ISO 14511:2001

Measurement of fluid flow in closed conduits — Thermal mass flowmeters

Mesure de débit des fluides dans les conduites fermées — Débitmètres massiques par effet thermique

General Information

• Status: Withdrawn
• Publication Date: 17-Oct-2001
• Withdrawal Date: 17-Oct-2001
• ICS: 17.120.10 - Flow in closed conduits
• Technical Committee: ISO/TC 30/SC 5 - Velocity and mass methods
• Drafting Committee: ISO/TC 30/SC 5 - Velocity and mass methods
• Current Stage: 9599 - Withdrawal of International Standard
• Completion Date: 10-Jan-2019

Relations

• Revised: ISO 14511:2019 - Measurement of fluid flow in closed conduits — Thermal mass flowmeters
• Effective Date: 23-Apr-2020

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 14511
First edition
2001-10-15
Measurement of fluid flow in closed
conduits — Thermal mass flowmeters
Mesure de débit des fluides dans les conduites fermées — Débitmètres
massiques par effet thermique
Reference number
ISO 14511:2001(E)
©
ISO 2001

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ISO 14511:2001(E)
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ISO 14511:2001(E)
Contents Page
Foreword.iv
Introduction.v
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
3.1 General terms.1
3.2 Specific terms .2
4 Selection of thermal mass flowmeters .4
5 Capillary thermal mass flowmeter (CTMF meter).5
5.1 Principles of measurement.5
5.2 Typical design.7
5.3 Applications and limitations of use .8
5.4 Meter selection.10
5.5 Installation and commissioning.12
6 Insertion and/or in-line thermal mass flowmeter (ITMF meter).13
6.1 Principles of measurement.13
6.2 Typical design.16
6.3 Applications and limitations of use .18
6.4 Meter selection.20
6.5 Installation and commissioning.22
7 Instrument specification sheet and marking .24
7.1 User specification sheet .24
7.2 Manufacturer's data sheet .24
7.3 Marking .25
8 Calibration .27
8.1 General considerations.27
8.2 Use of the desired gas under process conditions .27
8.3 Use of a surrogate gas .27
8.4 In-situ calibration.27
8.5 Insertion-ITMF meter .27
8.6 Calibration frequency.28
8.7 Calibration certificate .28
9 Pre-installation inspection and testing .29
10 Maintenance .29
10.1 General.29
10.2 Visual inspection .29
10.3 Functional test .30
10.4 Record keeping (maintenance audit trail) .30
Bibliography.31
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ISO 14511:2001(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.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
International Standard ISO 14511 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in
closed conduits, Subcommittee SC 5, Velocity and mass methods.
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ISO 14511:2001(E)
Introduction
This International Standard has been prepared to guide those concerned with the specification, testing, inspection,
installation, operation and calibration of thermal mass gas flowmeters.
A list of standards related to ISO 14511 is given in the bibliography.
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INTERNATIONAL STANDARD ISO 14511:2001(E)
Measurement of fluid flow in closed conduits — Thermal mass
flowmeters
1 Scope
This International Standard gives guidelines for the specification, testing, inspection, installation, operation and
calibration of thermal mass gas flowmeters for the metering of gases and gas mixtures. It is not applicable to
measuring liquid mass flowrates using thermal mass flowmeters.
This International Standard is not applicable to hot wire and other hot film anemometers, also used in making point
velocity measurements.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent editions of the normative documents 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 4006, Measurement of fluid flow in closed conduits — Vocabulary and symbols
ISO 7066-2, Assessment of uncertainty in the calibration and use of flow measurement devices — Part 2: Non-
linear calibration relationships
Guide to the expression of uncertainty in measurement (GUM). BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML, 1st edition,
corrected and reprinted in 1995
IEC 61000-4, Electromagnetic compatibility (EMC) — Part 4: Testing and measurement techniques
3 Terms and definitions
For the purposes of this International Standard, the terms and definitions given in ISO 4006 and the following
apply.
