prEN IEC 61757-2-2:2026

• Status: Not Published
• Publication Date: 01-Feb-2028
• ICS: 33.180.99 - Other fibre optic equipment
• Technical Committee: CLC/SR 86C - Fibre optic systems and active devices
• Current Stage: 4020 - Enquiry circulated - Enquiry
• Start Date: 10-Jul-2026
• Due Date: 18-Dec-2026
• Completion Date: 10-Jul-2026
• Ref Project: oSIST prEN IEC 61757-2-2:2026 - Fibre optic sensors - Part 2-2: Temperature measurement - Distributed sensing

• Revises: EN 61757-2-2:2017 - Fibre optic sensors - Part 2-2: Temperature measurement - Distributed sensing
• Effective Date: 02-Dec-2025

Overview

prEN IEC 61757-2-2:2026 is a committee draft international standard developed by CLC and the IEC for fibre optic sensors-specifically, performance parameters and testing procedures for distributed temperature sensing (DTS) using fibre optic technologies. This standard is part of the broader IEC 61757 series, which addresses generic specifications and individual requirements for various fibre optic sensors. The focus of Part 2-2 is on distributed temperature sensing, including key terminology, measurement methods, and test setups for widely used techniques based on Raman, Brillouin, and Rayleigh scattering.

Distributed fibre optic temperature sensing systems provide continuous temperature measurement along the length of an optical fibre, offering high spatial resolution and reliability for demanding environments. This standard facilitates interoperability, consistency, and metrological reliability in industrial, energy, and infrastructure applications worldwide.

Key Topics

• Distributed Fibre Optic Temperature Sensing (DTS): Coverage of DTS systems, their functionality, and performance evaluation.
• Measurement Methods: Standardized test setups, measurement procedures, and calculation of key parameters.
• Key Performance Parameters:
  • Temperature measurement error
  • Spatial resolution and sample spacing
  • Temperature repeatability and uncertainty
  • Attenuation range and distance measurement range
  • Environmental temperature stability and repeatability
• Test Setups: Guidelines for single-ended and loop configurations, including temperature-controlled enclosures and reference sensors.
• Terminology & Definitions: Harmonized use of technical terms, symbols, and abbreviated terms to aid global understanding and harmonization.
• Documentation Requirements: Comprehensive checklist for recording all influential factors, test conditions, system details, and calibration requirements.

Applications

Distributed fibre optic temperature sensing systems standardized under prEN IEC 61757-2-2:2026 are crucial in diverse sectors where accurate, real-time, and spatially resolved temperature data are critical, including:

• Energy and Power Grids: Monitoring of cable temperature, transformer winding, and substation environments for preventive maintenance.
• Oil & Gas: Downhole well monitoring, leak detection in pipelines, and reservoir management.
• Industrial Automation: Process control in chemical plants and refineries to ensure operational safety and process optimization.
• Civil Infrastructure: Structural health monitoring in bridges, tunnels, and buildings by embedded fibre sensors.
• Fire Detection: Early detection of overheating and fire along transport tunnels or high-risk facilities.
• Environmental Monitoring: Monitoring of permafrost or groundwater temperature profiles in geotechnical and environmental studies.

By providing harmonized performance parameters and repeatable measurement protocols, this standard supports system integrators, manufacturers, and end users in selecting and validating DTS solutions that meet strict regulatory and operational requirements.

Related Standards

• IEC 61757:2026 – Fibre optic sensors - Generic specification: Defines general requirements for all fibre optic sensing technologies, forming the foundation for part 2-2.
• IEC TR 61931 – Fibre optic - Terminology: Provides comprehensive vocabulary for the fibre optic field, ensuring consistency in terms.
• IEC 60050 – International Electrotechnical Vocabulary: Essential reference for all IEC standards.
• ISO/IEC Guide 99 – International vocabulary of metrology: Key reference for measurement-related terms and concepts.

