British Standard
I S B ) c ( , y p o C d e l l o r t n o c n U , 3 0 0 2 y l u J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b l : y p o C d e s n e c i L
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BRITISH STANDARD
BS EN ISO 6817:1997 BS 5792-1:1993 renumbered, incorporating Amendment No. 1
I S B ) c ( , y p o C d e l l o r t n o c n U , 3 0 0 2 y l u J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b l : y p o C d e s n e c i L
Measurement of conductive liquid flow in closed conduits — Method using electromagnetic flowmeters
The European Standard EN ISO 6817:1995 has the status of a British Standard
ICS 17.120.10
BS EN ISO 6817:1996
Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Industrial-process Measurement and Control Standards Policy Committee (PCL/-) to Technical Committee PCL/2, upon which the following bodies were represented: British Compressed Air Society British Gas plc Department of Trade and Industry (Gas and Oil Measurement Branch) Department of Trade and Industry (National Engineering Laboratory) Electricity Association Energy Industries Council Engineering Equipment and Materials Users’ Association GAMBICA (BEAMA Ltd.) Institute of Measurement and Control Institute of Petroleum Institute of Trading Standards Administration Institution of Gas Engineers Institution of Mechanical Engineers Society of British Gas Industries Water Research Centre Water Services Association of England and Wales
I S B ) c ( , y p o C d e l l o r t n o c n U , 3 The following body was also represented in the drafting of the standard, 0 through subcommittees and panels: 0 2 y l AEA Technology u J 5 2 , h g u o r o b h g u o L f o This British Standard, having y been prepared under the t i direction of the Industrial-process s r Measurement and Control e Standards Policy Committee, v i was published under the n authority of the Standards U , Board and comes r z into effect on Amendments issued since publication v 15 March 1993 c o Amd. No. Date Comments b l © BSI 03-1999 r z 9333 January 1997 Indicated by a sideline in the margin v c The following BSI references o relate to the work on this b l standard: : y Committee reference PCL/2 p Draft for comment 92/29124 DC o C d ISBN 0 580 22041 9 e s n e c i L
BS EN ISO 6817:1997
Contents
I S B ) c ( , y p o C d e l l o r t n o c n U , 3 0 0 2 y l u J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b l : y p o C d e s n e c i L
Page Committees responsible Inside front cover National foreword ii Foreword to EN ISO 6817 2 1 Scope 3 2 Normative references 3 3 Definitions 3 4 Symbols and units 4 5 Theorectical requirements 4 6 Construction and principle of operation 5 7 Installation design and practice 9 8 Equipment marking 13 9 Calibration and test conditions 14 10 Uncertainty analysis 14 Annex A (informative) Materials for construction of primary devices 18 Annex B (informative) Bibliography 19 Annex ZA (normative) Normative references to international publications with their relevant European publication 20 Figure 1 — Principle of an electromagnetic flowmeter 5 Figure 2 — Elements of an industrial electromagnetic flowmeter 6 Figure 3 — Exploded view of the primary device of an electromagnetic flowmeter 7 Figure 4 — Principle of pulsed d.c. (bipolar) system 9 Figure 5 — Shallow taper entry and exit reducers 11 Figure 6 — Cathodically protected pipelines: conductive links across flange joints 12 Figure 7 — Typical accuracy envelopes 15 List of references Inside back cover
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National foreword
I S B ) c ( , y p o C d e l l o r t n o c n U , 3 0 0 2 y l u J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b l : y p o C d e s n e c ii i L
This Part of BS 5792 has been prepared under the direction of the Industrial-process Measurement and Control Standards Policy Committee. It is identical with ISO 6817:1992 Measurement of conductive liquid flow in closed circuits — Method using electromagnetic flowmeters, published by the International Organization for Standardization (ISO). It supersedes BS 5792:1980 which is now withdrawn. This standard now forms Part 1 of two Parts of BS 5792 on electromagnetic flowmeters as follows: — Part 1: Method using electromagnetic flowmeters; — Part 2: Installation dimensions of flanged type electromagnetic flowmeters. In 1995 the European Committee for Standardization (CEN) accepted ISO 6817:1992 as European Standard EN ISO 6817:1995. As a consequence of implementing the European Standard this British Standard is renumbered as BS EN ISO 6817 and any reference to BS 5792-1:1993 should be read as a reference to BS EN ISO 6817. Cross-references International standard
Corresponding British Standard
ISO 4006:1991
BS 5875:1991 Glossary of terms and symbols for measurements of fluid flow in closed conduits
ISO 5168:1978
(Identical) BS 5844:1980 Methods of measurement of fluid flow: estimation of uncertainty of a flow-rate measurement
(Identical) BS 7118 Measurements of a fluid flow: a ssessment of ISO 7066-1:1989 ISO 7066-2:1988 ISO 9104:1991
uncertainty in the calibration and use of flow measurement devices Part 1:1990 Linear calibration relationships
(Identical) Part 2:1989 Non-linear calibration relationships (Identical) BS 7526:1991 Methods of evaluating the performance of electromagnetic flowmeters
(Identical) Informative reference is also made to the following standards: ISO 4185:1980 BS 6199 Measurement of liquid flow in closed conduits using weighing and volumetric methods Part 1:1981 Weighing method
ISO 7194:1983
(Identical) BS 1042: Measurement of fluid flow in closed conduits Section 2.3:1984 Methods of flow measurement in swirling or asymmetric flow conditions in circula r ducts by means of current-meters or Pitot static tubes
ISO 8316:1987
(Identical) BS 6199 Measurement of liquid flow in closed conduits
using weighing and volumetric methods Part 2:1988 Method for measurement by collection of the liquid in a volumetric tank
(Identical)
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BS EN ISO 6817:1997
International standard
Corresponding British Standard
IEC 359:1987
BS 4889:1990 Method for specifying the performance of electrical and electronic measuring equipment
IEC 381-1:1982 IEC 381-2:1978
(Technically equivalent) BS 5863 Analogue signals for process control systems Part 1:1984 Specification for direct current signals (Identical) Part 2:1980 Specification for direct voltage signals (Identical)
A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. I S B ) c ( , y p o C d e l l o r t n o c n U , 3 0 0 2 y l u J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b l : y p o C d e s n e c i L
Compliance with a British Standard does not of itself confer immunity from legal obligations.
