamping and re-clamping conditions. The reproduci-

bility of the measurements can then be measured bymeasuring the difference between the average errors in

the two sets of tests, one taken before unclamping and

re-clamping and the other after, i.e.

Reproducibility E ¯ 1 E ¯

2 (9)

where E ¯ 1 is the average error of the initial tests and

E ¯ 2 is the average error of the test undertaken after

unclamping and re-clamping. If more than two sets of

tests are available it may be possible to express thereproducibility as a function of the standard deviation of

the errors across the whole range of tests. In this casereproducibility would then be defined as:-

Reproducibility

2.83.sreproducibility where sreproducibility (10)

n

i 1

( E ¯ i E ¯

all)2

n1

where E ¯ i is the average error obtained in the ith repro-

ducibility test series, E ¯ all is the error average overall the

reproducibility test series, and n is the number test seriesundertaken to assess the reproducibility. Reproducibility

is generally expressed as a function of flow velocity.

1.4.4. Likely uncertainty, repeatability and

reproducibility under ideal fl ow pipe fl uid and

operational conditions

TTUFs are likely to be used over a very wide range

of pipe sizes and liquids and because the overall per-

formance achievable in the field depends on the care

with which the operators prepare the pipework andmount the transducers, coupled with an inability to pro-

vide traceable calibrations of clamp-on meters in the

field it is dif ficult to obtain figures for the uncertainty,

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repeatability and reproducibility of such flowmeters in

the field.

The values manufacturers quote for the accuracy of

clamp-on TTUFs range between ±1% to ±5% of reading

with this performance being over a restricted flow rangewhich is generally specified in terms of flow velocity.

At the lower flow rates the performance is generallyspecified as a fixed uncertainty expressed as a velocity

which may range from ±0.01 m/s to ±0.03 m/s

depending on the pipe size and the particular manufac-turer. In addition to these accuracy specifications, manu-

facturers often specify what is termed a calibrated accu-

racy. By this is meant the accuracy which is achievable

in situations where the meter is capable of being cali-

brated in situ by some other means, or where the clamp-

on transducers are being used as part of a spool piece.Typical values for the accuracy under these conditions

range between ±0.5% to ±2% of reading, dependent on

manufacturer and pipe size. Repeatabilities quoted by

manufacturers typically lie in the range ±0.15% to

±0.5% of reading, depending on manufacturer.

The evidence of users on performances achieved in

the field indicate that over a range of pipe and fluid con-

ditions, the specifications provided by manufacturers

may not always be achievable, although there is littlehard evidence on which to base such judgements.

These Guidelines suggest that the calibrations which

have been undertaken under laboratory conditions

should be taken as the basis for a judgement as to the

likely performance that could be achieved in the field

under operating conditions. The results obtainable in the

field are unlikely to be better than the performances achi-eved in the laboratory under fully developed flow con-ditions, and with extreme care being taken in the mount-

ing and positioning of the transducers. The results

obtainable under non-ideal flow conditions and poor

mounting of the transducers and incorrect spacing are

likely to increase the error of the measurement.

The calibrations on which estimates provided below

have been based have been undertaken over a range of

pipe materials and sizes, although the majority of pub-

licly available calibrations have been undertaken on pipesizes greater than 50 mm. These results indicate overall

that an accuracy within ±5% of reading can be obtained

for velocities greater than 1 m/s for a range of pipe sizes

and materials. A typical repeatability which can be

obtained is ±1% of reading and reproducibility under

unclamping and re-clamping conditions of ±2% of read-ing.

In some applications, such as nuclear power station

performance evaluation using the secondary cooling cir-

cuit, significantly better performance has been achieved

by providing a full scale model of the pipework andusing this to calibrate the clamp-on TTUF on a test

stand. This approach can be employed where the econ-

omic cost is justified and where the pipework material,

dimension and conditions can be expected to be accu-

rately known. Under these conditions and where the in

situ calibration is undertaken with the greatest of care,

the claimed accuracies of better than ±1% of reading are

probably justifiable.

1.5. Clamp-on Doppler ultrasonic fl owmeters

It is important that any potential user of non-invasive

ultrasonic metering techniques should be aware of the

difference between TTUFs and Doppler Ultrasonic

Flowmeters (DUFs) which are also commonly commer-

cially available. The technological basis of the two is

completely different and the overall accuracy which canbe obtained is much poorer using DUFs. Fig. 3 shows

the operation of a DUF. DUFs require gas bubbles or

solid particles to act as scatterers of ultrasound.

Assuming that all the scatterers are moving with a

velocity v, then the Doppler shift, f d , of the transmitted

ultrasound is given by:-

f d 2. f t.v

cl

.cosq (11)

where f t is the transmission frequency, cl is the velocity

of sound in the liquid and q is the angle of the ultrasonicbeam with respect to the pipe axis.

