ISO 14511:2019

Measurement of fluid flow in closed conduits - Thermal mass flowmeters

This document 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 document is not applicable to hot wire and other hot film anemometers, also used in making point velocity measurements.

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

General Information

• Status: Published
• Publication Date: 09-Jan-2019
• 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: 9093 - International Standard confirmed
• Start Date: 28-Jan-2025
• Completion Date: 13-Dec-2025

Relations

• Revises: ISO 14511:2001 - Measurement of fluid flow in closed conduits - Thermal mass flowmeters
• Effective Date: 23-Apr-2020

Overview

ISO 14511:2019 - Measurement of fluid flow in closed conduits - Thermal mass flowmeters provides guidance for the specification, testing, inspection, installation, operation and calibration of thermal mass gas flowmeters used to meter gases and gas mixtures. The standard covers both capillary (CTMF) and insertion/in-line (ITMF) thermal mass flowmeter types and explains measurement principles and practical limitations. It explicitly excludes measuring liquid mass flowrates and the use of hot‑wire or hot‑film anemometers for point velocity measurements.

Key topics and technical requirements

• Scope and definitions: Terms and classifications for thermal mass flowmeters and measurement units.
• Selection criteria: Performance, physical and environmental specifications, corrosion resistance and transmitter requirements.
• Measurement principles: Capillary meter principles and ITMF methods, including constant‑power and constant‑temperature‑differential techniques.
• Applications and limitations: Effects of gas properties, temperature, pressure, pulsation, contamination, mounting orientation, vibration and pressure loss on meter performance.
• Installation & commissioning: Pre‑installation inspection, mechanical stress, safety, cleaning and process adjustment guidance.
• Calibration & verification: Use of process gas vs surrogate gas, in situ calibration considerations, calibration frequency and required calibration certificates.
• Operation & maintenance: Routine visual/functional checks, record keeping and maintenance audit trail.
• Documentation: User specification sheets, manufacturer data sheets and mandatory/optional marking requirements.

Practical applications and users

ISO 14511:2019 is valuable for organizations and professionals involved in gas flow measurement and control, including:

• Instrument manufacturers and designers specifying thermal mass flowmeters.
• Plant engineers and process control specialists in chemical, petrochemical, power generation, HVAC, semiconductor and environmental monitoring sectors.
• Calibration laboratories and metrology services performing factory or in situ calibrations.
• Procurement and quality teams evaluating meter selection, installation and compliance.

Use cases include accurate gas metering for process control, emissions monitoring, leak detection and custody‑transfer scenarios where thermal mass flowmeters are suited to measure mass or normalized volumetric flow of gases.

Related standards

ISO 14511:2019 was developed in accordance with ISO/IEC Directives and complements other ISO standards on fluid flow measurement and metrology. Users should consult applicable national and industry standards for additional legal, safety and calibration requirements.

Keywords: ISO 14511:2019, thermal mass flowmeters, gas flow measurement, calibration, installation, ITMF, CTMF, flowmeter selection, thermal mass gas flowmeter.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 14511
Second edition
2019-01
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 2019
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Fax: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved

Contents Page
Foreword .v
Introduction .vi
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 . 8
5.3 Applications and limitations of use . 8
5.3.1 Gas property effects . 8
5.3.2 Application and fluid properties . 9
5.3.3 Temperature effects . 9
5.3.4 Pressure effects . 9
5.3.5 Pulsation effects . 9
5.3.6 Pressure loss . 9
5.3.7 Cleanness of the gas . 9
5.3.8 Mounting orientation effects . 9
5.3.9 Installation effects for flow profile .10
5.3.10 Vibrations — Hydraulic and mechanical .10
5.3.11 Valves .10
5.4 Meter selection .10
5.4.1 Principal requirement .10
5.4.2 Performance specifications .10
5.4.3 Physical specifications .10
5.4.4 Meter ratings .11
5.4.5 Application and fluid properties .11
5.4.6 Corrosion .11
5.4.7 Transmitter specifications .11
5.5 Installation and commissioning .11
5.5.1 General considerations .11
5.5.2 Safety .12
5.5.3 Mechanical stress .12
5.5.4 Process adjustment .12
6 Insertion and/or in-line thermal mass flowmeter (ITMF meter) .13
6.1 Principle of measurement .13
6.1.1 General.13
6.1.2 Constant power method .14
6.1.3 Constant-temperature-differential method .15
6.2 Typical design .16
6.2.1 ITMF basic design .16
6.2.2 In-line ITMF meter .16
6.2.3 Insertion-ITMF meter .17
6.2.4 Multi-point insertion-ITMF meter .18
6.3 Applications and limitations of use .18
6.3.1 General remarks .18
6.3.2 Normalized volume flowrate units.18
6.3.3 Fluid property effects .18
6.3.4 Temperature effects .19
6.3.5 Pressure effects .19
6.3.6 Fluid phase .19
6.3.7 Bi-directional flow .19
6.3.8 Pulsation effects .19
6.3.9 Pressure loss .19
6.3.10 Sensor contamination .19
6.3.11 Mounting orientation effects .19
6.3.12 Installation effects .20
6.3.13 Conduit vibrations .20
6.4 Meter selection .20
6.4.1 General.20
6.4.2 Performance specifications .20
6.4.3 Physical specifications .21
6.4.4 Meter ratings .21
6.4.5 Application and fluid properties .21
6.4.6 Corrosion .22
6.4.7 Transmitter specifications .22
6.5 Installation and commissioning .22
6.5.1 General considerations .22
6.5.2 Cleaning .22
6.5.3 Safety .22
6.5.4 Mechanical stress .23
6.5.5 Process conditions .24
7 Instrument specification sheet and marking .24
7.1 User specification sheet .24
7.2 Manufacturer's data sheet . .24
7.3 Marking .25
7.3.1 Mandatory .25
7.3.2 Optional .26
8 Calibration .26
8.1 General considerations .26
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 .27
8.7 Calibration certificate .27
9 Pre-installation inspection and testing .28
10 Maintenance .28
10.1 General .28
10.2 Visual inspection .29
10.3 Functional test .29
10.4 Record keeping (Maintenance audit trail) .29
Bibliography .30
iv © ISO 2019 – All rights reserved

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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed
conduits, Subcommittee SC 5, Velocity and mass methods.
This second edition cancels and replaces the first edition (ISO 14511:2001), of which it constitutes a
minor revision.
The changes compared to the previous edition are as follows:
— the sentence “The measurand temperature difference between the two sensors is proportional to the
mass flow rate” has been removed from 5.1;
— the references to the VIM and GUM Guides have been updated.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
Introduction
This document 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 this document is given in the Bibliography.
vi © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 14511:2019(E)
Measurement of fluid flow in closed conduits — Thermal
mass flowmeters
1 Scope
This document 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 document is not applicable to hot wire and other hot film anemometers, also used in making point
velocity measurements.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM: 1995)
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
IEC 61000-4, Electromagnetic compatibility (EMC) — Part 4: Testing and measurement techniques
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 4006 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
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 1 to entry: In this International Standard, the term “flowrate” is used as a synonym for mass flowrate,
unless otherwise stated.
3.1.2
mass flowrate
flowrate in which the quantity of fluid is expressed as a mass
Note 1 to entry: The term “flowrate” is used as a synonym for mass flowrate in this International Standard,
unless otherwise stated.
3.1.3
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
Note 1 to entry: To explain the closeness of agreement between the result of a measurement and a true value of
the measurand the expression “accuracy of measurement” is used.
Note 2 to entry: Accuracy is a qualitative concept.
3.1.4
repeatability
ability of a measuring instrument to provide closely similar indications for
repeated applications of the same measurand under the same conditions of measurement
Note 1 to entry: 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.
3.1.5
flow profile
graphic representation of the velocity distribution
Note 1 to entry: 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
3.2.3
thermal mass flowmeter
TMF meter
flow-measuring device which uses heat transfer to measure and indicate mass flowrate
Note 1 to entry: 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
2 © ISO 2019 – All rights reserved

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
mass flow controller
flow controlling device that comprises a TMF meter, a valve and controlling electronics
Note 1 to entry: 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 1 to entry: 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
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.
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 1 to entry: 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 1 to entry: 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 volume flowrate, (GB)
standardized volume 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 to entry: 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 to entry: 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, 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 to entry: 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 to entry: 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.
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.
4 © ISO 2019 – All rights reserved