Summary of pages
This document comprises a front cover, an inside front cover, pages i to iv, the EN ISO title page, pages 2 to 20, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. © BSI 03-1999
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EUROPEAN STANDARD
EN ISO 6817
NORME EUROPÉENNE September 1995
EUROPÄISCHE NORM ICS 17.120.10
Descriptors: Liquid flow, pipe flow, flow measurements, flowmeters, electromagnetic equipments, installation, specifications, marking
English version
Measurement of conductive liquid flow in closed conduits — Method using electromagnetic flowmeters (ISO 6817:1992) I S B ) c ( , y p o C d e l l o r t n o c n U , 3 0 0 2 y l u J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b l : y p o C d e s n e c i L
Mesure de débit d’un fluide conducteur dans les Durchflußmessung von leitfähigen conduites fermées — Méthode par débitmètres Flüssigkeiten in geschlossenen Leitungen — électromagnétiques Verfahren mit magnetisch-induktiven (ISO 6817:1992) Durchflußmeßgeräten (ISO 6817:1992) This European Standard was approved by CEN on 1995-08-31. CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CEN member. This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the official versions. CEN members are the national standards bodies of Austria, Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.
CEN European Committee for Standardization Comité Européen de Normalisation Europäisches Komitee für Normung Central Secretariat: rue de Stassart 36, B-1050 Brussels
© 1995 All rights of reproduction and communication in any form and by any means reserved in all countries to CEN and its members Ref. No. EN ISO 6817:1995 E
EN ISO 6817:1995
Foreword
This European Standard was taken over by CEN from the work of ISO/TC 30, Measurement of fluid flow in closed conduits, of the International Standards Organization (ISO). This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by March 1996, and conflicting national standards shall be withdrawn at the latest by March 1996. According to the CEN/CENELEC Internal Regulations, the following countries are bound to I implement this European Standard: Austria, SBelgium, Denmark, Finland, France, Germany, B ) Greece, Iceland, Ireland, Italy, Luxembourg, c ( Netherlands, Norway, Portugal, Spain, Sweden, , y Switzerland and the United Kingdom. p o C d e l l o r t n o c n U , 3 0 0 2 y l u J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b l : y p o C d e s n e c 2 i L
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ISO 6817:1992(E)
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1 Scope This International Standard describes the principle and main design features of industrial electromagnetic flowmeters for the measurement of flow-rate of a conductive liquid in a closed conduit running full. It covers their installation, operation, performance and calibration. This International Standard does not specify safety requirements in relation to hazardous environmental usage of the meter, nor does it apply to the measurement of magnetically permeable slurries, liquid metals nor usage in medical applications. This International Standard covers flowmeter types in both a.c. and pulsed d.c. versions.
3 Definitions For the purposes of this International Standard, the definitions given in ISO 4006 and the following definitions apply. Many of these are extracted from ISO 4006 for ease of reference.
2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards. ISO 4006:1991, Measurement of fluid flow in closed
device containing the following elements: — an electrically insulated meter tube through which the conductive liquid to be metered flows, — one or more pairs of electrodes, diametrically opposed, across which the signal generated in the liquid is measured, — an electromagnet for producing a magnetic field in the meter tube. the primary device develops a signal proportional to the flow-rate and in some cases the reference signal
conduits — Vocabulary and symbols.
ISO 5168:1978, Measurement of fluid flow — Estimation of uncertainty of a flow-rate measurement.
ISO 7066-1:1989, Assessment of uncertainty in the calibration and use of flow measurement devices — Part 1: Linear calibration relationships.
ISO 7066-2:1988, Assessment of uncertainty in the calibration and use of flow measurement devices — Part 2: Non-linear calibration relationships.
ISO 9104:1991, Measurement of fluid flow in closed conduits — Methods of evaluating the performance of electromagnetic flow-meters for liquids.
