EN ISO 15970:2014

SLOVENSKI STANDARD
01-maj-2014
=HPHOMVNLSOLQ0HUMHQMHQMHJRYLKODVWQRVWL9ROXPHWULþQHODVWQRVWLJRVWRWDWODN
WHPSHUDWXUDLQNRPSUHVLMVNLIDNWRU,62
Natural gas - Measurement of properties - Volumetric properties: density, pressure,
temperature and compression factor (ISO 15970:2008)
Erdgas - Messung der Eigenschaften - Volumetrische Eigenschaften: Dichte, Druck,
Temperatur und Kompressibilitätsfaktor (ISO 15970:2008)
Gaz naturel - Mesurage des caractéristiques - Caractéristiques volumétriques: masse
volumique, pression, température et facteur de compression (ISO 15970:2008)
Ta slovenski standard je istoveten z: EN ISO 15970:2014
ICS:
75.060 Zemeljski plin Natural gas
75.180.30 Oprema za merjenje Volumetric equipment and
prostornine in merjenje measurements
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 15970
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2014
ICS 75.060; 75.180.30
English Version
Natural gas - Measurement of properties - Volumetric properties:
density, pressure, temperature and compression factor (ISO
15970:2008)
Gaz naturel - Mesurage des caractéristiques - Erdgas - Messung der Eigenschaften - Volumetrische
Caractéristiques volumétriques: masse volumique, Eigenschaften: Dichte, Druck, Temperatur und
pression, température et facteur de compression (ISO Kompressibilitätsfaktor (ISO 15970:2008)
15970:2008)
This European Standard was approved by CEN on 16 February 2014.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management Centre has the same
status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United
Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2014 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 15970:2014 E
worldwide for CEN national Members.

Contents Page
Foreword .3
Foreword
The text of ISO 15970:2008 has been prepared by Technical Committee ISO/TC 193 “Natural gas” of the
International Organization for Standardization (ISO) and has been taken over as EN ISO 15970:2014.
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by September 2014, and conflicting national standards shall be
withdrawn at the latest by September 2014.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech
Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece,
Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.
Endorsement notice
The text of ISO 15970:2008 has been approved by CEN as EN ISO 15970:2014 without any modification.

INTERNATIONAL ISO
STANDARD 15970
First edition
2008-06-15
Natural gas — Measurement of
properties — Volumetric properties:
density, pressure, temperature and
compression factor
Gaz naturel — Mesurage des caractéristiques — Caractéristiques
volumétriques: masse volumique, pression, température et facteur de
compression
Reference number
ISO 15970:2008(E)
©
ISO 2008
ISO 15970:2008(E)
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©  ISO 2008
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Published in Switzerland
ii © ISO 2008 – All rights reserved

ISO 15970:2008(E)
Contents Page
Foreword. v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 2
3.1 Terms and definitions for density at reference conditions . 2
3.2 Terms and definitions for density at operating conditions. 2
3.3 Terms and definitions for pressure . 3
3.4 Terms and definitions for temperature. 4
3.5 Terms and definitions for compression factor . 4
4 Symbols and units. 4
4.1 Symbols and subscripts for density at reference conditions. 4
4.2 Symbols and subscripts for density at operating conditions. 5
4.3 Symbols and subscripts for compression factor. 5
5 Density at reference conditions . 6
5.1 Principle of measurement. 6
5.2 Performance assessment and acceptance tests. 10
5.3 Sampling and installation guidelines . 11
5.4 Calibration . 11
5.5 Verification . 11
5.6 Maintenance . 12
5.7 Quality control. 12
6 Density at operating conditions . 12
6.1 Principle of measurement. 12
6.2 Performance assessment and acceptance tests. 13
6.3 Sampling and installation guidelines . 16
6.4 Calibration . 20
6.5 Verification . 20
6.6 Maintenance . 21
6.7 Quality control. 21
7 Pressure. 21
7.1 Principle of measurement. 22
7.2 Performance assessment and acceptance tests. 24
7.3 Sampling and installation guidelines . 24
7.4 Calibration . 27
7.5 Verification . 28
7.6 Maintenance . 28
7.7 Quality control. 29
8 Temperature . 29
8.1 Principle of measurement. 29
8.2 Performance assessment and acceptance tests. 30
8.3 Installation guidelines . 31
8.4 Calibration . 33
8.5 Verification . 34
8.6 Maintenance . 34
8.7 Quality control. 34
9 Compression factor. 34
ISO 15970:2008(E)
9.1 Principle of measurement . 34
9.2 Working principle. 35
9.3 Performance assessment and acceptance tests. 38
9.4 Sampling and installation guidelines. 38
9.5 Calibration. 39
9.6 Verification. 40
9.7 Maintenance. 40
9.8 Quality control. 40
Annex A (informative) Guidance for instrument selection, instrument test
and operational procedures. 41
Annex B (informative) Instrument documentation. 45
Bibliography . 47

