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• Status: Withdrawn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
• Start Date: 10-Jan-2019
• Completion Date: 13-Dec-2025

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 2001
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ii © ISO 2001 – All rights reserved

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
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.
iv © ISO 2001 – All rights reserved

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.
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.
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
2 © ISO 2001 – All rights reserved

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.
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.
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,
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.
4 © ISO 2001 – All rights reserved

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
� 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 )
and the one downstream (T ) of the gas flow.
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 temperature difference and an amplifier
provides the flowrate output signal.
The measured temperature difference between the two sensors is proportional to the mass flowrate.
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
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�
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;
6 © ISO 2001 – All rights reserved

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.
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;
8 © ISO 2001 – All rights reserved

� 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 provision for in-situ calibration if desired and possible;
� class and type of conduit connections;
� material compatibility including O-ring(s), (valve) seat material;
� any non-destructive testing procedures;
10 © ISO 2001 – All rights reserved

� hazardous area classification;
� electrical connections;
� climatic and environmental effects on the flowmeter;
� any applicable national or international requirements.
5.4.4 Meter ratings
The following meter ratings shall be taken into account:
� maximum allowable process pressure;
� pressure drop at the maximum flowrate;
� ambient and process gas temperature;
� outboard leak integrity rating.
5.4.5 Application and fluid properties
The following application and fluid properties shall be taken into account:
� operating flowrates;
� properties of the metered gas, including gas type or composition of gas mixture;
� range of operational pressures;
� range of operational temperatures;
� reference process conditions when normalized volume units of flowrate are used [i.e. m /h (normalized),
l/min (normalized)].
5.4.6 Corrosion
Corrosion, including galvanic corrosion, of the wetted materials can adversely affect the performance and ultimately
shorten the operating life of the CTMF meter. Care should be taken when selecting the materials of construction to
ensure that the process gas, including purge or cleaning fluids are compatible. All process-wetted materials shall
be specified.
5.4.7 Transmitter specifications
The following transmitter specifications shall be taken into account:
� electrical, electronic, climatic and safety compatibility;
� required output options.
5.5 Installation and commissioning
5.5.1 General considerations
In general, consideration should be given to the following points:
� the process gas should be relatively clean and dry;
� the space required for the TMF meter installation (including provisions for the TMF meter verification by means
of master-meter connections, should in-situ calibration be required; see 8.4 for details);
� the class and type of pipe connections, materials and the dimensions of the equipment to be used;
� any hazardous area classification;
� any climatic and environmental effects on the sensor, e.g. temperature, humidity, corrosive atmospheres,
mechanical shock, vibration and electromagnetic field.
5.5.2 Safety
Consideration should be given to the following points on safety:
� the operation of the meter shall be limited to process conditions which remain within the meter's specifications;
� the meter shall comply with any necessary hazardous area classifications as well as all applicable national or
international requirements;
� only appropriately qualified and trained personnel shall install, operate and/or maintain the meter.
5.5.3 Mechanical stress
Consideration should be given to the following points on mechanical stress:
� the meter should be selected to withstand temperature, pressure and conduit vibration and should be
pressure-tested with a suitable fluid in accordance with the appropriate standard;
� to ensure leak-tightness in critical applications, all meters should be tested, preferably with helium gas under
vacuum conditions to an appropriate standard.
5.5.4 Process adjustment
5.5.4.1 General
If the actual operating conditions are not in compliance with the calibrated configuration of the meter, additional
adjustment may be required.
5.5.4.2 Zero adjustment
Once the meter installation is completed and checked for compliance with all manufacturer-specified
recommendations and statutory requirements, a zero adjustment procedure at the operating process conditions
may be necessary with some process gases to compensate for the effects previously described. It is essential that
this procedure be carried out at the actual process temperature and pressure conditions. These conditions shall be
stable and there shall be absolutely no gas flowrate. Depending on the quality of the gas and/or the nature of the
application, it may be desirable to repeat this operation at regular intervals.
12 © ISO 2001 – All rights reserved