3.1 electromagnetic flowmeter
Flowmeter which creates a magnetic field perpendicular to the flow, so enabling the flow-rate to be deduced from the induced electromotive force (e.m.f.) produced by the motion of a conducting liquid1) in the magnetic field. The electromagnetic flowmeter consists of a primary device and one or more secondary devices. 3.1.1 primary device
3.1.2 secondary device
equipment which contains the circuitry which extracts the flow signal from the electrode signal and converts it to a standard output signal directly proportional to flow-rate. This equipment may be mounted on the primary device 3.2 meter tube
pipe section of the primary device through which the liquid to be measured flows; its inner surface is usually electrically insulated 3.3 meter electrodes
one or more pairs of contacts by means of which the induced voltage is detected 3.4 magnetic field
magnetic flux, generated by the electromagnet in the primary device, which passes through the meter tube and through the liquid
1) In the present International Standard, for electromagnetic flowmeters, the more correct term “liquid” replaces the word “fluid”
(covering liquids and gases) of the general definition in ISO 4006. This usage also aligns with that in ISO 9104. © BSI 03-1999
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3.5 electrode signal
Total potential difference between the electrodes, consisting of the flow signal and the signals not related to flow such as in-phase, quadrature and common mode voltages. 3.5.1 flow signal
that part of the electrode signal which is proportional to the flow-rate and the magnetic field strength and which is dependent on the geometry of the meter tube and the electrodes 3.5.2 I in-phase voltage
S Bthat part of the electrode signal in phase with the ) flow signal but which does not vary with the c ( flowrate , y NOTE 1 This definition applies only to primary devices with p o a.c.-energized electromagnets. C3.5.3 d e quadrature voltage l l o r that part of the electrode signal which is 90° out of t n phase with the flow signal and which does not vary o c with the flow-rate n U3.5.4 , 3 common mode voltage 0 0 voltage which exists equally between each electrode 2 and a reference potential y l u 3.6 J reference signal 5 2 , signal, proportional to the magnetic flux created in h the primary device, which is compared in the g u secondary device with the flow signal o r o 3.7 b output signal h g u output from the secondary device which is a function o L of the flow-rate f o 3.8 y calibration factor of the primary device t i s r a number which enables the flow signal to be related e v to the volume flow-rate (or average velocity) under i n defined reference conditions for a given value of the U , reference signal r z v c o b l r z v c o b l : y p o C d e s n e c 4 i L
3.9 full-scale flowrate
flow-rate corresponding to the maximum output signal 3.10 cathodic protection
electrochemical means of preventing electrolytic corrosion of conduits 3.11 reference conditions
conditions for calibration of a flowmeter in accordance with clause 8 of this International Standard 4 Symbols and units The following symbols are used in this International Standard. Symbol
B D K Le U V k qv
Quantity
Magnetic flux density Inside diameter of meter tube Calibration constant Distance between measuring electrodes Mean axial liquid velocity Flow signal (electromotive force) Constant Volume flow-rate of the liquid
Units
tesla (T) metres (m) metres (m) metres (m) metres per second (m/s) volts (V) (dimensionless) cubic metres per second (m3/s)
5 Theoretical requirements 5.1 General
When a liquid moves in a magnetic field, voltages (e.m.f.s) are generated in accordance with Faraday’s law (see Figure 1). If the field is perpendicular to an electrically-insulated pipe which contains the moving liquid and if the electrical conductivity of the liquid is not too low, a voltage may be measured between two electrodes on the wall of the pipe. This voltage is proportional to the magnetic flux density, the average velocity of the liquid and the distance between the electrodes. Thus the velocity and hence the flow-rate of the liquid may be measured.
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5.2 Basic equation
I S B ) c ( , y p o C d e l l o r t n o c n U , 3 0 0 2 y l u J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b l : y p o C d e s n e c i L
6 Construction and principle of operation
In accordance with Faraday’s law of induction, the strength of the induced voltages is given by the 6.1 General simplified expression as As indicated schematically in Figure 1 and Figure 2, V = kBLeU ...(1) a pipe is so placed with respect to the magnetic field The volume flow-rate in the case of a circular pipe is that the path of the conductive liquid, flowing in the pipe, is normal to the magnetic field. In accordance 2 ; D ...(2) with Faraday’s law, motion of the liquid through the q v = ----------- U 4 magnetic field induces an electromotive force in the which combined with equation (1) gives liquid in a path mutually normal to the field and the direction of liquid motion. By placing electrodes in 2 V ; D q v = ------------- ---- ...(3) insulated mountings or by using insulated 4 kL e B electrodes with capacitance-type coupling in the pipe in a diametrical plane normal to the magnetic or field, a potential difference proportional to the flow V q v = K ---...(4) velocity is produced which can be processed by a B secondary device. Meters based on this principle are Equation (4) may be interpreted in various ways to capable of measuring flow in either direction through the meter tube. produce a calibration factor which in practice is usually determined by wet calibration, as described in clause 9 and in ISO 9104.
Figure 1 — Principle of an electromagnetic flowmeter
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The electromagnetic flowmeter consists of a primary device through which the process liquid flows, and a secondary device which converts the low-level signal generated by the primary device into a standardized signal for suitable acceptance by industrial instrumentation (see, for example, IEC 381). The system produces an output signal proportional to volume flow-rate (or average velocity). Its application is generally limited only by the requirement that the metered liquid shall be electrically conductive and non-magnetic. The primary and the secondary devices can be combined in a single assembly.
Other specific designs are also available, for example, a cast steel case with the coils insulated inside the case and liners fitted internally to this again. Flanges are usually provided to connect the primary device to the plant pipework, although flangeless meters are available in smaller sizes. The coils producing the magnetic field may be energized from the normal single-phase supply, or from some other supply. The coil assembly is either mounted externally or encapsulated within the pipe. In the latter case, the pipe may be made of magnetic material. In industrial electromagnetic flowmeters, the coils in the primary device can be either — a.c. energized, or — d.c. energized. The pulsed direct current (d.c.) meter is one in which the field windings of the primary device are energized from a source creating a pulsating current. The meter samples the signal at zero magnetic field and zero adjusts, but does not differentiate against all other spurious signals. General guidance on various aspects of the primary device is set out in 7.1 and physical features are considered in Annex A.