iv © ISO 2008 – All rights reserved

ISO 15970:2008(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 2.
The main task of technical committees is to prepare International Standards. 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 document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 15970 was prepared by Technical Committee ISO/TC 193, Natural gas.
ISO 15970:2008(E)
Introduction
The transmission of natural gas can involve passage across national boundaries; at border stations and
elsewhere, knowledge of the physicochemical properties of the fluid is of great operational and economic
importance. The energy flow and properties of the gas are required at several stages of the overall production
and custody transfer process: production, blending, transmission, metering, distribution and supply.
International standardization of the performance specifications for various types of measuring instruments can
facilitate comparison of, and increase confidence in, measurement results for contracting partners. In many
cases, it is possible to calculate the properties of natural gas with sufficient accuracy, given the composition.
However, it is often also possible to measure the property using techniques that do not require a
compositional analysis for their implementation.
This International Standard considers only those methods for determining physical properties of natural gas
that do not rely upon a detailed component analysis of the gas. Such measurements consider the “whole”
sample of the gas.
This International Standard defines performance characteristics necessary to specify instrumentation for
measurement of some natural gas properties. It provides guidelines for the installation, traceable calibration,
performance, operation, maintenance and acceptance testing of these measurement instruments.
The principle of measurement of various properties included in this International Standard is typical for a
number of applications.
It is required that the calibration of the instruments dealt with in this International Standard be traceable to
national standards or International Standards.
It is required that the measuring instruments, including their installation and the devices used for field
calibration, verification and maintenance comply with local legal regulations on application in hazardous areas.
Annex A presents general guidelines for instrument selection, instrument test and operational procedures of
the instruments considered in this International Standard.
Annex B lists the data of particular importance for the instrument documentation.

vi © ISO 2008 – All rights reserved

INTERNATIONAL STANDARD ISO 15970:2008(E)

Natural gas — Measurement of properties — Volumetric
properties: density, pressure, temperature and compression
factor
1 Scope
This international Standard gives requirements and procedures for the measurement of the properties of
natural gas that are used mainly for volume calculation and volume conversion: density at reference and at
operating conditions, pressure, temperature and compression factor.
Only those methods and instruments are considered that are suitable for field operation under the conditions
of natural gas transmission and distribution, installed either in-line or on-line, and that do not involve the
determination of the gas composition.
This International Standard gives examples for currently used instruments that are available commercially and
of interest to the natural gas industry.
NOTE Attention is drawn to requirements for approval of national authorization agencies and to national legal
regulations for the use of these devices for commercial or official trade purposes.
The density at reference conditions (sometimes referred to as normal, standard or even base density) is
required for conversion of volume data and can be used for other physical properties.
Density at operating conditions is measured for mass-flow measurement and volume conversion using the
observed line density and can be used for other physical properties. This International Standard covers
density transducers based on vibrating elements, normally suitable for measuring ranges of 5 kg/m to
250 kg/m .
Pressure measurement deals with differential, gauge and absolute pressure transmitters. It considers both
analogue and smart transmitters (i.e. microprocessor based instruments) and, if not specified otherwise, the
corresponding paragraphs refer to differential, absolute and gauge pressure transmitters without distinction.
Temperature measurements in natural gas are performed within the range of conditions under which
transmission and distribution are normally carried out (253 K < T < 338 K). In this field of application,
resistance thermometer detectors (RTD) are generally used.
The compression factor (also known as the compressibility factor or the real gas factor and given the
symbol Z) appears, in particular, in equations governing volumetric metering. Moreover, the conversion of
volume at metering conditions to volume at defined reference conditions can properly proceed with an
accurate knowledge of Z at both relevant pressure and relevant temperature conditions.
2 Normative references
The following referenced documents are indispensable for the application 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 2186, Fluid flow in closed conduits — Connections for pressure signal transmissions between primary
and secondary elements
ISO 15970:2008(E)
ISO 5167-1, Measurement of fluid flow by means of pressure differential devices inserted in circular cross-
section conduits running full — Part 1: General principals and requirements
ISO 6976, Natural gas — Calculation of calorific values, density, relative density and Wobbe index from
composition
ISO 10715, Natural gas — Sampling guidelines
ISO 12213-1, Natural gas — Calculation of compression factor — Part 1: Introduction and guidelines
IEC 60079-0, Explosive atmospheres — Part 0: Equipment — General requirements
IEC 60079-1, Explosive atmospheres — Part 1: Equipment protection by flameproof enclosures “d”
IEC 60079-11, Explosive atmospheres — Part 11: Equipment protection by intrinsic safety “i”'
IEC 60079-14, Explosive atmospheres — Part 14: Electrical installations design, selection and erection
IEC/TR 60079-15, Electrical apparatus for explosive gas atmospheres — Part 15: Construction, test and
marking of type of protection 'n' electrical apparatus
IEC 60381-1, Analogue signals for process control systems — Part 1: Direct current signals
IEC 60381-2, Analogue signals for process control systems — Part 2: Direct voltage signals
IEC 60751, Industrial platinum resistance thermometer sensors
IEC 60770-1, Transmitters for use in industrial-process control systems — Part 1: Methods for performance
evaluation
3 Terms and definitions
For the purpose of this document, the following terms and definitions apply.
3.1 Terms and definitions for density at reference conditions
3.1.1
density at reference conditions
mass of a gas divided by its volume at specified reference conditions of pressure and temperature
3.1.2
relative density at reference conditions
ratio of the mass of a gas, contained within an arbitrary volume, to the mass of dry air of standard composition
in accordance with ISO 6976, which would be contained in the same volume at the same references
conditions
3.2 Terms and definitions for density at operating conditions
3.2.1
density
mass of a gas divided by its volume at operating conditions of pressure and temperature (operating and
reference conditions)
2 © ISO 2008 – All rights reserved