For a full understanding of distributed sensing performance criteria and their evaluation methods, prEN IEC 61757-2-2:2026 should be used in conjunction with these foundational standards.

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Keywords: fibre optic sensors, distributed temperature sensing, DTS, IEC 61757, Raman scattering, Brillouin scattering, Rayleigh scattering, performance parameters, temperature measurement, spatial resolution, test setups, standardization, instrumentation, industrial monitoring.

SLOVENSKI STANDARD
01-september-2026
Optična zaznavala - 2-2. del: Merjenje temperature - Razpršeno zaznavanje
Fibre optic sensors - Part 2-2: Temperature measurement - Distributed sensing
Lichtwellenleitersensoren - Teil 2-2: Temperaturmessung - Ortsaufgelöste faseroptische
Messung
Capteurs à fibres optiques - Partie 2-2: Mesure de température - Détection répartie
Ta slovenski standard je istoveten z: prEN IEC 61757-2-2:2026
ICS:
17.200.20 Instrumenti za merjenje Temperature-measuring
temperature instruments
33.180.99 Druga oprema za optična Other fibre optic equipment
vlakna
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

86C/2021/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 61757-2-2 ED2
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2026-07-10 2026-10-02
SUPERSEDES DOCUMENTS:
86C/2006/CD, 86C/2013B/CC
IEC SC 86C : FIBRE OPTIC SYSTEMS, SENSING AND ACTIVE DEVICES
SECRETARIAT: SECRETARY:
United States of America Mr Fred Heismann
OF INTEREST TO THE FOLLOWING COMMITTEES: HORIZONTAL FUNCTION(S):

TC 17,TC 18,TC 20,TC 38,TC 45,TC 65,TC 85
ASPECTS CONCERNED:
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft
for Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of
which they are aware and to provide supporting documentation.
Recipients of this document are invited to submit, with their comments, notification of any relevant “In Some Countries”
clauses to be included should this proposal proceed. Recipients are reminded that the CDV stage is the final stage for
submitting ISC clauses. (SEE AC/22/2007 OR NEW GUIDANCE DOC).

TITLE:
Fibre optic sensors - Part 2-2: Temperature measurement - Distributed sensing