I 6.2 Primary devices S BThe primary device of an electromagnetic flowmeter ) consists of the coils, a yoke of ferromagnetic c ( , material, the meter tube through which the liquid y flows and the electrodes. The primary device may p o contain circuitry for deriving the reference signal. C d Figure 3 shows an exploded view of an industrial e l l primary device. The coils and the yoke are arranged o r to produce a magnetic field, the meter tube is a t n non-magnetic material such as plastic, ceramic, o aluminium, brass or non-magnetic stainless steel. c n An insulating lining is used with metallic tubes to U , prevent the metal tube from short-circuiting the 3 electrode signal. The lining may be glass, elastomer, 0 0 plastic, ceramic, etc. (see Annex A). The materials 2 y used for the lining and the electrodes are chosen to l u be compatible with the liquid to be metered. J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b Figure 2 — Elements of an industrial electromagnetic flowmeter l : y p o C d e s n e c 6 i L
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Figure 3 — Exploded view of the primary device of an electromagnetic flowmeter
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6.3 Secondary devices
Secondary devices carry out the following processes: a) amplify and process the electrode and reference signals to obtain a signal proportional to flow; b) eliminate, as far as possible, spurious e.m.fs. These include common mode and quadrature signals; c) provide means of compensating for supply voltage and frequency variations where necessary; d) provide means of compensating or minimizing magnetic field strength variations in the primary I S device. This is important since it directly affects B repeatability of the voltage at the measurement ) c electrodes. ( , y Compensation is achieved by the following means: p o a) a gain-compensated amplifier in which the C gain is proportional to the supply frequency and d inversely proportional to the supply voltage; e l l o b) a system in which the output is proportional to r t the ratio of the flow signal and a reference signal n o derived from the field current. At a given c n flow-rate both signals may vary with supply U voltage and frequency, but their ratio will remain , 3 constant; 0 0 c) a system in which the field current is 2 stabilized. y l u J For alternating current (a.c.) energized systems 5 with unregulated coil current, the secondary device 2 , measures the ratio of V/B (see clause 5). Voltages h other than the flow signal (V ) may be picked up by g u electrode leads. These voltages may be generated by o r the varying flux intersecting a loop composed of the o b electrode leads, the electrodes, and the liquid h g connecting the electrodes (transformer effect). Such u a voltage will be approximately 90° out of phase o L with the flow signal. That portion which is 90° out of f o phase is called “quadrature”. The remainder is y called the “in-phase” component. The “in-phase” t i s r component is zeroed at no-flow during initial e installation, unless the flowmeters have a device v i n which provides this function automatically. U , If the coil current is regulated, the magnetic field is r z considered to be constant and it is only necessary to v c measure the electrode signal. If the coil current is o not regulated, then, in order to compensate for b l r variations in the magnetic field, the secondary z device may use a reference signal obtained from the v c primary element. This reference signal may be o b l derived from the supply voltage, the supply current, : the flux density in the metal or the flux density in y p the air gap. o C d e s n e c 8 i L
In a pulsed d.c. system, under ideal or reference conditions, the peak-to-peak value of the electrode signals, (V p + V n), is proportional to the flow velocity in the pipeline and V p is also equal to V n [see Figure 4 a)], where V p = positive voltage and V n = negative voltage. In a practical situation, if the zero or “no-flow” signal is offset in the positive direction by an amount V e then the positive signal is (V p + V e) and the negative signal is (V n – V e) [Figure 4 b)]. Hence the overall value of the electrode signal is (V p + V n) and the offset zero is eliminated. The same applies if the offset is in the negative direction. The system thus eliminates zero errors automatically at all times and zero adjustment is not usually required, either at start-up/commissioning or at any time during subsequent operation. General guidance on the function and installation of secondary devices is presented in 7.2. 6.4 System output
The system output can be one or more of the following: a) analog direct current in accordance with IEC 381-1; b) analog direct voltage in accordance with IEC 381-2; c) a frequency output in the form of scaled or unscaled pulses; d) digital. 6.5 Effect of the liquid conductivity
If the electrical conductivity of the liquid is uniform in the measuring section of the meter, the electric field distribution is independent of the liquid conductivity and therefore the meter output is generally independent of the liquid conductivity. Minimum operational conductivity requirements should be obtained from the manufacturers. The internal impedance of the primary device obviously depends upon the liquid conductivity, and very large changes in this impedance may produce errors in the output signal. If the conductivity is not uniform throughout the meter, errors may also occur. A heterogeneous fluid composed of small particles uniformly distributed in a medium can be considered as a homogeneous liquid. Deposition of electrically conducting layers on the inside surface of the liner may also lead to errors. 6.6 Reynolds number effect
In industrial electromagnetic flowmeters, the effect of Reynolds number is usually so small that for practical purposes it can be ignored.
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6.7 Velocity profile effect
Distortions in velocity profiles may be caused by pipe fittings (bends, valves, reducers, etc.) placed upstream or downstream from the flowmeter; the resulting flow patterns may have an influence on the performance of the meter. In general, the user should comply with the manufacturer’s recommendations for installation in order to minimize these effects. Flow pattern effects are described in 7.1.2.1.
I S B ) c ( , y p o C d e l l o r t n o c n U , 3 0 0 2 y l u J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b l : y p o C d e s n e c i L
7 Installation design and practice 7.1 Primary devices 7.1.1 Size
Usually the bore of the primary device tube will be the same as that of the adjacent pipework. If, in this case, the mean axial velocity corresponding to the maximum flow-rate is less than that recommended by the manufacturer, a primary device with a smaller bore should be used. A primary device with a bore smaller than that of the adjacent pipework may also be used for other reasons, e.g. to reduce cost or in the interests of rationalization. Information on the allowable tolerances for matching the pipe and meter tube bores is given in ISO 9104.
Figure 4 — Principle of pulsed d.c. (bipolar) system
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7.1.2 Layout
7.1.2.2 Full pipe requirement
There is no theoretical restriction on the attitude at which a primary device may be mounted, provided the pipe remains full at all times. Locations close to electrical equipment which may interfere with the flow measurement signal, or locations where currents may be induced in the primary device, should be avoided.
The primary device shall be mounted in such a position that it will be completely filled with the liquid being metered, otherwise the measurement will not be within the manufacturer’s stated accuracy. If necessary, warning devices should be installed to preserve measurement integrity. Partially-filled primary device meters are used, for example in sewage applications, but these merit special consideration outside the scope of this International Standard.