ISO 15970:2008(E)
3.2.2
vibrating element density transducer
device that contains a vibrating element that is maintained at its natural frequency, made such that the
element contains or is surrounded by gas, the gas and the element forming a system where the density of the
gas is the main property of the gas determining the natural frequency of the element
NOTE The natural frequency to the first approximation is determined by the gas density.
3.2.3
main density transducer constants
constants that, to a first approximation, define the relationship between the natural frequency of the vibrating
element and the density of the gas
3.2.4
raw density
density as determined by a vibrating-element density transducer from its vibrating frequency by use of the
main density transducer constants before any corrections for temperature, pressure and composition are
applied
3.2.5
correction density transducer constants
constants applicable to a density transducer to correct for the deviation between the calibration condition
under which the main constants were determined and the operating conditions
3.2.6
temperature-corrected density
raw density corrected for difference in temperature to which the vibrating element is exposed in operation and
the temperature at which the density transducer was calibrated
3.2.7
compositional-corrected density
temperature-corrected density, corrected for difference in gas properties between gas to which the vibrating
element is exposed in operation and the gas properties of the gas used for calibration
NOTE Normally, the gas property relevant for this purpose is velocity of sound, hence this term is often referred to as
velocity-of-sound-corrected density.
3.2.8
line density
compositional-corrected density, corrected for difference in operating conditions, e.g. pressure and
temperature, to which the vibrating element is exposed and the operating conditions in the line where the
density is measured
3.3 Terms and definitions for pressure
3.3.1
pressure transmitter
device that responds to a measured pressure to produce a standard output signal for transmission, which has
a prescribed continuous relationship to the value of the measured pressure
3.3.2
lower range value
LRV
lowest value of the pressure that a transmitter is adjusted to measure
3.3.3
upper range value
URV
highest value of the pressure that a transmitter is adjusted to measure
ISO 15970:2008(E)
3.3.4
span
algebraic difference between the upper and lower range values
3.3.5
static pressure
pressure that would be measured by a pinpoint observer travelling with a particle of the fluid
3.3.6
absolute static pressure
static pressure of a fluid measured with reference to an absolute vacuum
3.3.7
gauge pressure
difference between the absolute static pressure of a fluid and the atmospheric pressure at the place and time
of the measurement
3.4 Terms and definitions for temperature
3.4.1
temperature transmitter
device that responds to a measured temperature to produce a standard output signal for transmission, which
has a prescribed continuous relationship to the value of the measured temperature
3.5 Terms and definitions for compression factor
3.5.1
least squares method
method used to compute the coefficients of the equation when a particular form of equation is chosen for
fitting a curve data
NOTE The principle of least squares is the minimization of the sum of squares of deviations of the data from the
curve.
4 Symbols and units
4.1 Symbols and subscripts for density at reference conditions
Symbol Quantity Unit
k Ratio of Z(p, T) and Z
—
Z n
p Absolute pressure Pa
Density
ρ kg/m
T Thermodynamic temperature K
Z Compression factor —
Subscripts
A Standard air
m Measured gas/measuring chamber
n Reference conditions
r Reference chamber
4 © ISO 2008 – All rights reserved