PROPOSED STABILITY DATE: 2030
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IEC CDV 61757-2-2 © IEC 2026
1 CONTENTS
2 FOREWORD . 3
3 INTRODUCTION . 5
4 1 Scope . 6
5 2 Normative references . 6
6 3 Terms, definitions, abbreviated terms, and symbols . 6
7 3.1 Terms and definitions . 6
8 3.2 Abbreviated terms . 10
9 3.3 Symbols . 10
10 4 General test setups for measurement of performance parameters . 11
11 4.1 General and test setup requirements . 11
12 4.2 General required information to be documented . 14
13 5 Measurement procedures for performance parameters . 15
14 5.1 Temperature measurement error . 15
15 5.1.1 Test procedure and conditions . 15
16 5.1.2 Parameter calculation . 15
17 5.2 Spatial resolution . 17
18 5.2.1 Test procedure and conditions . 17
19 5.2.2 Parameter calculation . 18
20 5.3 Temperature repeatability . 19
21 5.3.1 Test procedure and conditions . 19
22 5.3.2 Parameter calculation . 19
23 5.3.3 Formulas . 19
24 5.4 Spatial temperature uncertainty . 20
25 5.4.1 Test procedure and conditions . 20
26 5.4.2 Parameter calculation . 20
27 5.5 Environmental temperature stability . 21
28 5.5.1 Test procedure and conditions . 21
29 5.5.2 Parameter calculation . 22
30 5.6 Warm-up time . 23
31 5.6.1 Test procedure and conditions . 23
32 5.6.2 Parameter calculation . 24
33 5.7 Attenuation range . 24
34 5.7.1 Test procedure and conditions . 24
35 5.7.2 Parameter calculation . 25
36 Annex A (informative) Measurement parameter performance table . 26
37 Annex B (informative) Point defect effects . 28
38 B.1 General . 28
39 B.2 Point defect . 28
40 B.3 Test procedures and conditions . 28
41 Bibliography . 31
43 Figure 1 – General test setup: single-ended configuration . 12
44 Figure 2 – General test setup: loop configuration . 13
45 Figure 3 – Temperature measurement error calculation: step a) . 16
46 Figure 4 – Temperature measurement error calculation: steps b) and c) . 17
IEC CDV 61757-2-2 © IEC 2026
47 Figure 5 – Temperature measurement error calculation: steps d) and e) . 17
48 Figure 6 – Spatial resolution illustration . 18
49 Figure 7 – Temperature repeatability calculated from Figure 3 . 19
50 Figure 8 – Spatial temperature uncertainty calculated from Figure 3 . 21
51 Figure 9 – Environmental temperature stability parameter calculation method. 23
52 Figure 10 – Example illustrating calculation of warm-up time . 24
53 Figure B.1 – Point defect measurement (example) . 30
55 Table 1 – Environmental temperature cycle . 22
56 Table A.1 – Blank measurement parameter performance table . 27
IEC CDV 61757-2-2 © IEC 2026
58 INTERNATIONAL ELECTROTECHNICAL COMMISSION
59 ____________
61 Fibre optic sensors -
62 Part 2-2: Temperature measurement - Distributed sensing
65 FOREWORD
66 1) The International Electrotechnical Commission (IEC) is a worldwide organization for
67 standardization comprising all national electrotechnical committees (IEC National Committees).
68 The object of IEC is to promote international co-operation on all questions concerning
69 standardization in the electrical and electronic fields. To this end and in addition to other
70 activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
71 Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC
72 Publication(s)"). Their preparation is entrusted to technical committees; any IEC National
73 Committee interested in the subject dealt with may participate in this preparatory work.
74 International, governmental and non-governmental organizations liaising with the IEC also
75 participate in this preparation. IEC collaborates closely with the International Organization for
76 Standardization (ISO) in accordance with conditions determined by agreement between the two
77 organizations.
78 2) The formal decisions or agreements of IEC on technical matters express, as nearly as
79 possible, an international consensus of opinion on the relevant subjects since each technical
80 committee has representation from all interested IEC National Committees.
81 3) IEC Publications have the form of recommendations for international use and are accepted
82 by IEC National Committees in that sense. While all reasonable efforts are made to ensure that
83 the technical content of IEC Publications is accurate, IEC cannot be held responsible for the
84 way in which they are used or for any misinterpretation by any end user.
85 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC
86 Publications transparently to the maximum extent possible in their national and regional
87 publications. Any divergence between any IEC Publication and the corresponding national or
88 regional publication shall be clearly indicated in the latter.
89 5) IEC itself does not provide any attestation of conformity. Independent certification bodies