7.1.2.1 Effect of layout on velocity distribution
Ideally, the magnetic field should be so arranged that the calibration factor is always the same, irrespective of the flow pattern. Though this can be 7.1.2.3 Electrode position done in flowmeters with special electrode Since any gas bubbles will rise and collect at the top I of the pipe, or sediment may collect at the bottom of Sarrangements, it cannot be achieved if small the pipe, the primary device should be mounted so Belectrodes are used. In practice, when a flow velocity ) profile which is significantly different from that in that neither electrode is in these positions c ( the original calibration is presented to the electrode (see also 7.1.3.1). , y plane, an electromagnetic flowmeter may exhibit a p 7.1.2.4 Zero-checking provision o shift in calibration. The arrangement of pipe fittings C upstream of the primary device is one of the factors In order to check the flowmeter zero, means should d be provided to stop the flow through the primary e which can contribute to the creation of a particular l l device, leaving it filled with stationary liquid. velocity profile. o r t However, in the case of a synchronous d.c. pulsed n Precise data on the effects of flow disturbances is not o field supply with an automatically adjusting zero, c always available, but for most electromagnetic n flowmeters it is recommended that any source of this provision may not be necessary. U , flow disturbance, such as a bend, should be at least 7.1.2.5 Multiphase flow through the primary device 3 0 ten pipe diameters upstream of the electrode plane 7.1.2.5.1 Entrained solids 0 if the performance is not to be altered by more 2 For the measurement of liquids containing abrasive y than 1 %. When the distance is unavoidably less l materials, vertical mounting is recommended to u than this, the manufacturer’s advice should be J ensure evenly distributed lining wear. Where there sought. 5 is a possibility that material may settle in the 2 Swirling flow can also alter the calibration factor , primary device, it should be mounted vertically or h because, although flow components perpendicular g provision should be made to flush it through. u to the pipe axis cannot contribute to the flow-rate, o r they may contribute to the signal. Furthermore, the A ring to protect the leading edge of the magnetic o flowmeter is sometimes used. This ring shall be b amount and distribution of swirl arising from h various upstream pipe configurations, such as designed to ensure streamlined flow. g u several bends in different planes, is difficult to 7.1.2.5.2 Entrained gases o L predict from the geometry of the pipework. When An electromagnetic flowmeter measures total f o swirling flow is suspected, it is good practice to volume flow. Entrained gases cause measurement y insert a swirl reducer upstream of the primary t inaccuracies in direct relation to the volume i s r device; some types of swirl reducers are described in percentage of gas to liquid. Precautions should be e ISO 7194. taken to reduce this effect by increasing the liquid v i n When the primary device is connected to the circuit pressure, e.g. by locating the primary device on the Uby means of conical pieces, the effect on the high-pressure side of a restrictor such as a control , r calibration factor due to the irregular flow pattern valve, or by eliminating the entrained gas. z v may be either reduced or amplified according to the c 7.1.2.5.3 Phase slippage o type of irregularity (swirl, asymmetry, etc.) and the b l In the case of entrained solids and/or gases, relative r design of the connecting piece (convergence, z divergence, value of total angle, etc.). average motion of the phases can affect the v c performance. This condition is particularly likely if o the tube is mounted vertically. In such situations b l : the user should consult the manufacturer. y p o C d e s n e c 10 i L
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7.1.3 Pipework connections
7.1.3.3 Connecting pieces
7.1.3.1 Design
To minimize pressure loss and flow disturbances in cases where an undersized meter is installed, it is advisable to connect the primary device into the pipework by means of shallow tapered cone pieces (recommended maximum included angle 15°) (see Figure 5). In this case, the inlet and outlet straight pipe sections shall be the same size as the flowmeter (see 7.1.2). Eccentric taper pipes shall be used when the pipeline is horizontal, to prevent air pockets from forming.
When designing the piping system, access for installing and removing the primary device as well as access to the electrical connections should be provided. Means should be provided for adjusting and aligning the adjacent pipework. Extra care should be taken during pipework construction to prevent excessive strain on the primary device, both during and after installation. Every effort should be made to minimize piping loads and resulting strains at the primary device connecting flanges, particularly in plastic meters which are not intended to sustain piping loads. Permissible values should be checked with the manufacturer. 7.1.3.2 Pipework adjustment
There should be means for adjusting the distance between pipework flanges used for mounting the flowmeter and for aligning the adjacent pipework. It is essential that the primary device is correctly aligned on the pipe axis when it is bolted into the pipework. Wafer types require special care. Flange bolts should be tightened evenly and in moderation in order to avoid damage to the lining. The manufacturer should state the maximum permissible torque. Care should be taken when handling the primary device; slings around the primary device, or lifting lugs, should be used. Lifting by any means that could damage the liner, for example, hooks in the bore, shall not be used.
7.1.4 Electrical installation 7.1.4.1 General requirements
The metered liquid and the primary device body should be at the same potential, preferably earth potential. In the case where cathodic protection is used to protect buried pipework, this precaution becomes essential (see 7.1.4.3). The connection between the liquid and the primary device body may be made by contact with the adjacent pipework; or, where insulated or non-conductive pipework is used, by conductive (earthing) rings or electrodes. Equipotential conductive links (usually copper braids) should be fitted across both flange joints (see Figure 6). The manufacturer’s instructions should be carefully followed for interconnections between the primary device and the secondary device. The power supply should be taken from a point that is as free as possible from transient voltages. Instructions in relation to electrical grounding of the flowmeter system shall be rigidly observed.