In general the Doppler shift is not a single frequency

because broadening of the spectrum occurs as the scat-

terers have a range of velocities; the beam has a finite

width; the angle of the ultrasonic beam to the direction

of liquid flow has a range of values and the effects of

turbulence. An estimate has therefore to be made of theaverage Doppler shift from a spectrum which typicallylooks like that in Fig. 3.

The performance of Doppler flowmeters is limited,

due to the nature, size, spatial of the scatterers which

varies the attenuation of the ultrasonic beam. The sensed

volume is not well defined. In large pipes it is likely to

be close to the wall. The relationship between the sensed

velocity and the mean velocity in the pipe is unknown.

The velocity being sensed is that of the scatterers, and

because of slippage it might not correspond to theliquid velocity.

Upstream disturbances, in the form of bends, valves,

pipework and probes causing vortices can all cause the

meter to read in error. Errors can also be caused by

vibration. In order to obtain an accurate volumetric

flowrate measurement, the internal diameter of the pipe-work needs to be measured accurately. The uncertainty

with such flow meters can be high and typically is likely

to be greater than ±10% of reading. However, under

closely defined flow conditions it is possible for the DUF

to provide a repeatability of ±2% of reading. Typicalapplication areas for such meters are the measurement

of raw sewage, sludge, slurries, paper pulp, tar, sands,

and oil-water-gas mixtures.

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conditions may not apply to another. The overall accu-

racy of the technique should probably be reckoned to be

±5%, although for a particular application where the

meter can be calibrated under similar conditions accu-

racies of ±1% of flowrate or better may be expected. Theapplication areas to which such flowmeters have been

employed are in the measurement of single phaseliquids, liquid-solid mixtures, liquid-gas mixtures,

liquid-liquid mixtures, low pressure gas and high press-

ure gas.

2. Installation effects on clamp-on TTUFs

A clamp-on TTUF, when used with only a single

beam measurement, is sensitive to velocity profile effectsin both fully developed and disturbed flow conditions,

because as Eq. (1) shows the device estimates the aver-

age velocity across the whole of the pipe cross section

by measuring the velocity profile along the beam. Even

in fully developed flow the value of k is not unity and

depends on Reynolds number and pipe wall roughness.

This may or may not be corrected for by manufacturers.

Fig. 5 shows the k factor variation with Reynolds num-

ber for smooth pipes. A correction based on ReynoldsNumber may or may not be incorporated by the manu-

facturer. Fig. 5 shows that the corrections required are

greatest in the laminar regime. In the transition regime

however the correction factor may not be well determ-

ined, and this uncertainty on the correction factor for the

meter may add significantly to the overall meter uncer-

tainty.In the case of disturbed profiles, the k factor for fully

developed flow is no longer applicable. The flow profile

can be disturbed by upstream pipework configurations

such as bends, expansion and contractions and the pres-

ence of valves and pumps. Manufacuturers often specify

a requirement to have 10D of straight pipework upstream

Fig. 5. k factor against Reynolds Number for fully developed flow

in smooth pipes. (Taken from reference [3], used with permission of

Academic Press.)

of the meter and 5D downstream of the meter. Although

the downstream conditions are probably satisfactory, the

upstream requirement is probably over optimistic.

2.1. Installation conditions for the above accuracies to

be achieved

In providing advice as to the effect of the disturb-

ances, two approaches have been adopted. The first is to

provide estimates of distances at which the effect of thedisturbance is less than ±2% reading. If this assumed,

together with an inherent uncertainty of the measurement

of ±5% and the two sets of errors add in a mean square

sense, the overall effect of the disturbances at distances

greater than these specified distances can be reckoned to

be negligible.The data which has been used to provide these dis-

tances is based largely on the performance of wetted

sensor single beam TTUFs since it is believed that this

data is more comprehensive and reliable. They are gen-

erally consistent with the limited data which has been

obtained from clamp-on TTUFs. Table 2 provides the

distances for which a series of disturbances for which

the errors of measurement are likely to be less than ±2%.

2.2. Effects of upstream pipework

If the upstream disturbance is known and it is not

possible to obtain the required distances specified in

Table 2 then the following data, which is provided in

graphical form in Figs. 6–13, based on the data in refer-

ence [16], can be used to provide an estimate of theadditional error created by the disturbance. It should benoted that in the majority of cases a single beam dia-

metrical meter under reads the flowrate.