90 provide conformity assessment services and, in some areas, access to IEC marks of conformity.
91 IEC is not responsible for any services carried out by independent certification bodies.
92 6) All users should ensure that they have the latest edition of this publication.
93 7) No liability shall attach to IEC or its directors, employees, servants or agents including
94 individual experts and members of its technical committees and IEC National Committees for
95 any personal injury, property damage or other damage of any nature whatsoever, whether direct
96 or indirect, or for costs (including legal fees) and expenses arising out of the publication, use
97 of, or reliance upon, this IEC Publication or any other IEC Publications.
98 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced
99 publications is indispensable for the correct application of this publication.
100 9) IEC draws attention to the possibility that the implementation of this document may involve
101 the use of (a) patent(s). IEC takes no position concerning the evidence, validity or applicability
102 of any claimed patent rights in respect thereof. As of the date of publication of this document,
103 IEC had not received notice of (a) patent(s), which may be required to implement this document.
104 However, implementers are cautioned that this may not represent the latest information, which
IEC CDV 61757-2-2 © IEC 2026
105 may be obtained from the patent database available at https://patents.iec.ch. IEC shall not be
106 held responsible for identifying any or all such patent rights.
107 IEC 61757-2-2 has been prepared by subcommittee 86C: Fibre optic systems, sensing and
108 active devices of IEC technical committee 86: Fibre optics. It is an International Standard.
109 This second edition cancels and replaces the first edition published in 2016. This edition
110 constitutes a technical revision.
111 This edition includes the following significant technical changes with respect to the previous
112 edition:
113 a) updates of the terms and definitions;
114 b) addition of a symbol list;
115 c) updates of Figure 1, Figure 2, Figure 6, Figure 9, Figure 10, and Figure B.1;
116 d) addition of Table 1.
117 The text of this International Standard is based on the following documents:
Draft Report on voting
XX/XX/FDIS XX/XX/RVD
118 Full information on the voting for its approval can be found in the report on voting indicated in
119 the above table.
120 The language used for the development of this International Standard is English [change
121 language if necessary].
122 This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
123 accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
124 at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
125 described in greater detail at www.iec.ch/publications.
126 A list of all parts in the IEC 61757:2026, published under the general title Fibre optic sensors,
127 can be found on the IEC website.
128 The committee has decided that the contents of this document will remain unchanged until the
129 stability date indicated on the IEC website under webstore.iec.ch in the data related to the
130 specific document. At this date, the document will be
131 – reconfirmed,
132 – withdrawn, or
133 – revised.
IEC CDV 61757-2-2 © IEC 2026
134 INTRODUCTION
135 This document is part of the IEC 61757:2026, which is dedicated to fibre optic sensors. Generic
136 specifications for fibre optic sensors are defined in IEC 61757:2026.
137 The individual parts of the IEC 61757:2026 are numbered as IEC 61757:2026-M-T, where M
138 denotes the measurand and T the technology of the fibre optic sensor. The IEC 61757-2-T
139 series is concerned with temperature measurements.
IEC CDV 61757-2-2 © IEC 2026
140 1 Scope
141 This part of IEC 61757 defines the terminology, structure, and measurement methods of
142 distributed fibre optic sensors for temperature sensing (DTS). DTS includes the use of Raman
143 scattering, Brillouin scattering and Rayleigh scattering effects. In addition, Raman scattering
144 and Rayleigh scattering based measurements are performed with a single-ended fibre
145 configuration only. Brillouin scattering based measurements are performed with a single -ended
146 fibre or fibre loop configuration. The technique accessible from both sides at same time (e. g.
147 Brillouin optical time domain analysis, BOTDA) is referred to here as a loop configuration.
148 This document specifies the most important DTS performance parameters and defines the
149 procedures for their determination. In addition to the group of performance parameters, a list of
150 additional parameters has been defined to support the definition of the measurement