NOTE The term “gas” is used as a synonym for single gases, gas mixtures and vapours.
3.1 General terms
3.1.1
flowrate
quotient of the quantity of fluid passing through the cross-section of a conduit and the time taken for this quantity to
pass through this section
NOTE In this International Standard, the term “flowrate” is used as a synonym for mass flowrate, unless otherwise stated.
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ISO 14511:2001(E)
3.1.2
mass flowrate
flowrate in which the quantity of fluid is expressed as a mass
NOTE The term “flowrate” is used as a synonym for mass flowrate in this International Standard, unless otherwise stated.
3.1.3
accuracy of measurement
closeness of the agreement between the result of a measurement and a true value of the measurand
NOTE Accuracy is a qualitative concept.
3.1.4
uncertainty of measurement
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that can
reasonably be attributed to the measurand
3.1.5
repeatability
�measuring instrument� ability of a measuring instrument to provide closely similar indications for repeated
applications of the same measurand under the same conditions of measurement
NOTE These conditions include:
� minimized variations resulting from the observer;
� the same measurement procedure;
� the same observer;
� the same measuring equipment, used under the same conditions;
� the same location;
� repeated measurements within a short period of time.
3.1.6
flow profile
graphic representation of the velocity distribution
NOTE The point flow velocity across the cross-section of a conduit is not constant. It varies as a consequence of upstream
and downstream disturbances and with the Reynolds number of the flow stream. For a fully developed flow, the point flow
velocity varies from 0 m/s at the pipe wall to a maximum value at the conduit centre. The flow profile describes the variation of
the flow velocity across the conduit cross-section and may be expressed mathematically or graphically.
3.2 Specific terms
3.2.1
sensor
element of a measuring instrument or measuring chain that is directly affected by the measurand
3.2.2
laminar flow element
element inserted into the gas stream to establish a constant ratio between the main flow stream and the bypass
flow through the sensor
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ISO 14511:2001(E)
3.2.3
thermal mass flowmeter
TMF meter
flow-measuring device which uses heat transfer to measure and indicate mass flowrate
NOTE The term thermal mass flowmeter also applies to the measuring portion of a thermal mass flow controller and not
the control function.
3.2.4
capillary thermal mass flowmeter
CTMF meter
TMF meter normally consisting of a laminar flow element, bypass tube (capillary), temperature sensors (some
designs include a separate heater) with supporting electronics and housing
3.2.5
insertion and/or in-line thermal mass flowmeter
ITMF meter
TMF meter normally consisting of one or two temperature sensing sensors (some designs have a separate heater)
with supporting structure, electronics and housing, of which the sensors are exposed to the full gas stream
3.2.5.1
insertion-ITMF meter
ITMF meter with the sensors mounted on a probe, inserted through the process conduit wall, into the gas stream
3.2.5.2
in-line ITMF meter
ITMF meter with the sensors mounted in a flow body which serves as an integral part of the conduit
3.2.6
thermal mass flow controller
flow controlling device that comprises a TMF meter, a valve and controlling electronics
NOTE The output of the TMF meter is compared against an adjustable setpoint and the valve is correspondingly opened or
closed to maintain the measured flowrate at the setpoint value.
3.2.7
transmitter
associated electronics providing the heater with electrical power and transforming the signals from the temperature
sensors to give output(s) of the measured parameters
NOTE The transmitter can be integrally mounted to a TMF meter. However, for some applications the transmitter can be
remotely installed away from the flow sensor.
3.2.8
retractor mechanism
�insertion-ITMF meters� mechanical arrangement including an isolation valve that allows the positioning and/or
extraction of the flow sensor within the conduit
3.2.9
rangeability
statement of the minimum and maximum limits of which an individual sensor can measure and indicate
EXAMPLE For a maximum flowrate � 1 000 kg/h and a minimum flowrate � 10 kg/h, the rangeability � 10 kg/h to
1 000 kg/h.