Figure 5 — Shallow taper entry and exit reducers
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ISO 6817:1992(E)
I S B ) c ( , y p o C d e l l o r t n o c Figure 6 — Cathodically protected pipelines: conductive links across flange joints n U The first requirement is that the primary device , 7.1.4.2 Power factor (a.c. systems only) 3 body and the liquid be at the same potential. This 0 As the primary device has coils to provide the 0 may be achieved simply by adequate electrical 2 magnetic field, it is an inductive device and causes bonding between the primary device body and the y the field current to lag the supply voltage by an l adjacent piping, or, where insulated or u angle approaching 90°, thus giving a poor power J non-conductive pipework is used, one of the 5 factor. Typical values range from 0,1 to 0,4 2 conductive “earthing” rings or electrodes. , depending on the size of the primary device. To Series-mode voltage cannot be rejected by the h improve the power factor, correction capacitors may g secondary device. u be connected in parallel with the supply and may be o r fitted externally or within the primary device Under bonded conditions with cathodic protection o the electric supply earth should not be connected to b enclosure, by arrangement with the manufacturer. h the primary device body, otherwise the protection g 7.1.4.3 Precautions to be observed where cathodic u current will be bypassed to the supply earth. o protection is used L With cathodic protection systems on long pipelines, f When an electromagnetic flowmeter primary device o the cathodic current is often obtained from several is installed in a cathodically protected pipeline, y sources. These may be a considerable distance t i special precautions are necessary to ensure that the s apart, and at different potentials owing to variation r e d.c. component of the cathodic current does not in earth resistance along the length of the pipeline. v affect the accuracy and stability of the flowmeter i This may cause high currents to flow in the pipeline, n Usystem. In such a case, the flowmeter manufacturer which, if allowed to flow through a primary device , should always be consulted for installation advice. r body, may cause inaccuracy in measurements. z v The precautions necessary will depend on the Provision of an insulating flange and conductive c o location of the primary device relative to other parts links, as shown in Figure 6 (upper drawing), b l of the cathodic protection system. obviates this effect. r z the flowmeter connected to the pipework or removed v 7.1.5 Cleaning and maintenance of the primary c from it (see 7.1.5.1). o device b l Bullet-shaped electrodes to reduce coating, : If insulating materials are likely to be deposited y from the conducting liquid onto the electrodes or the ultrasonic cleaning methods and capacitative signal p o walls of the meter tube, provision should be made pick-up may minimize such effects. C for mechanical, electrical or chemical cleaning, with d e s n e © BSI 03-1999 c 12 i L
ISO 6817:1992(E)
Currently used cleaning methods are described in 7.1.5.1 to 7.1.5.4. 7.1.5.1 Withdrawable electrodes
Withdrawable electrodes can be provided by using mechanical valving and sealing arrangements so that the electrodes can be withdrawn (usually at full pipeline pressure) for external inspection and cleaning. 7.1.5.2 Mechanical scraper
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In this system a rotary scraper is fitted to each electrode such that its scraping edge is perpendicular to the electrode face. The scraper is driven by an external electric motor via fluid pressure seals. It may be used continuously or intermittently. This method is becoming less commonly used in modern electromagnetic flowmeters. 7.1.5.3 Ultrasonic cleaning
A high energy ultrasonic wave is induced in each electrode shaft by means of an external oscillator and transducer. The shaft length and the frequency of ultrasound are chosen to produce an antinode at the electrode face. Deposits are removed by the resultant local cavitation at the electrode. This approach is generally used on crystalline-type coatings. 7.1.5.4 Electrolytic or “burn-off” method
In this method a voltage from the mains supply is connected between the electrodes (the secondary instrument being automatically disconnected during this operation), causing electrolysis on the surface of each electrode. The resultant rapid gas evolution causes removal of deposits. This approach is generally used on oily, greasy and sludge-type coatings. Heating of the electrodes can also be used to remove fat or grease deposits from sewage. 7.2 Secondary devices 7.2.1 Location
Secondary devices should be installed in an accessible position free from excessive vibration, due regard being given to the manufacturer’s specifications for ambient temperature and humidity. In particular, direct solar irradiation shall be avoided.
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7.2.2 Electrical installation
The cables carrying the electrode and reference signals should be of the type approved by the manufacturer. These cables should be as short as possible and not exceed the limit imposed by the manufacturer. Care should be taken to ensure that signal cables are not routed in proximity to high current cables. Good earthing practice should be observed with particular attention being paid to the prevention of “earth loops”. 8 Equipment marking 8.1 Primary device 8.1.1 Mandatory data
The following data shall be impressed either on the primary device or on a name plate: a) instrument type and serial number; b) rated pressure and temperature; c) power supply: voltage, frequency and power (when independently powered). 8.1.2 Optional data
The following data may be optionally provided: a) enclosure protection rating (in conformity with ISO/IEC publications); b) nominal diameter; c) calibration factor; d) lining material; e) electrode material. NOTE 2 Additional information such as trademark, mass of unit, date of manufacture, flow direction arrow, etc., may be included if the size of the name plate permits. 8.2 Secondary device 8.2.1 Mandatory data
The following data shall be impressed on a name plate: a) instrument type and serial number; b) power supply: voltage, frequency and power; c) output signals; d) limiting load impedance.
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ISO 6817:1992(E)
8.2.2 Optional data
9.3 Interpretation of results
The following data may be optionally provided: a) enclosure protection rating (in conformity with ISO/IEC publications).
9.3.1 Reference accuracy envelope
NOTE 3 Additional information such as trademark, date of manufacture, etc., may be included if the size of the name plate permits.
