ISO 11631:1998
(Main)Measurement of fluid flow — Methods of specifying flowmeter performance
Measurement of fluid flow — Methods of specifying flowmeter performance
Mesure de débit des fluides — Méthodes de spécification des performances des débitmètres
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 11631
First edition
1998-11-01
Measurement of fluid flow — Methods of
specifying flowmeter performance
Mesure de débit des fluides — Méthodes de spécification des performances
des débitmètres
A
Reference number
ISO 11631:1998(E)
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ISO 11631:1998(E)
Contents
1 Scope .1
2 Normative reference .1
3 Terms and definitions .1
4 General requirements.7
5 Traceability.8
6 Uncertainty of flow measurement.8
7 Conditions of use .8
8 Traceability group.9
Annex A (informative) Illustrations of linearity.11
Annex B (normative) Repeatability.13
Bibliography.15
© ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced
or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm, without permission in writing from the publisher.
International Organization for Standardization
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Internet iso@iso.ch
Printed in Switzerland
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ISO ISO 11631:1998(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide
federation of national standards bodies (ISO member bodies). The work of
preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which
a technical committee has been established has the right to be represented
on that committee. International organizations, governmental and non-
governmental, in liaison with ISO, also take part in the work. ISO
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in
the ISO/IEC Directives, Part 3.
Draft International Standards adopted by the technical committees are
circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting
a vote.
International Standard ISO 11631 was prepared by Technical Committee
TC 30, Measurement of fluid flow in closed conduits, Subcommittee SC 9,
General topics.
Annex B forms a normative part of this International Standard. Annex A is
for information only.
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INTERNATIONAL STANDARD ISO ISO 11631:1998(E)
Measurement of fluid flow — Methods of specifying
flowmeter performance
1 Scope
This International Standard applies to technical specifications and descriptions issued by manufacturers of
flowmeters. It specifies methods of describing the performance of any flowmeter, for use in either closed conduits or
open channels. It indicates how flowmeters may be classified according to their traceability group, and specifies
how manufacturer's statements on traceability, quality assurance and conditions of use should be expressed,
although further statements may be required for other conditions of use.
NOTE The terms and definitions given in clause 3 include a large number of associated terms to encourage common
usage of these terms in technical specifications.
2 Normative reference
The following normative document contains provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, this publication do
not apply. However, parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent edition of the normative document indicated below. For undated references,
the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of
currently valid International Standards.
ISO/TR 5168, Measurement of fluid flow — Estimation of uncertainty of a flowrate measurement.
3 Terms and definitions
For the purposes of this International Standard, the terms and definitions given in ISO 5168 apply, adapted where
appropriate to refer specifically to flowmeters and to describe the characteristics of flowmeters.
3.1
accuracy (deprecated)
^of a flowmeter& ability of a flowmeter to give responses close to a true value
NOTE Accuracy is a commonly used term and can include the effects of systematic and random errors, hysteresis,
deadband, etc. Although it is convenient to combine all these errors under the heading of "accuracy", it is a qualitative term: no
numerical value is attached to it, and it is not used in the performance specification of a flowmeter.
3.2
accuracy class
class of flowmeters which meets certain metrological requirements that are intended to keep errors within specified
limits
NOTE An accuracy class is usually denoted by a number or symbol which is adopted by convention and is called the
class index.
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ISO 11631:1998(E) ISO
3.3
bias
^of a flowmeter& systematic error of the indication of a flowmeter
3.4
calibration
set of operations which establish, under specified conditions, the relationship between values of quantities indicated
by a flowmeter and the corresponding values indicated by a standard of reference
NOTE 1 The result of a calibration permits either the assignment of values of measurands to the indications or the
determination of corrections with respect to indications.
NOTE 2 A calibration may also determine other metrological properties, such as the effect of influence quantities.
NOTE 3 The result of a calibration may be recorded in a document, sometimes called a calibration certificate or a calibration
report.
NOTE 4 The result of a calibration is often expressed as a calibration factor, or as a series of calibration factors, or as a
calibration curve.
NOTE 5 Calibration does not include adjustment.
3.5
confidence limits
lower and upper limits within which the true value is expected to lie with a specified probability, assuming negligible
systematic error
3.6
confidence level
probability that the value will lie between the specified confidence limits, assuming negligible systematic error
NOTE This is generally expressed as a percentage, e.g. 95 %.
3.7
conformity
^of a curve& closeness to which a curve approximates a specified curve (e.g. linear, logarithmic, parabolic, cubic,
square root, etc.), expressed quantitatively
NOTE 1 Conformity is usually measured in terms of nonconformity and expressed as conformity; e.g. the maximum deviation
between an average curve and a specific curve. The average curve is determined after making two or more full-measuring-
range calibrations in each direction. The value of conformity is referred to the output span unless otherwise stated.
NOTE 2 As a performance specification, conformity may be expressed as independent conformity, terminal-based
conformity, or zero-based conformity.
3.7.1
independent conformity
maximum deviation of the actual characteristic (average of upscale and downscale readings) from a specified curve
so positioned as to minimize the maximum deviation
NOTE The maximum deviation can be minimized by using the method of least squares (see ISO 7066-1 and ISO 7066-2).