151 specifications and their associated test procedures. The definitions of these additional
152 parameters are provided for informational purposes and should be included with the sets of
153 performance parameters.
154 A general test setup is defined in which all parameters can be gathered through a set of tests.
155 The specific tests are described within the clause for each measurement parameter. This
156 general test setup is depicted and described in Clause 4 along with a list of general information
157 that will be documented based upon the specific DTS device and test setup used to measure
158 these parameters in accordance with this standard.
159 Annex A provides a blank performance parameter table which should be used to record the
160 performance parameter values for a given DTS device and chosen optical test setup
161 configuration.
162 2 Normative references
163 The following documents are referred to in the text in such a way that some or all of their content
164 constitutes requirements of this document. For dated references, only the edition cited applies.
165 For undated references, the latest edition of the referenced document (including any
166 amendments) applies.
167 IEC 60050 (all parts), International Electrotechnical Vocabulary
168 IEC 61757:2026, Fibre optic sensors - Generic specification
169 IEC TR 61931, Fibre optic - Terminology
170 ISO/IEC Guide 99, International vocabulary of metrology - Basic and general concepts and
171 associated terms (VIM)
172 3 Terms, definitions, abbreviated terms, and symbols
173 3.1 Terms and definitions
174 For the purposes of this document, the terms and definitions given in IEC 61757:2026, IEC
175 60050 (all parts), IEC TR 61931, ISO/IEC Guide 99 (VIM) and the following apply.
176 ISO and IEC maintain terminology databases for use in standardization at the following
177 addresses:
178 – IEC Electropedia: available at https://www.electropedia.org/
179 – ISO Online browsing platform: available at https://www.iso.org/obp
IEC CDV 61757-2-2 © IEC 2026
180 3.1.1
181 attenuation range
182 total cumulated optical loss (one way loss) tolerated by the DTS system without affecting the
183 specified measurement performance more than a given factor at a given location, spatial
184 resolution, and measurement time
185 Note 1 to entry: Part of the total cumulative loss can be the fibre attenuation, point defect losses introduced by
186 components such as connectors, splices, kink in the fibre, attenuators.
187 Note 2 to entry: The attenuation range is expressed in decibels (dB).
188 3.1.2
189 distributed fibre optic temperature sensing system
190 DTS
191 measurement set-up consisting of a distributed fibre optic sensor connected to a DTS device,
192 including processor, data archive, and user interface, which provides a spatially resolved
193 temperature measurement
194 Note 1 to entry: Synonyms for DTS device include DTS unit, DTS interrogator, DTS controller, and DTS instrument.
195 3.1.3
196 distance measurement range
197 maximum distance from the DTS device output connector along the fibre optic sensor within
198 which the instrument measures a temperature with specified measurement performance under
199 defined conditions
200 Note 1 to entry: Defined conditions are spatial resolution, spatial temperature uncertainty, and measurement time.
201 Note 2 to entry: This supporting parameter is closely related to the total accumulated optical loss (one way) tolerated
202 by the interrogation unit without affecting specified measurement performance. In test cases used to prove or verify
203 the reported specifications, the total fibre length is equal to or greater than the specified distance measurement
204 range, for the tolerated total accumulated optical loss.
205 Note 3 to entry: The distance measurement range is expressed in length units (m or km).
206 3.1.4
207 environmental temperature repeatability
208 difference of the measured constant fibre optic sensor temperature at a specified DTS device
209 temperature (e. g. nominal operating temperature) before and after temperature cycling of the
210 DTS device across the entire DTS device operating temperature range
211 Note 1 to entry: This parameter is derived from environmental temperature stability.
212 3.1.5
213 environmental temperature stability
214 difference of the measured constant fibre optic sensor temperature before, during and after
215 temperature cycling of the DTS device across the entire DTS device operating temperature
216 range
217 Note 1 to entry: Worst case environmental temperature effect, high/low environmental temperature effect, and
218 environmental temperature repeatability are derived from this definition.
219 3.1.6
220 high/low environmental temperature effect