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ISO 14511:2001(E)
3.2.9.1
turndown
numerical ratio of the maximum to minimum limits of which an individual sensor can measure
EXAMPLE For a maximum flowrate � 1 000 kg/h and a minimum flowrate � 10 kg/h, the turndown ratio �
1 000/10 � 100:1.
NOTE In practice, the terms rangeability and turndown are used interchangeably and can be associated with an uncertainty
statement.
3.2.10
k-factor
numerical factor unique to each TMF meter which is associated with the mass flowrate derived during the
calibration and when programmed into the transmitter ensures that the meter performs to its stated specification
NOTE When a surrogate gas has been used for calibration purposes, the manufacturer’s gas factor list or database has
been applied for conversion to the desired gas under process conditions.
3.2.11
normalized volumetric flowrate (GB)
standardized volumetric flowrate (US)
flowrate for which the quantity of fluid is expressed in terms of volume, with the fluid density calculated at a known
and fixed pressure and temperature condition
NOTE 1 The values used to define these reference conditions (also known as “standard reference conditions”) are industry
and country specific and therefore shall always be specified when these units are used. Typical reference conditions are 0 °C
and 101,325 kPa.
3
NOTE 2 Normalized volumetric units or volumetric units specified to standard reference conditions, such as “Nm /h”,are
commonly used with CTMF and ITMF meters, however this practice is not recommended as they are neither SI units nor
symbols and their use without knowledge of the reference conditions will lead to significant errors. In this International Standard,
3
volumetric units specified to standard reference conditions are followed by the expression “(normalized)”,e.g. m /h
(normalized).
3.2.12
normalized velocity (GB)
standardized velocity (US)
flowrate for which the quantity of fluid is expressed in terms of the speed of flow, with the fluid density calculated at
a known and fixed pressure and temperature condition
NOTE 1 The values used to define these reference conditions (also known as “standard reference conditions”) are industry
and country specific and therefore shall always be specified when these units are used. Typical reference conditions are 0 °C
and 101,325 kPa.
NOTE 2 Normalized volumetric units or volumetric units specified to standard reference conditions, such as “Nm/s”,are
commonly used with CTMF and ITMF meters, however this practice is not recommended as they are neither SI units or symbols
and their use without knowledge of the reference conditions will lead to significant errors. In this International Standard,
volumetric units specified to standard reference conditions are followed by the expression “(normalized)”, e.g. m/s (normalized).
4 Selection of thermal mass flowmeters
TMF meters fall into two basic design categories:
a) capillary TMF meters (CTMF meters);
b) full bore TMF meters, consisting of the following two types (ITMF meter):
1) insertion type;
2) in-line type.
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ISO 14511:2001(E)
The choice of appropriate design for a particular application is primarily dependent on:
� the required flowrate and range;
� the cleanliness of the gas;
� the conduit dimensions.
The two basic types of TMF meters have a number of overlapping characteristics for flowrate and conduit
dimensions as shown in Table 1. Other factors may influence the final choice of the meter depending on the
application. Table 1 is a guideline only and the manufacturer’s specifications should be consulted for the absolute
limits.
Table 1 — Preliminary TMF meter selection criteria
ITMF meter
Characteristic CTMF meter
In-line Insertion
a, b
3
� 2 000 m /h at reference
a, b a, b, c
Typical flow range 0,22 kg/h to 7 000 kg/h ��5kg/h
conditions of 0 °C and 101,325 kPa
Typical conduit size 3 mm to 200 mm 8 mm to 200 mm � 80 mm
d
Gas condition Clean and dry only Preferably clean and dry
Gas temperature � 70 °C � 500 °C
a
Flow range is dependent on the conduit size.
b
Quoted flow range in air or nitrogen.
c
This is the minimum flowrate in a 80 mm conduit. Flowrates in excess of 100 t/h can be achieved in large conduits.
d
The ITMF meter can operate in the presence of dirty and/or wet gases. However, its performance is impaired.