Table 1 — Preliminary TMF meter selection criteria
ITMF meter
Characteristic CTMF meter
In-line Insertion
3 a,b
< 2 000 m /h at reference condi-
a,b a,b,c
Typical flow range 0,22 kg/h to 7 000 kg/h > > 5 kg/h
tions 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 ) and the one downstream (T ) of the gas flow.
1 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 ) towards the downstream sensor (T ). A bridge circuitry interprets the
1 2
temperature difference and an amplifier provides the flowrate output signal.
a) Two temperature sensors and separate heater b) Two self-heating temperature sensors
Key
1 upstream temperature sensor T
2 downstream temperature sensor T
3 upstream temperature sensor T (with heater)
4 downstream temperature sensor T (with heater)
5 constant power supply P
6 heater
7 bridge circuitry
8 amplifier
9 flow signal output (typically 0 V to 5 V DC 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 formulae apply:
 TT− 
Pq=×c +L (1)
 
mp
f
 CTMF 
or after rearranging Formula (1):
PL− × f
()
CTMF
q = (2)
m
cT −T
()
p 21
where
6 © ISO 2019 – All rights reserved

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
p
constant pressure;
T − T is the temperature difference, expressed in kelvins;
2 1
P is the constant input power, expressed in watts;
L is the end conduction loss, expressed in watts;
NOTE  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.
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 that forces proportional fraction of the total mass flow rate to go through the flow sensor
(capillary).
The full-scale (FS) flow range of a CTMF meter is directly influenced by the specific heat c (at constant
p
pressure) 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 value of the process gas.
p
NOTE Each manufacturer of a CTMF product is intended to 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)
mm,,proc,FSref,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 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 heater 4 laminar flow element
a
2 temperature sensors Direction of flow.
3 bypass loop/capillary tube
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 Formulae (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.
8 © ISO 2019 – All rights reserved

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;
— 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 gas
Solids settlement, coating, or trapped condensate can affect the meter performance. These conditions
should be avoided. The preferred cleanness specification of the manufacturer shall be followed. Most
manufacturers specify the allowed particle size or provide an inlet filter as standard to the CTMF meter.
5.3.8 Mounting orientation effects
Most manufacturers specify the effect for mounting orientations. For most instruments, this effect
is negligible. The preferred mounting position for high pressure applications, however, is horizontal,
because of possible zero-offset. In order to achieve the specified performance, the installation guidelines
from the manufacturer shall be followed.
5.3.9 Installation effects for flow profile
The performance of a CTMF meter is not affected by fluid swirl and non-uniform velocity profiles
induced by upstream and downstream piping configurations. No special straight-piping lengths are
normally required but depend on the manufacturer. Nonetheless, good installation practices should be
observed at all times.
Filters, or other protective devices should be provided upstream of the meter to remove solids or liquid
droplets that can possibly cause measurement errors.
5.3.10 Vibrations — Hydraulic and mechanical
Vibrations in the process-line normally have no effect on the performance of the CTMF meter. However,
all vibrations should remain within the limits of common practice.
5.3.11 Valves
Valves upstream and downstream of a CTMF meter, installed for the purpose of isolating and zeroing
the meter, can be of any type. However, they should provide tight shut-off to ensure that true zero
flowrate can be achieved. Control valves in series with a CTMF meter shall be installed close together
with the TMF meter so as to minimize dead volume.
5.4 Meter selection
5.4.1 Principal requirement
The principal requirement of a metering system is that it shall measure gas mass flowrate with the
specified uncertainty. Consideration shall be given to the points given in 5.4.2 to 5.4.7 when choosing a
suitable meter.
5.4.2 Performance specifications
The following performance specifications shall be taken into account:
— uncertainty:
— compliance of the actual installation and operating conditions with the manufacturer’s data;
— calibration procedure used and traceability of calibration equipment;
— repeatability;
— rangeability;
— stability;
— temperature effect;
— pressure effect;
— mounting orientation effect.
5.4.3 Physical specifications
The following physical specifications shall be taken into account:
— space required (meter dimensions, upstream, downstream conduit diameters) for installation of
the flowmeter, including p
...