2.3. Reducing effects of disturbed fl ows

One method which is often suggested for improving

the measurement under disturbed flow conditions is to

Table 2

Distances downstream of a disturbance for less than ±2% increased

uncertainty

Disturbance Number of diameters required to

reduce error to less than ±2%

Conical contraction 4

Conical expansion 18

Single 90° Bend 30

Two bends 90° in ‘U’ 22

Two 90O bends in perpendicular 47

planes

Butterfly valve 2 / 3 open 18

Globe valve 2 / 3 open 15

Gate valve 2 / 3 open 20

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Fig. 6. Errors downstream of a conical contraction.

Fig. 7. Errors downstream of a conical expansion.

Fig. 8. Errors downstream of a single 90° bend.

Fig. 9. Errors downstream of two 90° bends in ‘U’ configuration.

Fig. 10. Errors from two 90° bend in perpendicular planes.

Fig. 11. Errors downstream of a butterfly valve 2 / 3 open.

Fig. 12. Errors from a butterfly valve 2 / 3 open.

Fig. 13. Errors downstream of a gate valve 2 / 3 open.

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Fig. 14. Two beam measurements.

undertake the measurements in perpendicular planes as

shown in Fig. 14 and take the average of the twomeasurements. Some manufacturers enable this to be

undertaken with simultaneous measurements from the

two beams being provided using two sets of transducers

and one set of processing electronics. If only one set of

electronics is available, then it is necessary to provide

an independent measure of the mean flowrate to accountfor any changes which may occur in the mean flowrate

between the two measurements. Most manufacturers rec-

ommend that the average of the two measurements

should be used as the estimate of the flowrate. In the

light of the above indications it is probably better to take

the maximum reading as the best estimate of the flowr-

ate.

3. Pipework effects

3.1. Typical ranges of pipe sizes, pipe materials,

lining materials and wall thickness to which thetechnology can be applied

Clamp-on TTUFs are required to operate over a range

of pipe sizes, materials, wall thicknesses and liningmaterials. Particular manufacturers will claim to be able

to work with particular pipe sizes, pipe materials, wall

thicknesses and lining materials and not all manufac-

turers will be able to provide a metering system which

will operate over the complete range of pipe sizes,

materials, wall thicknesses and lining materials shown

in Table 3.

The claim made by manufacturers is that the pipe has

Table 3

Ranges of pipe sizes, materials, wall thicknesses and lining materials with which clamp-on TTUFs can be used

Pipe sizes over which operation of clamp-on flowmeters has been 8 mm id to 6000 mm id

claimed by manufacturers

Pipe materials for hich manufacturers have used Clamp-on TTUFs Pig iron, cast iron, carbon steel, stainless steel, copper, aluminium,

Hastelloy, asbestos, concrete, glass, LDPE, HDPE, PP, PVC,PTFE,

PVDF, ABS, FPR, acrylic

Maximum pipewall thickness for the operation of clamp-on TTUFs Most manufacturers do not claim an upper limit, although one

manufacturer sets this at 25 mm

Lining materials and external coatings with which manufacturers Tar, epoxy, mortar, rubber

claim to be able to use clamp-on TTUFs

to be sonically conducting, although the experience of

users indicate that whilst some manufacturers may be

able to work with particular pipes other cannot. The fol-

lowing commentary should be taken as indicative and

not necessarily as definitive and identifies pipe materials

where the problems most frequently occur. Occlusions

or pores in cast iron can attenuate the ultrasound. Con-crete and cement pipe can cause problems but often if

the pipe is fully soaked, the meters may work better.

There is evidence that with stainless steel and GRP some

manufacturers experience dif ficulty in achieving

adequate signal transmission. For some manufacturers,

the pipe material, the pipe size and material may limit

the particular mode in which the flowmeter can be oper-

ated. (see Section 5.2).

Although manufacturers do not in general specify the

range of pipe wall thicknesses with which their flowme-

ters will operate, at least one manufacturer identifies a

maximum pipe wall thickness and there is evidence insmall diameter pipes having thick walls that there can

be significant errors.

Several problem areas have been identified with coat-

ings and pipe linings. Clamp-on TTUFs will only suc-

cessfully work where the coating/lining material is

bonded to the pipe material in such a way that there are

no air gaps between the two materials, since air gaps

will prevent successful transmission of the ultrasound.

If the meter is to be clamped on to the coating material

then the thickness of the coating must be accurately

known in order that the pipe o.d. can be accurately

determined. If the pipe is lined, then the thickness of thelining is required to be known accurately. The correct

sound speeds for the wave which is being transmitted

through coating/lining materials are required. Methods

for undertaking these measurements are provided in Sec-

tion 5.10.

