ISO/TR 12148:2009

TECHNICAL ISO/TR
REPORT 12148
First edition
2009-02-15
Natural gas — Calibration of chilled
mirror type instruments for hydrocarbon
dewpoint (liquid formation)
Gaz naturel — Étalonnage des instruments du type à miroir refroidi pour
points de rosée hydrocarbures (formation de liquide)

Reference number
©
ISO 2009
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

©  ISO 2009
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2009 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Symbols . 2
5 Performance characteristics of automatic weighing method in accordance with
ISO 6570:2001. 2
5.1 Working principle. 2
5.2 Functional requirements. 4
5.3 Measurement uncertainty . 5
6 Performance requirements for a chilled-mirror-type instrument for hydrocarbon-dew-
point determination . 6
6.1 Working principle. 6
6.2 Precision. 7
7 Requirements/Points of interest to carry out a calibration . 8
7.1 Process gas. 8
7.2 Sampling system . 8
7.3 Selection of the PHLC reference value. 9
7.4 Compositional range of gases covered by the calibration procedure. 10
7.5 Calibration interval . 10
8 Execution of calibration procedure . 10
8.1 General. 10
8.2 Method A — Change in trip point value. 11
8.3 Method B — Direct change of dew point value . 12
8.4 Which method should be used? . 12
Annex A (informative) Condensation behaviour of natural gas . 13
Annex B (informative) Example of a hydrocarbon-dew-point analysis . 16
Annex C (informative) Examples of a calibration. 19
Annex D (informative) Performance of different types of chilled-mirror instruments for
hydrocarbon dew point determination .29
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 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.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
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/TR 12148 was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee SC 1, Analysis
of natural gas.
iv © ISO 2009 – All rights reserved

Introduction
Under certain conditions, higher hydrocarbons present in natural gas or similar gases can condense and the
hydrocarbon liquids formed can cause difficulties in the operation of gas transport and distribution systems.
Hydrocarbon dew-point measurements, by condensation on a mirror, can give an indication of the conditions
under which condensation starts. Theoretically, the hydrocarbon dew point is the temperature at which the first
small droplets of liquid are formed at a fixed pressure. In practice, all dew-point-measurement methods are
based on the observation of the formation of a film of hydrocarbon liquids on the surface of an illuminated,
cooled mirror. The observation can be done visually (manual mirror) or by an electronic sensor (automatic
chilled mirror). The cooling can be achieved in two ways: expansion of natural gas, compressed air or carbon
dioxide or by applying a Peltier-cooling device. Either manual or automatic chilled mirrors can be applied to
measure the hydrocarbon dew point of natural gas.
It is not possible to calibrate commercially available hydrocarbon dew point analysers in a traceable way,
because no hydrocarbon dew-point reference material or reference instrument is available. Because of
differences in working principles, analysers from different manufacturers can give different values for the
hydrocarbon dew point for a given gas. In practice, the dew point of an automatic dew point monitor is often
“tuned” to match the value measured by a manual chilled mirror, or “tuned” to the value calculated from a
known gas composition using a thermodynamic model.
Modern automatic hydrocarbon-dew-point chilled-mirror instruments have the possibility of adjusting the value
of the presented hydrocarbon dew point. Sometimes this adjustment is carried out by changing a physical
setting in the instrument itself; in other cases the setting can be changed by entering a new value into the
instrument’s controller. The availability of such an adjustable parameter is a prerequisite for using the
calibration procedure described in this Technical Report.
NOTE 1 Changing the setting of such a parameter can result in a major change in the presented hydrocarbon-dew-
[2]
point value. For example, a study carried out under the auspices of the National Physical Laboratory shows from real
measurements that the hydrocarbon dew point for a natural gas measured at the same pressure varies between −6,28 °C
and 8,54 °C [Gas B, with pressure equal to 3,5 MPa (35 bars), detector sensitivity varying between 110 mV and 275 mV].
[2]
In 2007, the results of a comprehensive study of the measurement of the hydrocarbon dew point of real and
synthetic natural gases was published. In this study, six analytical methods were examined: one automatic
chilled-mirror instrument, one manual chilled-mirror instrument, two laboratory gas chromatographs and two
process gas chromatographs. In this study, it is concluded that the role of accurate synthetic gas mixtures in
the calibration of chilled-mirror instruments is limited. Furthermore, it is stated that a standard composed of n-
butane in nitrogen is indeed a straightforward and inexpensive way to calibrate a chilled-mirror instrument, yet
forms an atypical, rapidly condensing hydrocarbon film. Therefore, such a calibration gas has limited use in
calibrating a hydrocarbon-dew-point instrument.
NOTE 2 Several studies showed the importance of not only the component concentration in a calibration mixture but
[4]
also knowledge of the nature of the components used. For example, a GERG study shows that adding aromatic and/or
cyclic hydrocarbons has a significant influence on the hydrocarbon dew point.
Based on the working principle of chilled-mirror devices, there are five major sources that can be responsible
for significant systematic errors in the measured hydrocarbon dew point and for which no adjustment can be
made because no proper calibration method exists. These five sources are
a) the often significant amount of liquid that it is necessary to form before the instrument is able to detect the
dew point temperature;
b) the cooling rate, which is often too fast to ensure that the temperature measured by a temperature sensor
somewhere in the mirror equals the temperature of the mirror surface and the temperature of the gas in
the measuring cell;
c) the way the gas flow passes through the measurement cell during the actual measuring phase
(continuous versus stop-flow principle);
d) the measurement of the mirror temperature, which doesn’t take place at the mirror surface itself but near
the mirror surface;
e) the hydrocarbon dew point that, when measured at a certain pressure setting, doesn’t necessarily
corresponds to the cricondentherm pressure valid for the actual gas composition.
In this Technical Report, a calibration procedure is presented which allows the adjustment and even the
calibration of a hydrocarbon-dew-point chilled-mirror analyser against the indirect automatic weighing method
according to ISO 6570. By using this procedure, the measured hydrocarbon dew point corresponds
unambiguously to a given value for the potential hydrocarbon liquid content (PHLC) at this measured dew-
point temperature. In this way, a traceable and much more objective measurement of the hydrocarbon dew
point is possible. By doing an on-site comparison/calibration against the indirect automatic weighing method, it
is even possible to correct for the gas-dependent performance of the hydrocarbon-dew-point analyser, which
exists to some extent for different natural gases.
Measurements carried out according to ISO 6570 consist of cooling down a well defined gas flow to a
specified and accurately measured temperature. The gas has enough time to form liquid and to establish a
gas-liquid equilibrium at a certain pressure and temperature. This process is similar to the process that occurs
in practice, when in a pipeline the pressure and/or temperature are reduced, the complete gas flow is cooled
down and depending on the gas quality in the worst case can result in hydrocarbon liquid drop out. By
calibrating a dew point analyser against I
...