221 difference of the measured constant fibre optic sensor temperature at the high and low
222 temperature limit of the DTS device temperature operating range
223 Note 1 to entry: This parameter is derived from environmental temperature stability.
224 3.1.7
225 hot spot
226 short length of fibre optic sensor (L ) which is exposed by a measurable temperature change
HS
227 (ΔT) which is significantly bigger than the DTS device temperature repeatability and which is
228 confirmed by reference temperature devices in two thermal chambers
IEC CDV 61757-2-2 © IEC 2026
229 Note 1 to entry: See Clause 4 and Figure 6.
230 3.1.8
231 location
232 L
233 optical distance (specified in length units) from the DTS device output connector to a desired
234 temperature sample point along the fibre optic sensor
235 Note 1 to entry: The furthest location from DTS device output connector for the particular test is quantified as L m
Z
236 and is often chosen to be the same as the distance measurement range for purposes of comparing the measurement
237 results with quoted specifications.
238 3.1.9
239 measurement time
240 time between independent temperature measurements when making successive measurements
241 on a single fibre optic sensor
242 Note 1 to entry: This parameter includes acquisition time and processing time for measured data. This parameter
243 is selectable by the user typically in some limited fashion. Multiple independent temperature measurements may be
244 averaged together to provide an overall measurement time.
245 Note 2 to entry: Equivalently, it is the time interval between successive temperature trace timestamps under these
246 conditions.
247 3.1.10
248 point defect
249 local variation in the optical properties of a fibre optic sensor, occurring at a single location or
250 over a length substantially shorter than the DTS device spatial resolution
251 Note 1 to entry: The definition of a point defect covers a variety of possibilities that can have similar effects on the
252 temperature measurement. Examples include
253 – a poor fibre splice or a splice between two fibres of different core diameter;
254 – a back reflection from a fibre connection;
255 – a local fibre position with high loss, e.g., a bend or kink.
256 3.1.11
257 point defect temperature offset
258 difference between the average values of the temperature sample points in two zones on the
259 temperature trace, one each side of a point defect, where the actual fibre optic sensor
260 temperatures are the same
261 Note 1 to entry: The point defect temperature offset can be positive, negative or zero.
262 3.1.12
263 sample spacing
264 distance between two consecutive temperature sample points in a single temperature trace
265 Note 1 to entry: See Example of a temperature trace with temperature sample points:
IEC CDV 61757-2-2 © IEC 2026
267 Note 2 to entry: Sample spacing can be a user-selectable DTS device parameter.
268 Note 3 to entry: The sample spacing is expressed in length units.
269 3.1.13
270 spatial resolution
271 smallest length of a temperature-affected fibre optic sensor for which a DTS device can
272 measure the reference temperature of the hot spot fibre condition within the specified
273 temperature measurement error of the DTS device
274 3.1.14
275 spatial temperature uncertainty
276 uncertainty of location of temperature data in a single temperature trace expressed by twice the
277 standard deviation of a specified number of adjacent temperature sample points, with the fibre
278 optic sensor held at constant temperature
279 3.1.15
280 temperature dead zone
281 limited zone of a temperature trace, where the temperature sample points deviate from the
282 undisturbed parts of the trace by a specified limit due to a point defect
283 3.1.16
284 temperature measurement error
285 maximum difference between a centred and uniformly weighted moving average of the
286 measured temperature and a reference temperature for all data points of the fibre optic sensor
287 over the full operating temperature range and all acquisition times
288 Note 1 to entry: Single value (worst case) is specified in temperature units (e.g. ± 0,8 °C).
289 Note 2 to entry: The number of elements used for the moving average is defined in Clause 5. In practical
290 applications, other methods of smoothing might be applicable.
291 3.1.17
292 temperature repeatability
293 precision of temperature data based on repeated temperature traces at a given location
294 expressed by twice the standard deviation of corresponding temperature sample points in each
295 temperature trace, with the fibre optic sensor held at constant temperature
IEC CDV 61757-2-2 © IEC 2026
296 3.1.18
297 temperature sample point