If there is more than one lining material, a lining

material and a pipe coating, or a lining material together

with a coating which has built up over a period of time,

then account of all these in terms of calculation of pipe

id from od and separation of trandsucers. Some manu-

facturers do not enable data from lining materials to be

taken in separately, in which case a thickness corre-

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sponding to the sum of the wall thickness and the lining

thickness has to be input, together with a speed of sound

which represents a weighted average of the speed of

sound in the two materials, based on their relative thick-

nesses. For manufacturers who take into account liningmaterials and coatings, some allow the user to input

more than one lining material or coating. Where this isnot possible, it is necessary for the user to input an

appropriate thickness and speed of sound which accounts

for the total thickness and an average speed of sound.

3.2. Effects of pipe wall roughness on measurement

performance

The roughness of the internal wall of the pipe is likely

to cause two sets of problems. The fully developed pro-file in a pipe depends on the pipe wall roughness. Since

the velocity profile is affected, so the k factor required

to correct the measured velocity along the beam to the

average velocity across the pipe cross section. Fig. 15

shows the correction factor required for fully developed

flow for a series of pipe relative roughness, k/d

(roughness to diameter ratio) and shows that pipe wall

roughness can cause an error of up to 4% in very

rough pipes.In addition to affecting the profile, the pipe internal

roughness can also cause significant scattering of the

ultrasonic signal. The degree of scattering is related to

the absolute level of roughness compared to the wave-

length of the ultrasound being employed. Thus it is

related to the ratio of λ /k where λ is the wavelength of

ultrasound in pipe wall material and k is the roughness.Since λ is inversely proportional to the frequency of excitation of the flowmeter—a typical wavelength in a

metal pipe at 1 MHz is 6mm whereas at 500 kHz the

wavelength is 12 mm. Therefore, as a general guideline,

the rougher the inner surface of the pipework, the lower

should be the frequency of operation of the flowmeter.

Rougher pipe work will also limit the number of wall

reflection (see Section 5.2) which can be used. If the

Fig. 15. Effect of pipe wall roughness on k factor.

flowmeter will not work with three reflections, then two

should be tried and if two will not work then a single

pass configuration with no reflections should be

attempted. Pipe wall roughness can also affect the esti-

mation the pipe id from the pipe od and pipe wall thick-ness measurements (see Section 5.6).

3.3. Ultrasonic properties of pipework

In order to obtain the correct separation of the trans-

ducers, both the thickness of the pipe wall and the speedof sound in the material need to be known. Manufac-

turers have available tables of speeds of sound in differ-

ent material. The nature of the ultrasonic wave which is

transmitted through the pipe may either be a longitudinal

wave, a shear wave or a Lamb wave. It is therefore parti-

cularly important to input the correct speed of sound in

the wall material during the set up phase of the meter.

In the case of materials where the speed of sound isuncertain, an ultrasonic thickness gauge applied to asample of the material of known thickness can be used

to provide a measurement of the speed of sound.

4. Fluid effects

4.1. Effects of fl uid density and viscosity on meter

performance

As was shown in Section 2.2 in fully developed flow

the k factor for the meter depends on the Reynolds num-ber Re given by:-

Re v.d . r

m(12)

Thus, operating a clamp-on TTUF requires a knowledge

of the density r and the viscosity m or alternatively the

kinematic viscosity n which is given by m / r. It is parti-

cularly important to know the viscosity of high viscosity

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products for which measurements are being undertaken

in small pipes since these flows may be in the laminar

flow regime.

4.2. Effects of speed of sound on meter performance

Clamp-on TTUFs require the user to input the speedof sound into the meter at the initialisation stage as part

of the procedure to set the correct distance between the

two transducers. Subsequently, depending on the designof the flowmeter, this speed of sound will then be used

in the defining equation of the flowmeter or alternatively

a new speed of sound will be estimated from the transit

time measurements. The latter enables compensation for

the variations which may occur as a consequence of

changes in temperature of the fluid. The correct separ-ation of the transducers and the correct speed of sound

is important to the correct operation of the flowmeter.

In the case of liquids, whose speed of sound is uncertain

then manufacturers generally have facilities for its

measurement.

4.3. Effects of air bubbles and solids on meter

performance

TTUFs are designed for operation with liquids which

have only low content of air bubbles or solids. The

effects of air bubbles is to scatter the ultrasound but also

to affect the speed of sound in the liquid by affecting

the compressibility of the liquid. These have the effect

of attenuating the signal and degrading the signal to

noise ratio in the flowmeter. The degree of scatteringand attenuation is dependent on the size, number anddistribution of the air bubbles. Manufacturers provide

typical levels of entrained air with which their meter will

continue to operate, typically less than 10%, although

generally without any specification of the size of the

bubbles. The effect of solid material is to scatter the

ultrasound. Low densities of solids having particles

diameters less than λl /8, where λl is the wavelength of

ultrasound in the liquid is likely to cause little effect on

the meter.