5 Capillary thermal mass flowmeter (CTMF meter)
5.1 Principles of measurement
A typical CTMF meter consists of a meter body and flow sensor. The flow sensor is mounted integrally into the
meter body. A defined portion of the gas flow from the meter body is diverted through the (bypass) flow sensor,
through which the gas flowrate is measured.
Figure 1 shows a simplified CTMF meter with a typical flow sensor consisting of a thin tube and two temperature
sensors. Depending upon the meter manufacturer, the heater can either be combined with each temperature
sensor or be located separately in the middle of the flow sensor, i.e. between the temperature sensor upstream (T )
1
and the one downstream (T ) of the gas flow.
2
A precision power supply delivers constant heat to the flow sensor. Under stopped-flow conditions, both sensors
measure the same temperature. As the flowrate increases, heat is carried away from the upstream sensor (T )
1
towards the downstream sensor (T ). A bridge circuitry interprets the temperature difference and an amplifier
2
provides the flowrate output signal.
The measured temperature difference between the two sensors is proportional to the mass flowrate.
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ISO 14511:2001(E)
a) Two temperature sensors and separate heater b) Two self-heating temperature sensors
Key
1 Upstream temperature sensor T
1
2 Downstream temperature sensor T
2
3 Upstream temperature sensor T (with heater)
1
4 Downstream temperature sensor T (with heater)
2
5 Constant power supply P
6 Heater
7 Bridge circuitry
8Amplifier
9 Flow signal output (typically 0 V to 5 V d.c. for 4 mA to 20 mA)
Figure 1 — Simplified CTMF meter
The flow sensor measures the mass flowrate as a function of temperature difference. This can be expressed
according to first law of thermodynamics (heat in � heat out, for no losses) for which the following equations apply:
��
TT�
21
Pq��c� �L (1)
��mp
f
��CTMF
or after rearranging equation (1)
PL��f
� �
CTMF
q � (2)
m
cT� �T �
p 21
where
q is the mass flowrate, expressed in kilograms per second;
m
c is the specific heat, expressed in joules per kilogram per kelvin [J/(kg�K)], of the gas at constant
p
pressure;
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ISO 14511:2001(E)
T � T is the temperature difference, expressed in kelvins;
2 1
P is the constant input power, expressed in watts;
1)
L is the end conduction loss , expressed in watts;
f is the constant meter factor related to the CTMF meter design.
CTMF
Flow-through thermal mass flow sensors are only accurate at relatively low flowrates. Therefore, so as to measure
accurately higher flowrates, it is necessary to split the total flow. This flow diversion is carried out in the meter body
by means of a laminar flow element. This element produces a linear pressure drop which is proportional to the
mass flowrate.
The full-scale (FS) flow range of a CTMF meter is directly influenced by the specific heat c (at constant pressure)
p
of the process gas.
Not all calibrations can be performed using the desired process gas. If the gas is corrosive or hazardous, it is
necessary to use a reference gas for calibration, i.e. air or nitrogen. In this case, it is necessary to calculate the c
p
value of the process gas.
NOTE Each manufacturer of a CTMF product should be able to provide a list of gas conversion factors or to make
reference to an appropriate database.
The k-factor is defined as:
c
p,
ref
k � (3)
c
p,
proc
The full-scale flowrate (FS) of the process gas is given by:
qq,,��k (4)
mmproc,FS ref,FS
where
k is the k-factor;
c is the specific heat, at constant pressure, of the reference gas;
p,ref
c is the specific heat, at constant pressure of the process gas;
p,
proc
q is the mass flowrate, on a full-scale basis, of the reference gas;
m,
ref,FS
q is the mass flowrate, of the process gas on a full-scale basis.
m,
proc,FS
Using this relationship as well as the gas tables available for specific heat, the user and the manufacturer can
easily (re)calibrate the meter by using a “safe” gas.