298 measured temperature value associated with a single point at a known location along a fibre
299 optic sensor
300 Note 1 to entry: Due to thermodynamic effects, the measured value represents the temperature along a very small
301 section of the fibre optic sensor that includes the point.
302 3.1.19
303 temperature trace
304 set of temperature sample points distributed along a fibre optic sensor and spaced by the
305 sample spacing
306 Note 1 to entry: All the sample points are associated with a common time of measurement, often called the trace
307 timestamp. The measured values represent the temperature during a period that includes the timestamp.
308 Note 2 to entry: All the sample points in a temperature trace are measured values produced by the DTS device,
309 and not interpolated or smoothed values produced by subsequent processing outside the DTS device.
310 3.1.20
311 total fibre length
312 L
Z
313 distance from the DTS device output connector to the final end of the fibre optic sensor
314 Note 1 to entry: Final end of the fibre optic sensor can either be a purposefully cut or terminated end of the fibre
315 physically far from the DTS device (in a single-ended configuration), or the end of a loop consisting of a connector
316 that is connected to the same DTS device (in a loop configuration).
317 Note 2 to entry: This parameter is either equal to or greater than the distance measurement range.
318 Note 3 to entry: The distance measurement range is expressed in length units.
319 3.1.21
320 worst case environmental temperature effect
321 maximum difference of the measured constant fibre optic sensor temperature at different
322 locations along the sensor during a complete temperature cycling of the DTS device across the
323 entire instrument operating temperature range
324 Note 1 to entry: This parameter is derived from environmental temperature stability.
325 3.2 Abbreviated terms
DTS distributed fibre optic temperature sensing system
TE thermal enclosures (temperature-controlled)
x
VOA variable optical attenuator
326 3.3 Symbols
th
A(i) spatial temperature uncertainty at i sample point location
L location
L
fibre sensor length (loose and strain-free wound) < spatial resolution
A
L
fibre sensor length (loose and strain-free wound) = claimed spatial resolution
B
L
fibre sensor length (loose and strain-free wound) > 4 times spatial resolution
C
L
fibre sensor length (loose and strain-free wound)
D
L
short length of fibre optic sensor which is exposed by a measurable
HS
temperature change (∆T)
L ,L fibre length (normal spool)
W W/2
L
total fibre length
Z
IEC CDV 61757-2-2 © IEC 2026
N
number of traces
n
is the number of data points for calculation
S point centred and uniformly weighted moving average curve
i
th
S (i) standard deviation for each data point (i sample point location within a trace)
x
over time (N consecutive traces)
th
S(i) temperature repeatability at i sample point location
T
constant optical fibre temperature
const
∆T measurable temperature range
T
designated DTS device operating temperature
op
T
high environmental temperature effect of the DTS device
eh
T
low environmental temperature effect of the DTS device
el
T
environmental temperature repeatability
er
T
worst case environmental temperature effect of the DTS device
ew
th th
T (i) temperature data at i sample point location of j trace
j
T
minimal DTS device operating temperature
op-min
T
maximal DTS device operating temperature
op-max
T , T , lowest, middle and highest temperature of the fibre optic sensor temperature
low mid
range
T
high
T
traceable calibrated reference temperature
ref
t ,t ,t shortest, middle and longest possible DTS device measurement time
min mid max
σ
temperature standard deviation
327 4 General test setups for measurement of performance parameters
328 4.1 General and test setup requirements
329 General test setups for single and loop configurations are schematically shown in Figure 1 and
330 Figure 2 respectively. Their aim is to provide a common base for determining the measurement
331 specifications while at the same time minimizing complexity, cost, reconfiguration requirements,
332 and test execution time. Further information about general test setups and measurement
333 procedures can be found in SEAFOM-MSP-01:2016 [1].
IEC CDV 61757-2-2 © IEC 2026
335 Key
1 thermal enclosure (temperature-controlled) TE
DEV
2 DTS device
3 DTS device output connector
4 fibre connection
5 optional variable optical attenuator
fibre sensor length L (loose and strain-free wound)
D
fibre length L or L (normal spool)
W W/2
fibre sensor length L (loose and strain-free wound)
D
9 thermal enclosure (temperature-controlled) TE
D
fibre sensor length L (loose and strain-free wound) > 4 × spatial resolution
C