ISO 15970:2008(E)
4.2 Symbols and subscripts for density at operating conditions
Symbol Quantity Unit
C Velocity of sound m/s
C Velocity of sound in calibration gas m/s
c
C Velocity of sound in gas in density transducer m/s
g
−1
F
Frequency
s
a
b
Density transducer constants
K K K
1 2 N
ρ Raw density
kg/m
r
Temperature corrected density
ρ kg/m
t
ρ Compositional corrected density
kg/m
c
Line density
ρ kg/m
L
T Calibration temperature K
c
t
Calibration temperature °C
c
T
Temperature in density transducer K
d
T
Temperature in pipe K
L
t Temperature in density transducer
°C
d
t Temperature in pipe
°C
L
p Pressure in density transducer Pa
d
p Pressure in pipe Pa
L
a
The number of constants (n) can vary for the different types of density transducers. The manufacturers are allowed to use a
numbering system for constants different from the one used throughout this International Standard.
b 3
The unit of the various constants shall be such that all terms in Equations (4) and (5) come out with unit kg/m .
Subscripts
L Pipe or line
d Density transducer
4.3 Symbols and subscripts for compression factor
Symbol Quantity Unit
V Volume of the small vessel in the Z-meter m
V Volume of the large vessel in the Z-meter m
V Sum of volumes V and V m
3 1 2
p Line pressure Pa
p Pressure before expansion Pa
p Pressure after expansion Pa
Z Compression factor at conditions p and T —
1 1
Z Compression factor at conditions p and T —
2 2
ISO 15970:2008(E)
Z Compression factor at conditions p and T —
3 3
k Ratio of volumes V and V —
V 2 1
Coefficients of the polynomial of the compression
a
B (T),B (T)
1 2
factor as a function of the pressure
T Temperature of the Z-meter K
t Temperature of the Z-meter °C
Z Compression factor —
k Ratio of Z(p,T) and Z —
Z n
Coefficients in the function for the transfer in
a, b —
temperature
Coefficients in the function for the transfer in
b
e, f, g
pressure
a
The units of B(T ) and C(T ) shall be such that all resulting terms in Equations (9) and (10) are dimensionless.
b
The units of e, f and g shall be such that all resulting terms in Equation (12) are dimensionless.

Subscripts
i initial conditions
f final conditions
n reference conditions
5 Density at reference conditions
5.1 Principle of measurement
5.1.1 General
Two basic principles are used for measuring the density at reference conditions:
a) direct measurement, for example determining the buoyancy force of a defined volume of gas with a
balance system;
b) indirect measurement, for example determining the natural frequency of a vibrating element, which is
influenced by the density of the medium in which the element vibrates.
5.1.2 Balance system
The apparatus measures the buoyancy force of a closed, gas-filled glass bulb in an atmosphere of gas whose
density at reference conditions is being determined (see Figure 1).
The glass bulb is fitted to a balance beam with an open glass bulb as a counterweight. This weighing system
is mounted in a chamber through which the gas being tested is passed. Either the displacement of the
balance beam or the force that is necessary to compensate the displacement can be taken as a measure for
the density.
A correcting system compensates for the temperature and pressure fluctuations of the measuring chamber.
5.1.3 Vibrating element system
Two different systems are commonly used. Each consists of two chambers. One chamber is filled with a
reference gas that is similar to the gas being measured and sealed from the atmosphere. The gas being
tested is continuously passed through the other chamber. A pressure equalizer ensures that the pressure in
6 © ISO 2008 – All rights reserved

ISO 15970:2008(E)
the measuring chamber is equal to the pressure in the reference chamber. The housings of the systems are
designed in such a way that both gas chambers have the same temperature (see Figures 2 and 3).