4.4. Limitations caused by highly attenuative fl uid

media

Highly viscous fluid generally has associated with

them high levels of attenuation of ultrasound. So forexample, water has a kinematic viscosity of

1.003 × 106 m2s1 at 20°C and an attenuation at 1MHzof a pressure wave of 0.22dB/m whereas a product such

as castor oil has a kinematic viscosity of

1.1 × 102 m2s1 and an attenuation of 95dB/m at 1

MHz. Thus it is possible that the attenuation in a high

viscosity fluid may be so high as prevent correct oper-

ation of the flowmeter. This is usually detected by the

flowmeter electronics and flagged up. Since viscous

attenuation is generally a function of frequency, lower-

ing the frequency of operation of the flowmeter may

enable measurements to be made.

5. Operational effects

5.1. User training

The simple appearance of the meter and the simplified

principle often presented belies the fact that it is a very

sophisticated flowmeter and there are many complex

interrelated phenomena involved in producing a

measurement. Users who undertake the measurement

must be aware of these and take them into account whenundertaking the measurement. It must be stressed that

clamp-on meters should be installed by trained person-

nel.

5.2. Transducer con figuration

There are three configurations to mount the trans-

ducers. The two main methods are direct transmission

(Z) and single reflection (V). Multiple reflections (W)are also used. Direct transmission (Z), Fig. 16a, is used

Fig. 16. a. Z configuration. b V configuration. c W configuration.

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for large pipe sizes as the distance between the path of

the ultrasound is shorter than the other configurations

and thus less signal loss. They are however much more

dif ficult to install correctly, to measure the distance

between the transducers and align. Single reflection orreflex mode (V), Fig. 16b, is the recommended instal-

lation method. The path length is longer giving bettertime resolution. The set up and alignment of the trans-

ducers are much easier as they are on the same side of

the pipe. The measurement of the separation is easierand potentially more accurate. The down side is that the

axial separation between the transducer could be a prob-

lem in confined spaces. Multiple reflections (W), Fig.

16c, may be used on small pipes to increase the lapse

time, by increasing the ultrasound path between the

transducers. Care must be taken to ensure that the correctdistance is used, as it is very easy to end up with a V,

W or more reflections without realizing that the distance

is incorrect.

5.3. Location of the transducers with respect to the

pipe

For pure liquid with no particles nor bubbles, the

orientation of the transducers is irrelevant. For horizontal

pipe runs, gas has a tendency to float to the top and

particles settle at the bottom. Thus it is preferable tomount the transducers around the side of horizontal pipes

(i.e. at 3 and 9 o’clock positions). It should be empha-

sised that if there were gas at the top and sediments onthe bottom, the effective flow area will be different from

the pipe area and hence the volume flow would be in

error. For gas in vertical mains the transducers should

mount on the down leg, with flow from top to bottom.

For particles in vertical main, mount the transducers on

the up leg, with the fluid travelling upwards.

5.4. Positioning of the transducers

The method of positioning depends on the transducer

design and the method of transmission. Most transducersare clamped in position by a clamping mechanism, such

as a strap and then located by measurement of the dis-

tance apart. Most suppliers also offer a ‘calibrated track ’to help with the measurement and location.

If when setting up the transducers there appears to be

no signal, some manufacturers suggest a process calledscanning can be carried out. The transducers are moved

relative to each other until a signal is found, or the signal

becomes stronger. It is better to start too far apart and

move the transducers towards each other as they scan

better forwards rather than backwards. If a signal isfound by scanning, the implications are that the pipe

diameter, wall thickness, materials selected and fluid

could be wrong as there is only one ‘correct’ position.

A careful examination should be carried out before pro-

ceeding with the measurement.

As mentioned above, it is dif ficult to achieve good

accuracy for direct transmission. The following methods

may be used to set up the meter:

A sheet of paper is wrapped around the pipe to mark positions. The sheet is then removed and the final

locations calculated and marked. The sheet is replaced

on the pipe and the transducers located in position.In a confined, dirty space this is very dif ficult.

Some suppliers provide locating spigots on the clamp-

ing mechanisms. These spigots are located into appro-

priate holes in a spacer to give the correct trans-

ducer spacing.

A transducer is fixed in its position. The second trans-ducer is moved (scanned) along the opposite side for

the best signal. When this is found, the transducer is

clamped in place and the distance apart measured.

5.5. Method of clamping

There are a variety of methods used for holding the

transducers in position.

The simplest method is to use straps, usually nylon

with quick release and grip buckles. These are easy

to locate and tension. They do however stretch with

time and should not be used for long term measure-

ment.

