ISO 19970:2025

International
Standard
ISO 19970
Second edition
Refrigerated hydrocarbon and non-
2025-07
petroleum based liquefied gaseous
fuels — Metering of gas as fuel on
LNG carriers during cargo transfer
operations
Hydrocarbures réfrigérés et combustibles gazeux liquéfiés à
base non pétrolière — Mesurage du gaz comme carburant sur
les transporteurs de GNL pendant les opérations de transfert de
cargaison
Reference number
© ISO 2025
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Design requirements . 3
4.1 General .3
4.2 BOG measurement system .3
4.2.1 General .3
4.2.2 Flowmeter .4
4.2.3 Associated measuring instruments .4
4.2.4 Flow computer . .4
4.2.5 Indicating device .4
5 Uncertainty requirement . 4
6 Calibration and uncertainty verification . 5
6.1 General .5
6.2 Calibration at laboratory or factory before installation .5
6.3 Calibration and verification after installation .5
6.4 Periodic uncertainty verification of BOG measurement system .5
7 Metering and calculation . 6
7.1 General .6
7.2 Metering .6
7.3 Calculation of heating value . .6
Annex A (informative) Typical installation of fuel gas flowmeters . 7
Annex B (informative) Calculation of energy transferred . 8
Annex C (informative) Uncertainty calculation examples . 9
Bibliography .13

iii
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
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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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
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This document was prepared by Technical Committee ISO/TC 28, Petroleum and related products, fuels and
lubricants from natural or synthetic sources, Subcommittee SC 5, Measurement of refrigerated hydrocarbon
and non-petroleum based liquefied gaseous fuels.
This second edition cancels and replaces the first edition (ISO 19970:2017), which has been technically
revised.
The main changes are as follows:
— boil-off gas (BOG) measurement system has been defined;
— uncertainty calculation examples have been added to Annex C.
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.

iv
Introduction
[14]
Concerns over the emission of pollutants from ship exhausts prompted IMO to enact MARPOL Annex VI
to the IMO Protocol of 1997, which sets limits on sulfur oxide (SOx) and nitrogen oxide (NOx) emissions.
MARPOL Annex VI also specifies a global cap which limits the permitted sulfur content of fuel oil to a mass
fraction of 3,5 % and special SOx emission control areas (SECAs) where the sulfur contents must not exceed a
mass fraction of 1,0 %. In order to comply with these requirements, ships generally require special facilities
to be installed to limit SOx emissions, unless they use low sulfur fuel oil.
Liquefied natural gas (LNG) carriers are capable of utilizing boil-off gas (BOG) in ships and shore tanks.
They return gas during the cargo transfer operation as fuel for their own power generation, if commercial
value of such energy consumed on board is accountable as an element of custody transfer measurement. This
document provides the procedures for metering gas and assessing its calorific value, and the requirements
for metering devices.
Aspects of safety are not dealt with in this document. It is the responsibility of the user to ensure that the
system meets applicable safety regulations.

v
International Standard ISO 19970:2025(en)
Refrigerated hydrocarbon and non-petroleum based liquefied
gaseous fuels — Metering of gas as fuel on LNG carriers
during cargo transfer operations
1 Scope
This document specifies the minimum requirements to quantify boil-off gas (BOG) consumed on liquefied
natural gas (LNG) carriers for their own functions, in particular for power generation and during cargo
transfer operations.
This document provides requirements for the metering of BOG and the subsequent calorific value
calculations, which can be taken into account when the energy transferred during cargo transfer is
determined. This document also gives performance requirements and calibration of the elements included
in the BOG measurement system.
This document, with some modifications, can also be applied to the measurement of BOG consumed by LNG
carriers at sea.
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
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
boil off
process of evaporation of a liquid resulting from heat ingress or a drop in pressure
[SOURCE: ISO 10976:2023, 3.1.6]
3.2
boil-off gas
BOG
vapour (3.11) produced by boil off (3.1)
Note 1 to entry: BOG in this document refers to the low molecular gas returned from shore tanks to ship tanks and the
gas produced in ship cargo tanks during cargo transfer operation.
[SOURCE: ISO 10976:2023, 3.1.7 — Note 1 to entry has been added.]