Key
1 instrument housing
2 gas outlet
3 closed glass bulb
4 open glass bulb
5 gas inlet
6 pressure sensor
7 temperature sensor
8 magnet
9 compensation coil
10 photo sensor
11 PID regulator
12 display
Figure 1 — Gas density balance system
ISO 15970:2008(E)
Key
1 measuring chamber
2 reference chamber
3 vibrating element
4 diaphragm
5 pressure control valve
6 gas inlet
7 gas outlet
Figure 2 — Gas densitometer with one vibrating element
8 © ISO 2008 – All rights reserved

ISO 15970:2008(E)
Key
1 gas inlet
2 vibrating element
3 measuring chamber
4 reference chamber
5 pressure equalizer
6 gas outlet
Figure 3 — Gas densitometer with two vibrating elements
Generally, the density, ρ, is as given by Equation (1):
T
p 1
n
ρρ =  (1)
n
p Tk
nZ
where
p is the pressure;
T is the thermodynamic temperature;
k is the ratio (Z/Z ) of the compression factors;
Z n
n is the subscript indicating that the values are at reference conditions.
With equal pressure and temperature in the reference chamber (subscript r) and measuring chamber
(subscript m), the ratio of the respective densities is given by Equation (2):
ρ
ρ
k
nm,
mr
= (2)
ρρ k
rn,r m
Assuming that k /k is a constant, which is a good approximation for low pressures and means that the gases
r m
are similar, the quotient of the densities of the gas to be measured and of the reference gas is directly
proportional to the density at reference conditions, as given in Equation (3):
ρρ
km
mm
ρ
ρ == k ⋅ (3)
nr,
nm,
ρρ
k
r
rr
ISO 15970:2008(E)
where k is a constant.
The difference between the two systems is that one uses two vibrating elements to measure the density, one
in the reference chamber and one in the measuring chamber, whereas the other system has one vibrating
element system inside the measuring chamber and uses the fact that the design of the system ensures that
the density of the sealed reference gas is constant. The function of a vibrating element system is discussed in
6.1.
5.2 Performance assessment and acceptance tests
5.2.1 Requirements
The necessary requirements depend on the purpose of the measurement. The requirements affect several
characteristics of the instrument such as
a) accuracy, sensitivity,
b) safety,
c) reliability, long-term reproducibility,
d) insensitiveness to disturbances,
e) installation and calibration features,
f) response time,
g) robustness,
h) handling and maintenance features,
i) data handling, connections,
j) costs.
5.2.2 Performance tests
The manufacturer shall perform extensive laboratory tests on a selected number of instruments to verify the
performance of the instrument type and to ensure that its own and the purchasers' requirements are met.
Guidance and recommendations are given in IEC 60770-2.
The tests shall be carried out by connecting the instrument to reference gases with appropriate densities at
reference conditions and varying the parameter of interest to establish the influence on the instrument being
tested. The parameter can be, for example
a) the temperature of the sample gas,
b) the operating pressure,
c) the ambient temperature and pressure,
d) the flow rate through the instrument and back-pressure effects,
e) the humidity of the sample gas,
f) the supply voltage,
g) the mounting position,
h) the magnetic and electric fields,
i) contaminents within the gas sample.
10 © ISO 2008 – All rights reserved