9 Calibration and test conditions 9.1 Wet calibration
The calibration factor should be determined by a wet calibration using water in the test facility at reference (nominal calibration) conditions (see 9.2). I The conditions under which this is carried out S Bshould be such that the measurements are traceable ) to national or International Standards, and hence c ( , that the calibration is to a known uncertainty. For y example, ISO 4185 and ISO 8316 describe suitable p o calibration methods. ISO 9104 can also be consulted C for methods used in evaluation of electromagnetic d e flowmeters. l l o r Where the primary device is too large to be installed t n in a manufacturer’s test facility, or where the o c facility has insufficient flow capacity, another n higher capacity test facility can be used or, if this is U , not possible, a site calibration may be carried out 3 0 using the user’s tank or reservoir or by a comparison 0 with another reference flowmeter in the system. The 2 y overall uncertainty shall be determined as specified l u in clause 10. J 5 The method of computing the primary device signal 2 based on magnetic field strength measurements and , h on physical dimensions, commonly referred to as g u “dry calibration”, is beyond the scope of this o r International Standard. o b 9.2 Nominal calibration conditions h g u Nominal calibration conditions are those conditions o L which shall exist at the time of calibration. These f nominal calibration conditions should be specified o y by the manufacturer. For comparison purposes, the t i flowmeter should be tested within the range of s r ambient and flow conditions defined in ISO 9104. e v i Provided that the flowmeter has reached thermal n Uequilibrium, it is normally assumed that influencing , factors have a negligible effect on the metrological r z v characteristics of the flowmeter, so far as they c o remain within the operating limits stated by the b l manufacturer. r z v c o b l : y p o C d e s n e c 14 i L
The manufacturer should provide the range of operating conditions, together with the effect of these on the performance. Reference should also be made to ISO 9104 for fuller information on this subject. It is current practice to specify a reference accuracy envelope over a designated flowrate range. Typical reference accuracy envelopes are shown in Figure 7. 9.3.2 Accuracy at reference conditions
Flowmeter accuracy, at reference conditions, is determined by the combined random and systematic uncertainties in the measurement of the flowmeter signal and the volume flow-rate. A summary of uncertainty analysis in this context is given in clause 10. The upper and lower uncertainty limits on each data point shall be within the manufacturer’s accuracy envelope (see Figure 7). 9.3.3 Deviation from reference conditions
Deviation from reference test conditions may affect flowmeter performance. While these effects are normally compensated for in the secondary device, limits of error for each influencing quantity should be specified by the manufacturer. 9.4 Pressure testing
The primary device or meter tube shall be subjected to testing in accordance with an appropriate pressure code standard if required. 10 Uncertainty analysis The calculation of the uncertainty in the measurement of flow-rate shall be carried out as specified in ISO 5168. However, it is useful to recall some general principles and to present the way in which they apply to measurement using an electromagnetic flowmeter. The fitting of curves to specific sets of calibration or user data from flow measurement devices is covered in ISO 7066-1 and ISO 7066-2. 10.1 General 10.1.1 Definition of the error
The error in the measurement of a quantity is the difference between the measured and the true values of the quantity.
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ISO 6817:1992(E)
I S B ) c ( , y p o C d e l l o r t n o c n U , 3 0 0 2 y l u J 5 2 , h g u o r o b h g u o L f o y t i s r e v i n U , r z v c o b l r z v c o b l : y p o C d e s n e c i L
Figure 7 — Typical accuracy envelopes
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ISO 6817:1992(E)
No measurement of a physical quantity is free from uncertainties arising either from systematic errors or from the random dispersion of measurement results. Systematic errors cannot be reduced by repeating measurements, since they arise from the characteristics of the measuring apparatus, the installation and the flow characteristics. However, a reduction in the random error may be achieved by repetition of measurements, since the random error of the mean of n independent measurements is n times smaller than the random error of an individual measurement. 10.1.2 Definition of the standard deviation I 10.1.2.1 If variable X is measured several times, Seach measurement being independent of the others, B ) then the standard deviation s X of the distribution of c ( n measurements, X i, is , y p o C d e l l o r t n o c n where U , is the arithmetical mean of the n 3 X 0 measurements of the variable X ; 0 2 X i is the value obtained by the ith y l measurement of the variable X ; and u J is the total number of measurements of X . n 5 2 , For brevity, s X is normally referred to as the h standard deviation of X . g u 10.1.2.2 If repeated measurements of a variable X o r o are not available or are so few that direct b computation of the standard deviation on a h g statistical basis is likely to be unreliable, and if the u o maximum range of the measurements may be L f estimated, the standard deviation may be taken as o one-quarter of this maximum range (i.e. as one-half y t i of the estimated uncertainty above or below the s r adopted value of X ). In the same way, it is assumed e v that a systematic component of the error may be i n characterized by a standard deviation equal to Uone-half of the plus or minus maximum expected , r z value range of that component. v c 10.1.3 Definition of the uncertainty o b l 10.1.3.1 For the purpose of this International r z Standard, the uncertainty in a measurement of a v c variable is defined as twice the standard deviation o of the variable. The uncertainty shall be calculated b l : and quoted under this appellation whenever a y p measurement is claimed to be in conformity with o this International Standard. C d e s n e c 16 i L
When partial errors, the combination of which gives the uncertainty, are independent of one another, are small and numerous, and have a Gaussian distribution, there is a probability of 0,95 that the true error is less than the uncertainty. 10.1.3.3 Having estimated the standard deviation sqV of the flow-rate measurement qV , the uncertainty eqV is given by eqV = ± 2sqV The relative uncertainty E qV is defined by 10.1.3.2
e
s
q V
q V
qV q V = -------= 2 -------± V
E q
The result of a flow measurement shall always be given in one of the following forms: a) flow-rate = qV ± eqV (at the 95 % confidence level); b) flow-rate = qV (1 ± E qV ) (at the 95 % confidence level); c) flow-rate = qv within ± 100 E qV % (at the 95 % confidence level). 10.2 Calculation of the uncertainty in flow-rate measurement 10.2.1 Sources of error
In the case of a flow-rate measurement carried out by an electromagnetic flowmeter, the possible sources of error are essentially as follows: a) systematic error in the measurement of the output signal, arising from the equipment used; b) random error in the measurement of the output signal; c) error due to the flow conditions, which are generally different from those prevailing during the calibration of the flowmeter; this error comprises both systematic and random components; d) error arising from the uncertainty in the relationship qV ( X ) between the flow-rate qV and the output signal X . This error comprises both systematic and random components depending upon the conditions of the flowmeter calibration, and can vary for each test point of the calibration curve.