5.5.4.3 Span adjustment
CTMF meters are normally calibrated with respect to the actual process conditions, however if the latter are
different than the calibration conditions, or if the gas type or quality is different from that specified for the calibration,
a span adjustment may be required. This can only be carried out if the installation provides the means of obtaining
a reference flowrate value for comparison with the CTMF meter reading (see clause 8). It is essential that this
procedure be carried out at the actual process temperature and pressure conditions. These conditions and the
actual flowrate shall be stable during any span adjustment procedure. Depending on the quality of the gas and/or
the nature of the application, it may be desirable to repeat this operation at regular intervals.
6 Insertion and/or in-line thermal mass flowmeter (ITMF meter)
6.1 Principles of measurement
6.1.1 General
A typical ITMF flowmeter consists of two temperature sensors (see Figure 3).
An amount of heating power P is applied to one of the sensors, causing its temperature to rise to a measured value
T . The other sensor measures the gas temperature T .
2 1
The gas mass flowrate can be determined from the temperature difference between the heated sensor and the gas
(�T = T � T ) and the amount of heating power P applied.
2 1
Figure 3 — Typical ITMF meter sensor configuration
The relationship between the heating power P, the difference in temperature (�T) and the mass flowrate (q )can
m
be expressed as follows:
P K
��KK�q (5)
� �
12 m
�T
where
K , K and K are design and calibration parameters;
1 2, 3
�T is the temperature difference (� T � T ), expressed in kelvins;
2 1
P is the input heating power, expressed in watts;
q is the mass flowrate, expressed in kilograms per second.
m
This is known as King’s Law. K and K depend on the geometry of the sensing sensors and also on specific gas
1 2
properties such as thermal conductivity, viscosity and specific heat capacity. K is Reynolds-number-dependent.
The numerical value of these factors are meter and gas specific, therefore the ITMF meter has to be calibrated for
the gas desired for flow measurement.
In practice, there are two methods of measuring the gas mass flowrate using either a constant power or constant
differential temperature method.
6.1.2 Constant power method
This method keeps the electrical heating power (P) constant while measuring the differential temperature (�T). The
simplified equation is:
K
��TK �K � �q (6)
�
45 m
where
K , K and K are design and calibration parameters.
4 5 6
Typically, temperature-dependent resistors measure both the gas temperature and the heated-sensor temperature.
The difference between the two temperatures is a function of the mass flowrate (q )(seeFigure4).
m
Key
1 Constant voltage supply
2Amplifier
3 Mass flow determination
Figure 4 — Simplified sensor circuitry for the “constant power” ITMF meter
14 © ISO 2001 – All rights reserved

6.1.3 Constant-temperature-differential method
The temperature differential (�T ) is maintained constant while measuring the change in heating power (P). The
simplified equation is:
K
��9
PT�� �K �K �q (7)
��
78 m
��
��
where
K , K and K are design and calibration parameters.
7 8 9
2)
Both the heated sensor and the gas temperature sensor are typically temperature-dependent resistors. A bridge
circuit maintains the heated sensor (T ) at a constant-temperature differential from the gas temperature sensor (T )
2 1
(see Figure 5).
R � Resistor 1
R � Resistor 2
A � Amplifier
Figure 5 — Simplified circuitry for “constant-temperature-differential” ITMF meter
2) Other configurations exist that utilize the thermal properties of silicon diodes, thermocouples, resistance thermometer
detectors (also called resistance temperature detectors) (RTD), etc.
6.2 Typical design
6.2.1 ITMF basic design
An ITMF meter is comprised of three basic functional components: the meter body or probe, sensors and
electronics module. The arrangement of these components characterizes different common meter designs.
The electronics module is considered as a secondary device and may be remote from the meter body. The
electronics module provides the power source for the sensors and the process electronics. The ITMF meter output
can usually be specified as a rate and/or total output in a range of units. Additional outputs can usually be provided,
e.g. a flow computer which can provide other performance and diagnostics information.
ITMF meters are available in different design formats generally described by their conduit fitting: flanged, threaded,
wafer or insertion type (see Figures 6 and 7). The first three types are all in-line meters with a flow of gas through the
meter body. The body of an insertion-ITMF meter simply contains and protects the sensors. Standard ITMF meters
are available in a wide range of conduit fittings, e.g. ANSI and DIN flanges, sanitary couplings, and NPT thread.
6.2.2 In-line ITMF meter
An integral section of straight conduit with threaded or flanged end fittings contains the sensors. The flowing gas
stream passes through the meter and over the sensors. Certain ITMF meters of this type may include straight
lengths of conduit both upstream and downstream of the sensor to reduce the effects of flow disturbances. Figure 6
shows examples of typical in-line ITMF meters.
a) Flanged type
b) Wafer type
Figure 6 — Typical flanged and wafer in-line ITMF-meter designs
16 © ISO 2001 – All rights reserved

A wafer in-line ITMF meter should fit between two existing conduit flanges. The mounting length, or meter body
width, is generally standardized at 65 mm.
The sensor’s design depends on the manufacturer, although all incorporate at least a heat source and two
temperature sensors. The location of the sensors within the cross-section of the conduit is fixed. Other components
may be included for flow conditioning or physical protection.
6.2.3 Insertion-ITMF meter
For larger conduits, an insertion-ITMF meter is commonly used, although some designs can be used for conduits
as small as 50 mm diameter. The sensors are installed at the end of a probe which is inserted into the flowing gas
stream. Total mass flowrate is determined from the measured point flowrate, cross-sectional area and
compensation for the flow profile. Some degree of physical protection of the sensors is usually included. Figure 7
depicts two typical insertion type ITMF meters, each with a different mounting arrangement. Other types include
compression fittings, packing gland, sanitary and ultra high-purity fittings.
Flanged type
Figure 7 — Typical insertion-ITMF meter designs
The location of the sensors within the cross-section of the conduit is crucial for optimum performance (see 6.5). If
the sensors cannot be installed in accordance with manufacturer's recommendations, then further adjustments are
necessary. Certain insertion-TMF-meter designs permit sensor location adjustment within the conduit so as to
easily optimize installation.
The installation of the insertion-ITMF meter into an existing conduit is made possible by means of an adapter
welded to the external surface of the conduit. (The weld quality is to be in compliance with safety requirements.)
The insertion-ITMF meter is to be installed in the conduit using this adapter. The fitting connection of the adapter is
to match that on the insertion probe.
6.2.4 Multi-point insertion-ITMF meter
A special version of th
...