Chains are often used, these are particularly effective

for permanent installation. Steel ropes, with quick release and grip mechanisms

are used but tend to be cumbersome.

Jubilee clips may be used for smaller pipes and are

very effective for permanent installation. With iron based pipes, magnetic clamps can be used.

These are very easy to position, but care must be

taken not to adjust the clamping mechanism too

tightly, as the magnets tend to loosen. Magnetic

clamps should not be used when the temperature

exceeds 50°C.

In the event of only an approximate flow rate being

required, it is often possible to just use the adhesion of

the couplant to hold the transducers in place.

5.6. Getting most information from the installation

Velocity profile and swirl have a large influence on

the measurement. The recommendation of Section 2 with

regard to mounting downstream and upstream of disturb-

ances should be adhered to. However, the followingsteps may be used to ensure the best results:

By moving the transducers around the pipe circumfer-

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ence and checking the velocity it is possible to deter-

mine whether there is a large asymmetry of pro file.

All the published experimental evidence suggests that

because the meter reads low under asymmetric con-

ditions, the highest reading found is likely to be theclosest to the actual mean velocity.

It is possible to get an indication of swirl by compar-ing the velocity from the single reflection and the sin-

gle transmission mode. If they are very different, sev-

ere cross flow is present. The measurement should betreated with caution.

5.7. Couplant and pipe preparation

The purpose of the couplant is to provide a reliable

transmission of ultrasound between the transducer and

pipe wall. Different couplants are used for long and short

term use. The long term types are essentially Araldites,

Eurothane resins and Epoxy resins without fillers that

will diffuse the sound. For permanent installations, reg-ular checking is required. The frequency of checking

depends on supplier and couplant type. Flowmeter diag-nostics may be used to indicate when couplant replace-

ment is due. Short term types are Silicon grease, axle

grease etc. With a short term couplant, it is important to

ensure that it does not dry out. It is recommended that

a thin bead of couplant, about 5 mm by about 3–4 mm

deep is run along the transducer and then compressedon to the pipe.

The following precautions should be taken with

couplants:

Always clean and degrease the transducer pipe area. lf there is a coating on the outside of the pipe it may

be necessary to remove it, particularly a coating con-

taining fibres or metal strengthener.

Care should be taken to ensure that air is not intro-

duced by excessive spreading or mixing.

Excessive amounts of couplants should not be used.

For smaller pipes, the transducers could becomeacoustically connected via the couplant.

If there is pitting in the pipe walls, enough couplant

should be used to cover the pits and make a full

acoustic path.

With plastic pipes, it may be necessary to roughenthe surface slightly to ensure adhesion of epoxy resin.

Care should be taken in dusty/ flaky environments.

Mixing with the couplant can reduce the effectiveness

of the couplant. Also it can make the couplants dry

out.

It must be ensured that users are aware of the Healthand Safety regulations as some of the couplants are

irritants.

The temperature compatibility of the couplant and the

process needs to be checked.

Pipe surface preparation must be carefully done to

preserve the original curvature of the pipe. It is important

that the transducer faces and the pipe axis are parallel as

one degree error could lead to approximately one percent

change in path length.

5.8. Process temperature

Process temperature has several interacting effects. It

affects the speed of sound, fluid density, viscosity and

hence the Reynolds number and the velocity profile.

These effects have been covered in Section 2. Changes

in the speed of sound in the fluid have the effect of

changing the angle of the beam in the fluid and hence

the sensitivity of the flowmeter as shown in Eq. (1). Forprocesses where the process fluid temperature is likely

to change it is important that a flowmeter with speed of

sound compensation is employed.

Process temperature also has an impact on the selec-

tion of transducers and the couplant. Special transducersare required for low and high temperatures. The trans-ducers often use potting to protect the piezoelectric crys-

tals. Temperature can reduce the effective transparencyto sound of some potting materials. Temperature can

cause expansion of the materials at different rates caus-

ing them to ‘split apart’ and either destroy them, or form

air bubbles. If the temperature is too high, the piezoe-

lectric crystals will reduce effectiveness and eventually

stop working. A typical temperature range for a standard

transducer is40°C to +100°C, although transducers are

available which enable measurements to be made from

190°C to 500°C. For high temperature applications thetransducer may be coupled using a metal couplant and

buffered from the process by being mounted on the ends

of long buffer rods.

Care must also be taken with the use of couplants with

temperature. Supplier of couplants should be consulted

for abnormal operation.

At high temperature water based couplants will evap-

orate. At low temperature water based couplants may

freeze and change characteristics.

At high temperature some oil based couplants may

become ‘runny’. At low temperatures some oil based

couplants change their characteristics.

5.9. Pipe size and transducer frequency

All suppliers support a large range of pipe sizes. Dif-

ferent transducers are often used to cover the range.