3.7.2
terminal-based conformity
maximum deviation of the actual characteristic (average of upscale and downscale readings) from a specified curve
coinciding with the actual characteristic at upper and lower range-values
3.7.3
zero-based conformity
maximum deviation of the actual characteristic (average of upscale and downscale readings) from a specified curve
so positioned as to coincide with the actual characteristic at the lower range-value and to minimize the maximum
deviation
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ISO ISO 11631:1998(E)
3.7.4
linearity
specific, but often used, case of conformity in which the specified curve is a straight line
NOTE Illustrations of linearity are given in annex A.
3.8
deadband
maximum interval through which a stimulus can be changed in both directions without producing a change in
response of the flowmeter
NOTE Some flowmeters (turbines for example) may have a “deadband” from zero flow to some flowrate but thereafter have
a small discrimination threshold; ie. they have a minimum starting flow. [See minimum detectable flow (3.20).]
3.9
discrimination
ability of a flowmeter to respond to small changes in the value of the stimulus
3.10
discrimination threshold
largest change in a stimulus that produces no detectable change in the response of a flowmeter, the change in the
stimulus taking place slowly and monotonically
NOTE The discrimination (threshold) may not be constant through the flowrange.
3.11
drift
slow change with time of a metrological characteristic of a flowmeter
NOTE Unlike “stability”, “drift” is always considered with respect to time.
3.12
error
result of a measurement minus a true value of the measurand
NOTE 1 Since a true value cannot be determined, in practice a conventional true value is used.
NOTE 2 When it is necessary to distinguish “error” from “relative error”, the former is sometimes called “absolute error of
measurement”. This should not be confused with “absolute value of error”, which is the modulus of the error.
3.13
experimental standard deviation
s
quantity characterizing the dispersion of the results of a series of n measurements of the same measurand
n
2
∑x−x
()
i
i=1
s=
n−1
where
x is the result of the i th measurement;
i
x is the arithmetic mean of the n results considered.
2
NOTE 1 Considering the series of values as a sample of a distribution, is an unbiased estimate of the mean μ, and s is
n x
2
an unbiased estimate of the variance s , of that distribution.
NOTE 2 The expression is an estimate of the standard deviation of the distribution of x and is called the
sn/
experimental standard deviation of the mean.
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NOTE 3 The term “experimental standard deviation of the mean” is sometimes incorrectly called “standard error of the
mean”.
3.14
flowmeter
flow-measuring device which indicates the measured flowrate
3.15
hysteresis
property of a flowmeter whereby its response to a given stimulus depends on the sequence of the preceding stimuli
NOTE 1 Hysteresis may be expressed quantitatively as the maximum difference between the value of the measurand when
the stimulus is increasing and the value of the measurand when the stimulus is decreasing. An example is shown in Figure 1.
NOTE 2 Hysteresis may be quoted in terms of the measurand or, more usually, as a percentage given by the equation:
H
Hysteresis = × 100(%)
Q −Q
i i
max min
where the terms H, Q and Q are expressed in cubic metres per second and shown in Figure 1.
i i
max min
NOTE 3 Hysteresis does not include the effects of deadband.
Key
1 Indicate flowrate Q
i
Q
2 Reference flowrate
Figure 1 — Typical hysteresis loop
3.16
K-factor
output signal of a flowmeter, expressed in number of pulses per unit quantity
NOTE Where required, this term may carry a subscript to show the unit quantity (e.g. K -factor for pulses per unit mass,
m
K -factor for pulses per unit volume).
v
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3.17
maximum scale value
full scale
full-scale deflection
FSD
full-scale reading
FSR
value of the quantity measured corresponding to the maximum limit of the scale
3.18
measuring range
working range
set of values of a measurand for which the performance of a flowmeter is intended to lie within specified limits
3.19
meter factor
numerical factor by which the meter output is multiplied to obtain the measurand
NOTE This may vary with flowrate and is determined by calibration.
3.20
minimum detectable flow
flowrate at which, when increased from zero, the flowmeter first shows a response
3.21
operating range
region, between the extreme lower and upper range-values, outside which irreversible change in the performance of
the flowmeter will occur
NOTE The operating range may be wider than the “specified measuring range”.
3.22
precision
closeness of agreement between the results obtained by applying the experimental procedure several times under
prescribed conditions
NOTE 1 The smaller the random part of the experimental errors which affect the results, the more precise the procedure.
NOTE 2 This term should not be used as a synonym for accuracy, and no numerical value should be attached to it.
3.23
rangeability
ratio between the maximum and minimum upper range-values
NOTE This term applies to those flowmeters for which the upper range value may be set, by the user, to any value
between upper and lower limits, nominated by the manufacturer, and still maintain the specified performance.
3.24
range of indication
set of values bounded by the extreme indications
NOTE 1 For an analog display, this may be called the “scale range”.
NOTE 2 The range of indication is expressed in the units marked on the scale, regardless of the units of the measurand, and
is normally stated in terms of its upper and lower limits, for example 10 l/s to 20 l/s.
3.25
lower range-value
lowest value of the measurand that a flowmeter is adjusted to measure
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3.26
upper range-value
highest value of the measurand that a flowmeter is adjusted to measure
3.27
repeatability
value below which the absolute difference between two single successive test results obtained with the same
flowmeter on the same fluid under the same conditions (same operator, same test facility, and a short interval of
time, but without disconnecting or dismounting the flowmeter) can be expected to lie with a probability of 95 %
NOTE The method for calculating repeatability is given in annex B.
3.28
resolution
smallest difference between indications of a flowmeter that can be meaningfully distinguished
NOTE For a digital-display device, the resolution is the change in the indication when the least sign
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