ISO 15970:2008(E)
To check for effects due to gas composition, tests should be carried out using reference gases with different
compositions but similar densities at reference conditions. Tests for checking repeatability and long-term drift
shall be performed under field conditions also.
The results of all tests shall be properly reported and made available to the user of the instrument.
Prior to delivery, the manufacturer shall carry out a metrologically traceable factory acceptance test to ensure
proper operation for each instrument yielding individual test certificates. An example of a scheme for the
factory acceptance testing is given in Annex A. During this test, the vibrating element systems may be
calibrated whereas the balance systems shall be calibrated and carefully tested additionally on site.
NOTE As most of the tests recommended in 5.2 for density at reference conditions are also valid for density at
operating conditions, refer to 6.2 for a more extensive description.
5.3 Sampling and installation guidelines
The installation guidelines of the manufacturer shall be observed. Since the instruments are sensitive to
fluctuations in temperature, special care shall be taken to meet the temperature-fluctuation requirements for
operation, calibration and verification.
In order to minimize the reaction time of the measurement, pressure reduction shall be close to the sampling
point and the whole piping shall be as short as possible. Another instrument (for example a calorimeter) may
be connected to the same sampling line but each instrument shall have a separate vent.
Depending on the sample gas condition, additional filters shall be installed to protect the instrument from
contamination. Precautions shall be taken to prevent a change in the gas composition due to a temperature
drop caused by a reduction in pressure.
The sample flow rate shall be controlled.
It shall be possible to test and calibrate the system by connecting reference gases to it. It shall be ensured, for
example by the use of a double block and bleed valve design, that valve leaks do not lead to a mixing of the
gases.
5.4 Calibration
The technical manuals and manufacturers' instructions for calibration and recalibration and the requirements
of the authorities stipulated in the approvals of the instruments shall be observed.
The systems shall be calibrated by connecting them to reference gases. These gases shall be either one-
component gases of high purity (> 99,95 %) or reference gas mixtures.
One of the gases shall have a density at reference conditions which is in the range of that of the operating gas
and, for vibrating elements, the same applies for Z /Z and the velocity of sound.
n
Balance systems, which are very sensitive to environmental influences, shall be carefully tested and calibrated
on site. Vibrating element systems, which are relatively robust, may be calibrated in a laboratory or on site.
However, as the calibration curve constants for vibrating elements depend on the gas in the reference
chamber, for those reference chambers that are refilled on site, a recalibration on site is required.
A minimum calibration interval is one year, but shall not exceed five years. The user shall consider the
instrument's accuracy during operations and its financial impact on the results.
5.5 Verification
Verification of the correct operation and accuracy shall be performed at regular intervals by connecting the
system to a reference gas that has operating properties similar to those of the operating gas. In particular, for
all instruments, the reference gas shall have a density at reference conditions that is similar and in addition,
for vibrating elements, the same applies for Z /Z and the velocity of sound properties should be similar.
n
This procedure may be performed automatically.
ISO 15970:2008(E)
The acceptable limits of the deviation shall be established on the basis of, for example, the manufacturer’s
recommendations using control charts or parties’ agreements.
If the deviations are within these limits, a correction may be applied to the measured values without
recalibrating the system; if the deviations are outside the limits, the system shall be recalibrated.
5.6 Maintenance
Filters shall be checked and replaced at regular intervals. The condition of the operating gas governs the
length of time between the checks.
5.7 Quality control
If there are two density-measuring systems at one station, the results should be compared continuously and
an alarm should be given if the deviation exceeds predetermined limits.
Alternatively, the proper functioning of the measurement system may be ensured by comparing, at regular
intervals, the measured density with that calculated from a component analysis of a gas sample taken from
the line, either on-line or off-line.
6 Density at operating conditions
6.1 Principle of measurement
6.1.1 Vibrating element
This principle utilizes the fact that an element in a fluid is excited to vibrate at its natural frequency which
depends on the density of the fluid. Hence, by measuring the natural frequency, the density can be
determined.
The basic correlation between density, ρ , and the natural frequency, f, is represented by the second order
r
equation as given in Equation (4):
⎛⎞11⎛ ⎞
ρ=+KK +K (4)
⎜⎟ ⎜ ⎟
r0 1 2
f f
⎝⎠ ⎝ ⎠
6.1.2 Temperature and pressure correction
The relationship between the density and the frequency is, in principle, also affected by temperature and
pressure. This requires the introduction of pressure and temperature correction terms.
The main reason for the temperature and pressure correction is that the material properties, such as elasticity,
of the vibrating element are affected by temperature and pressure. Normally, the pressure effect is negligible.
The temperature effects can influence both the zero and the span. The correction can normally be expressed
as a first-order correction term as given in Equation (5):
⎡⎤
ρρ=+1 K()TT− +K (T−T ) (5)
tr 3 d c 4 d c
⎣⎦
6.1.3 Velocity of sound (VOS) correction
The densitometer constants K , K and K in Equation (4) also depend on the type of gas with which the
0 1 2
vibrating element is in contact.
12 © ISO 2008 – All rights reserved

ISO 15970:2008(E)
The gas quality parameter that best describes this dependence is the velocity of sound in the gas at operating
conditions.
K , K and K in Equation (4) are determined by calibration using a particular gas. If the velocity of sound in
0 1 2
the gas for which the density is being determined is different from the velocity of sound in the calibration gas,
the density resulting from the simple relation in Equation (4) shall be corrected.
Equation (6) describes the correction:
⎡⎤
⎛⎞
f
⎢⎥
1+ K
⎜⎟
⎢⎥
c
⎝⎠c
⎢⎥
ρρ= (6)
ct
⎢⎥
⎛⎞
f
⎢⎥
1+⎜K ⎟
⎢⎥⎜⎟
c
g
⎝⎠
⎣⎦
where c is the velocity of sound in the calibration gas at the same conditions of pressure and temperature at
c
which the density, ρ , is referred or at which the vibrating element is vibrating with the corresponding
r
frequency.
For a given temperature and pressure, c depends on the natural gas composition. Typically, a change in the
g
composition of the pipeline gas, resulting in a change in c of 1
...