1 MHz transducers appear to be the standard. These

cover approximately the pipe size range of 50 mm to2000 mm.

2 MHz transducers are used for smaller pipe sizes,

below 100 mm diameter and down to 10 mm.

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0.5 MHz transducers are used for large pipe diam-

eters, approximately 500 mm to 5000 mm diameter. Transducers operating at 4 MHz are offered in very

small sizes.

In general where there are bubbles or particles in the

flow, a lower frequency is recommended. Also if thereare problems with getting sound through the pipe, there

is a tendency to use lower frequencies.

5.10. Pipe diameter and wall thickness measurement

The internal pipe area is required to calculate the volu-

metric flow rate from the measured velocity. As the

internal pipe diameter cannot be measured directly, it is

often inferred from the outside diameter and the wall

thickness. The uncertainty of internal diameter measure-

ment is a major contributor to the flow uncertainty.

Pipe wall thickness is also required, in combination

with the material, to work out the transducer positions.Thus wall thickness will also affect the velocity

measurement.The recommended procedure for pipe internal diam-

eter measurement is as follows:

Measure the circumference of the pipe using a trace-able and accurate tape, or pipe tape.

Check the ovality using calipers. Determine the pipe outside diameter.

Measure the wall thickness.

Subtract twice the wall thickness from the pipe out-

side diameter.

The wall thickness may be obtained by a number of methods:

From a set of drawings or pipe specification.

By using an ultrasonic wall thickness gauge. If the

pipe is lined it may not be possible to tell whether

the thickness includes the liner using this method.

There are devices that could show the different

reflections which could be of some assistance to the

skilled user. If there is build-up on the inside of thepipe, it may not give the correct reading.

Drilling a small hole or using a tapping to insert a

thickness gauge or direct diameter measuring device.At the same time, the pipe walls can be checked for

build-up.

The best method is often to find an off-cut and meas-ure it.

6. Manufacurer specific

6.1. Measurement of Time difference

Most manufacturers use a variant of the leading edge

measurement technique shown in Fig. 17a. In this con-

Fig. 17. a Leading edge measurement. b Phase measurement system.

figuration the transducers are pulsed and the time is mea-sured from the arrival of the receive pulse. In some

designs a phase measurement system, see Fig. 17b, isemployed in which a sinusoidal burst is sent out from

both transducers and the phase of the received signals

compared. Although such a technique enables multiplemeasurement of the phase over several cycles within the

receive burst, it may lead to limitations as to the range

of velocities or pipe sizes over which the measurements

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can be made before phase ambiguity occurs as a conse-

quence of exceeding 360° of phase shift.

6.2. Correction for Reynolds Number

As has been shown in Section 2, clamp-on TTUFs

are sensitive to Reynolds Number. Manufacturers mayor may not compensate for this. For those manufacturers

who do compensate, the compensation is generally based

on a knowledge of the particular liquid being monitored.For products whose temperature is changing signifi-

cantly during the measurement, then compensation for

Reynolds Number on-line is required. Enquiries should

be made of manufacturers as to whether or not they pro-

vide on-line correction.

6.3. Correction for speed of sound

All manufacturers require the user to input either the

speed of sound or identify the liquid (and perhaps its

temperature) in the initialisation stage of the measure-

ment as part of the process to determine transducer sep-

aration. Some manufacturers continue to use this value

for the flow measurement itself whereas other manufac-

turers provide on-line measurement of the speed of sound in the liquid under process conditions. For liquids

for which the temperature is significantly changing, it is

necessary to enquire of the manufacturer what means of

temperature compensation is provided.

6.4. Diagnostics

Nearly all manufacturers provide the user with anindication of the signal strength being received by the

transducers and incorporate some means of automatic

gain control to compensate for attenuation of the ultra-

sound in the pipe wall and in the fluid. The signal

strength indicator may be used in situations where the

separation of the transducers does not appear to be the

correct one and the pipe is then scanned to find the cor-

rect separation.

Additional diagnostics during the measurement phasemay be employed. These diagnostics may measure the

modulation of the received signal (indicating the pres-

ence of a second phase such as air bubbles or solids and

the need to switch operation from TTUF mode to DUF

mode) or the signal to noise ratio of the received signal

which provides an indication of the likely quality of the measurement.

Since most of the measurement systems are micropro-

cessor based they will include a range of diagnostic tools

which will enable the hardware and the software of the

system to be checked and error codes flagged whichwould enable the specific error to be identified. These

diagnostic functions will vary from manufacturer to

manufacturer and enquiries should be made of potential

suppliers as to the range of diagnostic functions they

provide.