5.2 Typical design
As too much heat is transferred at higher flowrates, only very low flowrates can accurately be measured using the
basic CTMF meter design. Consequently, the CTMF meter type is often used in conjunction with a laminar flow
1) End conduction loss is a design-dependent quantity and is the heat lost by conduction to the flow meter body through the
mounting arrangement of the transducer. It is so called since the transducer is normally mounted at one of its ends.
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ISO 14511:2001(E)
element, placed in the main conduit where it produces a small pressure drop. The ends of the capillary tube are
connected to the inlet and outlet of the laminar flow element so as to create a small diverted flow through the
capillary tube. This design ensures a fixed ratio of the total gas flowrate through the capillary for measurement. The
heater and temperature sensors are usually placed on the capillary tube rather than the main conduit. However,
other designs exist where the sensors are located directly on the main conduit, i.e. without the capillary and the
laminar flow element.
The CTMF meter is generally supplied with screw-threaded fittings although flange fittings can be provided. This
design of TMF meter is often combined with a flow controller downstream to the sensing sensor so as to obtain a
mass flow controller. Typically, the electronic interface is located within the same unit as the bypass loop. Figure 2
shows a typical CTMF meter design.
Key
1 Direction of flow 4 Bypass loop
2 Heater 5 Laminar flow element
3 Temperature sensors
Figure 2 — Typical CTMF meter
5.3 Applications and limitations of use
5.3.1 Gas property effects
CTMF meters should only be used to measure dry and clean gases. Saturated vapours capable of condensing
should be avoided so as to prevent clogging or contaminating the flow sensor.
From equations (1) to (4), it can be seen that the calibration of the CTMF meter is dependent on the properties of
the gas. Therefore the CTMF is an inferential mass-flowrate meter. Although the CTMF meter responds to changes
in mass flowrate, calibration is dependent on the gas being measured as well as the operating conditions.
Variations in fluid properties, such as varying gas mixture composition, process pressure and/or temperature, which
affect the value of specific heat, will in turn affect the performance of the CTMF meter. When this situation occurs, it
may be necessary to compensate for these variations.
5.3.2 Application and fluid properties
In order to identify the optimum meter for a given application, it is important to establish the range of conditions to
which it will be subjected. These conditions should include:
� the operating flowrates;
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ISO 14511:2001(E)
� the properties of the metered gas, the type of gas or the composition of the gas mixture;
� the range of pressures of the process;
� the range of temperatures of the process.
5.3.3 Temperature effects
Temperature changes affect the behaviour of the CTMF meter. Compensation for this effect is usually incorporated
into the transmitter. Temperature variations may also induce an offset in the meter output at zero flowrate.
Therefore, the meter should be zeroed at the actual temperature of the process.
Temperature effects are normally specified in percentage or degrees Celsius.
Furthermore, temperature changes may influence the specific heat value at which the CTMF meter was initially
calibrated. In these cases, this effect should be determined using values from known gas data furnished by the gas
manufacturer after calibration.
5.3.4 Pressure effects
Pressure changes may affect the specific heat of the gas and/or flow ratio between sensor and laminar flow
element. It may, therefore, alter the k-factor of the CTMF meter. Some manufacturers make meter calibrations
under pre-defined pressure conditions so as to minimize this effect. However, in other cases, reference to known
gas data furnished by the gas manufacturer after calibration is required.
5.3.5 Pulsation effects
If the flow is pulsating, care should be taken to ensure that the transmitter responds quickly enough to follow the
pulsation. Some manufacturers offer adaptive output damping when a constant output signal is required under
these conditions. The manufacturer's specification for flowrate output response and/or damping should be
observed.
5.3.6 Pressure loss
A loss in pressure occurs as the fluid flows through the CTMF meter. The magnitude of the pressure loss is a
function of the pressure difference across the laminar flow element with respect to the flowrate. Most manufacturers
specify this effect and typically the amount of pressure loss is smaller than 100 hPa (100 mbar) at full-scale
flowrate. The value of the pressure loss should include the CTMF meter, fittings and inlet filter.
5.3.7 Cleanness of the ga
...