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ISO 6817:1992(E)
10.2.2 Propagation of the individual uncertainties
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The uncertainty in the flow-rate measurement is assessed by combining the individual uncertainties arising from the various sources listed in 10.2.1. Although systematic errors have been distinguished from random errors, the probability distribution of the possible values of each systematic component is essentially Gaussian. The combination of the random and systematic errors may therefore be treated as though all were truly random and, according to the ISO 5168, the relative standard deviation of the flow-rate measurement may be taken as the square root of the sum of the squares of the relative standard deviations arising from the various sources. Thus the result of the flow-rate measurement is
at the 95 % confidence level, where sR X is the standard deviation associated with the systematic error in the output signal measurement; sR X is the standard deviation of the random error in the output signal measurement; sf is the standard deviation arising from flow conditions; and is the standard deviation in the calibration sc relationship. In the case where the calibration relationship offers the simple form qV = K 1 X , the above formula reads
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ISO 6817:1992(E)
Annex A (informative) Materials for construction of primary devices A.1 Introduction
It is very important to choose construction materials suitable for the liquid to be metered. It is essential that account is taken of any other chemicals liable to pass through the meter tube, such as sterilizing agents, cleaners and solvents. As the user knows the properties of the liquid to be metered, the ultimate decision on the materials to be used should rest with him. I It is also important that the lining material is not Ssubjected to temperatures outside the range Brecommended by the manufacturer. The maximum ) c ( permissible pressure of the primary device is , y normally decreased as the temperature of the p process fluid is increased. o C A.2 Meter tube linings d e The following are examples of types of lining l l materials that are available. o r t n A.2.1 Elastomers o c A.2.1.1 Hard rubber (ebonite) n UHard rubber is generally suitable for use within the , 3 temperature range 0 °C to 90 °C. It has excellent 0 abrasion resistance against small particles and good 0 2 chemical resistance, particularly to leaching agents, y acid and alkalis. l u J A.2.1.2 Abrasion-resistant rubbers (natural) 5 2 Abrasion-resistant natural rubbers are generally , h suitable for use within the temperature g range – 20 °C to + 70 °C. They exhibit excellent u o wear resistance and good chemical resistance. r o b A.2.1.3 Neoprene h g Neoprene is generally suitable for use within the u temperature range 0 °C to 100 °C. It has good o L chemical and wear resistance properties, f o particularly in the presence of oil and greases. y t i NOTE 4 All rubber-based materials are attacked by high s r concentrations of free halogens, aromatic and halogenated e hydrocarbons and high concentrations of oxidizing chemicals. v i n A.2.1.4 Polyurethane UPolyurethane is generally suitable for use within , r z the temperature range of – 50 °C to + 50 °C. It v exhibits excellent wear and impact resistance. c o A.2.1.5 Other elastomers b l r Other elastomers are generally suitable for use as z v lining materials and may be used, as agreed c o between the user and the manufacturer. b l : y p o C d e s n e c 18 i L
A.2.2 Plastics A.2.2.1 Polytetrafluoroethylene (PTFE)
Usually as an extruded sleeve form not bonded to the meter tube, PTFE is generally suitable for use within the temperature range – 50 °C to + 200 °C. It has excellent wear resistance against small particles, and is chemically inert. It may collapse when subjected to sub-atmospheric pressures. For medium temperatures above 120 °C, advice on the maximum permissible pressure should be sought from the manufacturer. A.2.2.2 Polyamide
Polyamide is generally suitable for use at temperatures below 65 °C. It has good wear resistance properties. A.2.2.3 Chlorinated polyether
Chlorinated polyether is generally suitable for use at temperatures below 120 °C. It has excellent chemical resistance to caustic soda, acids in concentrations up to 30 % and brine. A.2.2.4 Glass-reinforced plastics (GRP)
GRP may be used as a lining material or for the meter tube itself. It is generally for use within the temperature range – 20 °C to + 55 °C and is particularly suitable for the largest size of primary devices. A.2.3 Ceramics
This construction material requires no lining, exhibits high form and measuring stability under pressure and temperature variations, and possesses excellent abrasion resistance. Additionally, high chemical resistance to acids and alkaline solutions is characteristic of high-purity Al 2O3 ceramics. The service temperature range is from – 60 °C to + 250 °C with full vacuum resistance. A.2.4 Vitreous enamel
Vitreous enamel is generally suitable for temperatures up to 150 °C, with excellent chemical and wear resistance, but requires careful handling and avoidance of exposure to hydrofluoric acid. A.3 Examples of electrode materials A.3.1 For non-corrosive liquids
Stainless steel is generally used. A.3.2 For corrosive liquids
The following may be suitable, depending on the chemical properties of the liquid to be metered: — stainless steel; — some nickel-based alloys; — platinum; — platinum/iridium; — tantalum;
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ISO 6817:1992(E)
— titanium. A.4 Meter tube and enclosure
The materials used for the meter tube, flanges and enclosure are usually specified by the manufacturer. It is essential that they be compatible with the environmental conditions in which they are to be used. The materials listed in A.3 may be used for parts of meter tubes that come into contact with the metered liquid, i.e. partially lined tubes.
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Annex B (informative) Bibliography [1] ISO 4185:1980, Measurement of liquid flow in closed conduits — Weighing method . [2] ISO 7194:1983, Measurement of fluid flow in closed conduits — Velocity-area methods of flow measurement in swirling or asymmetric flow conditions in circular ducts by means of current-meters or Pitot static tubes .
[3] ISO 8316:1987, Measurement of liquid flow in closed conduits — Method by collection of the liquid in a volumetric tank.
[4] IEC 359:1987, Expression of the performance of electrical and electronic measuring equipment . [5] IEC 381-1:1982, Analogue signals for process control systems. Part 1: Direct current signals . [6] IEC 381-2:1978, Analogue signals for process control systems. Part 2: Direct voltage signals .
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ISO 6817:1992(E)
Annex ZA (normative) Normative references to international publications with their relevant European publications This European Standard incorporates by dated or undated reference, provisions from other publications. These normative references are cited at the appropriate places in the text and the publications are listed hereafter. For dated references, subsequent amendments to or revisions of any of these publications apply to this European Standard only when incorporated in it by amendment or revision. For undated references the latest edition of the publication referred to applies (including amendments). Publication
Year
Title
EN
ISO 4006
1991
EN 24006 1993
ISO 9104
1991
Measurement of fluid flow in closed conduits — Vocabulary and symbols Measurement of fluid flow in closed conduits — Methods of evaluating the performance of electromagnetic flow-meters for liquids
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Year
EN 29104 1993
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BS EN ISO 6817:1997
List of references See national foreword.
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BS EN ISO 6817:1997
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