7. Future work

These current guidelines have identified a number of areas where at the present time there is insuf ficient data

available to provide firmer advice to the users of this

technology. A programme of work has been identified,for inclusion in the next Department of Trade and Indus-

try Flow Programme, including the following:-

The evaluation of the performance of clamp-on met-

ers in sizes above 600 mm with water as the flow-

ing medium. The performance of clamp-on meters on small sizes

below 50 mm on water since there is a significant

area of application to these devices in water and

energy metering.

The performance of small meters on liquids other than

water since there is a widespread use of small clamp-

on meters in the oil and food sector.

The temperature characteristics of clamp-on flowme-

ters because of their application in process measure-ments where the fluid temperature can vary.

The effect of installation conditions on small bore

clamp-on flowmeter measurements. The effect of pipe roughness on the performance and

operability of clamp-on flowmeters since these meters

are often required to work in conditions where there

is corroded pipework. The sensitivity of meter output to incorrect set-up

information being supplied to the meters. It is clear

that different manufacturers employ different algor-

ithms to compute the flowrate and that the effect of such errors will be different for different manufac-

turers. This work will enable the best algorithms to

be identified and hence the overall performance of

such meters to be improved.

8. Conclusions

These Guidelines have been produced to improve the

use of clamp-on TTUFs. As extent of user experience

increases and further calibrations are undertaken under

a wider range of conditions, a greater understanding of

the performance of such devices is likely and clamp-on

TTUFs with improved performance are likely to emerge.

Acknowledgements

The authors are grateful to the Department of Trade

and Industry for its financial support in the production

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of the Guidelines and its permission for them to be

reprinted here. A CD version of the Guidelines, includ-

ing speed of sound data for pipe materials and liquids

and densities and viscosities of liquids, are available

from the Department of Process and Systems Engineer-ing on CD at the address of the authors, e-mail address:

[email protected].

Further reading

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cation 84, Proc.Symp. On flow in open channels and closed con-

duits. 1977, pp 277–291.

[2] T. Cousins, The Doppler Ultrasonic Flowmeter, Flow Measure-

ment of Fluids, North-Holland Publishing Co., 1978 pp 513-518.

[3] L.C. Lynnworth, Ultrasonic flowmeters, in: W.P. Mason, R. Thur-

ston (Eds.), Physical Acoustics, 14, 1979, pp. 407–525.

[4] D.E. Morris et al. Improve SiCl4 production with clamp-on flow-

meters that avoid corrosion problems. Chemical Engineering Pro-cessing (1981) 16–17.

[5] M.L. Sanderson, J. Hemp. Ultrasonic flowmeter—a review of the

state of the art. Proc.Int. Conf in Advances in Flow Measurement

Techniques, BHRA Fluid Engineering, Bedford, UK. 1981. pp.

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[6] H. Yada, A clamp-on ultrasonic flowmeter for high temperature

fluids in small conduits, in: W.W. Durgin (Ed.), Flow—Its

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Society of America, South Triangle Park, NC, 1981, pp. 546–553.

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[8] R. Keech, The KPC multichannel correlation signal processor for

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[13] K. Spendel. On non-invasive ultrasonic flowmeters. PhD thesis,

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[14] M.S. Beck, A. Plaskowski, Cross Correlation Flowmeters—Their

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[15] E. Hayes. Evaluation of Ultrasonic Flowmeters in Two Phase

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nology, Cranfield, Bedford, UK, 1986.[16] P. Hojolt. Installation effects on single and dual beam ultrasonic

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[18] J. Heritage. Performance of commercial ultrasonic flowmeters.

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[19] R.C. Baker et al. Installation effects electromagnetic and ultra-

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[22] J.E. Heritage. The performance of transit time ultrasonic flowme-

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[24] J. Halttunen. Installation effects on ultrasonic and electromag-

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[25] R. Motegi et al. Widebeam ultrasonic flowmeter. IEEE 1990

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[26] J. Gatke, Problems of transducer coupling with acoustic clamp-

on flowmeters, MSR 34 (5) (1991) 211–217.

[27] J. Szebeszczyk, S. Pietrasz, Clamp-on flowmeter for homo-

geneous liquids, MSR 34 (5) (1991) 208–211.

[28] J. Sweetland. An assessment of clamp-on transit time ultrasonic

flowmeters. CIT Report DJS/18/508/117, Cranfield Institute of

Technology, Cranfield, Bedford, UK, 1992.[29] M. Shortall. Ultrasonics are advancing. Control and Instrumen-

tation (1993).

[30] J. Baumoel. Use of clamp-on transit-time ultrasound flowmeters

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[31] F. Cascetta. Application of a portable clamp-on ultrasonic flow-

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[32] S.T. Lange. Making a choice. Which flowmetering technology?

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