3.3
custody transfer measurement
measuring of liquid level, liquid and vapour temperature, vapour pressure and analysis of the composition of
liquid natural gas to be delivered to or from a tank, by which volumetric and other data are determined as a
basis of payment of cost or assessment of duty
3.4
custody transfer measurement system
CTMS
structure that processes inputs from level gauges, thermometers, pressure gauges, etc., and provides
custody transfer measurement information on board, generating documents with regard to custody transfer
of liquid natural gas
[SOURCE: ISO 18132-1:2011, 2.1.4, modified]
3.5
flowmeter
flow measuring device which indicates the measured flowrate
3.6
gas dangerous space
gas dangerous zone
area which consists of the following places:
— a space in the cargo area which is not arranged or equipped in an approved manner to ensure that its
atmosphere is at all times maintained in a gas-safe condition;
— an enclosed space outside the cargo area through which any piping containing liquid or gaseous products
passes, or within which such piping terminates, unless approved arrangements are installed to prevent
any escape of product vapour into the atmosphere of that space;
— a cargo containment system and cargo piping;
— a hold space where cargo is carried in a cargo containment system requiring a secondary barrier;
— a hold space where cargo is carried in a cargo containment system not requiring a secondary barrier;
— a space separated from a hold space described in 4.1 [of the International Code for the Construction and
Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code)] by a gas-tight steel boundary;
— a cargo pump room and cargo compressor room;
— a zone on the open deck, or semi-enclosed space on the open deck, within 3 m of any cargo tank outlet,
gas or vapour outlet, cargo pipe flange or cargo valve or of entrances and ventilation openings to cargo
pump rooms and cargo compressor rooms;
— the open deck over the cargo area and 3 m forward and aft of the cargo area on the open deck up to a
height of 2,4 m above the weather deck;
— a zone within 2,4 m of the outer surface of a cargo containment system where such surface is exposed to
the weather;
— an enclosed or semi-enclosed space in which pipes containing products are located. A space which
contains gas detection equipment complying with 13.6.5 (of the IGC code) and a space utilizing boil-off
gas as fuel and complying with Chapter 16 are not considered gas-dangerous spaces in this context;
— a compartment for cargo hoses;
— an enclosed or semi-enclosed space having a direct opening into any gas-dangerous space or zone.
Note 1 to entry: This is defined by the International Code for the Construction and Equipment of Ships Carrying
[12]
Liquefied Gases in Bulk (IGC Code).

3.7
inherent error
difference of a measuring device when it is tested against a reference standard under controlled conditions
as specified by the manufacturer
3.8
LNG carrier
cargo ship specifically constructed and used for the carriage of liquid natural gas in bulk
3.9
maximum permissible error
extreme value of measurement error, with respect to a known reference quantity value, permitted by
specifications or regulations for a given measurement, measuring instrument, or measuring system
[SOURCE: ISO/IEC Guide 99:2007, 4.26, modified — The Notes to entry have been removed.]
3.10
uncertainty
non-negative parameter characterizing the dispersion of the quantity values being attributed to a
measurand, based on the information used
3.11
vapour
fluid in the gaseous state that is transferred to/from or contained within the cargo tank
3.12
verification
process of confirming the accuracy of an instrument by comparing to a source with known accuracy
4 Design requirements
4.1 General
Flowmeters shall be constructed so as not to leak gas into the atmosphere. In addition, all parts of a flowmeter
in contact with vapour that is caused by boil off shall be chemically compatible with the product, to avoid
both product contamination and corrosion of the flowmeter. Flowmeters installed in a gas dangerous space
or zone shall be of gastight construction.
All electric components of a flowmeter for use in electrically classified areas shall meet the electrical area
classification. They are expected to conform to applicable sections of the national and international electrical
safety standards. All flowmeters shall be maintained in safe operating condition and the manufacturers’
maintenance instructions should be complied with.
Flowmeters of any type may be used to measure BOG as long as they meet the uncertainty requirements
specified in Clause 5 of this document.
4.2 BOG measurement system
4.2.1 General
The BOG measurement system consists of the following constituents:
— flowmeter;
— associated measuring instruments;
— flow computer;
— indicating device.
...