fibre sensor length L (loose and strain-free wound) < spatial resolution
A
fibre sensor length L (loose and strain-free wound) = claimed spatial resolution
B
13 thermal enclosure (temperature-controlled) TE
C
14 thermal enclosure (temperature-controlled) TE
ABC
15 fibre termination
16 traceable calibrated reference temperature sensors
336 Figure 1 – General test setup: single-ended configuration
IEC CDV 61757-2-2 © IEC 2026
338 Key
1 thermal enclosure (temperature-controlled) TE
DEV
2 DTS device
3 DTS device output connector
4 fibre connection
5 optional variable optical attenuator
fibre sensor length L (loose and strain-free wound)
D
fibre length L or L (normal spool)
W W/2
fibre sensor length L (loose and strain-free wound)
D
9 thermal enclosure (temperature-controlled) TE
D
10 fibre sensor length L (loose and strain-free wound) > 4 × spatial resolution
C
fibre sensor length L (loose and strain-free wound) < spatial resolution
A
fibre sensor length L (loose and strain-free wound) = claimed spatial resolution
B
13 thermal enclosure (temperature-controlled) TE
C
14 thermal enclosure (temperature-controlled) TE
ABC
fibre length L or L (normal spool)
W W/2
16 DTS interrogation unit input connector
17 traceable calibrated reference temperature sensors
339 Figure 2 – General test setup: loop configuration
340 Individual evaluation procedures may be performed with a modified setup providing the required
341 measurement conditions. In this case, a detailed setup description and documentation is
342 required.
343 The DTS device shall be tested with a fibre optic sensor recommended by the DTS device
344 manufacturer, unless otherwise specified or agreed. The fibre lengths L , L , L of the fibre
A B C
IEC CDV 61757-2-2 © IEC 2026
345 coils in the thermal enclosures TE and TE shall be selected based upon the expected
ABC C
346 spatial resolution of the DTS device being tested. The fibre lengths L and L within and outside
D W
347 TE shall be chosen to make the total fibre lengthL match the claimed distance measurement
D Z
348 range of the DTS device being tested. In single ended configurations (see Figure 1) the total
349 fibre lengthL shall be equal to the total length of fibre from the DTS device output connector
Z
350 up to the terminated end of the spatial resolution fibre section represented by fibre lengths L ,
A
351 L , L .
B C
352 NOTE 1 The use of fibre length L , located before and after a long length L of fibre (which makes up the total
D W
353 fibre lengthL ), provides a test setup capable of accommodating various DTS device with different distance
Z
354 measurement ranges.
355 Fibre length L shall be equal to 10 % of the total fibre lengthL .
D Z
356 In loop configuration (Figure 2) the total fibre lengthL shall be equal to the total length of fibre
Z
357 from the DTS device output connector up to the DTS device input connector. In case of a DTS
358 device comparison with a single-ended configuration, the length of the normal spools shall be
359 (2 × L ). This guarantees the same overall attenuation. In all other cases the length of the
W/2
360 normal spools shall be (2 × L ).
W
361 Low insertion loss and back-reflections should be accomplished when connecting the fibres by
362 connectors or by fusion splices.
363 The fibres in the thermal enclosures shall be coiled in such a way (loose wound) that the fibre
364 is completely exposed by the surrounding temperature, and that there is no fibre strain. Normal
365 spool in this case means a fibre spool as delivered from the fibre supplier.
366 The realization of the required thermal enclosures can differ in certain respects, such as using
367 climate chambers or liquid filled calibration baths. It should be assured that the TE and TE
C ABC
368 provide a large and sharp enough temperature difference between the coils (at least 20 °C
369 occurring over no longer than 50 % of the rated spatial resolution). Traceable calibrated
370 reference temperature sensors shall be used to monitor the required coil temperatures. The
371 reference temperature measurement uncertainty shall be at least a factor of 5 smaller than the
372 temperature measurement error that is being assessed.
373 NOTE 2 Further information on how to use a crushed-ice bath for metrological comparison of Raman-distributed
374 temperature sensors is provided in Failleau, G. et al [2].
375 The DTS device being tested shall be calibrated according to manufacturer's recommendations
376 before performing any measurements.
377 4.2 General required information to be documented
378 The general required information to be documented is as follows:
379 – completion date of all testing;
380 – name of the organization executing the testing;
381 – test setup configuration;
382 – detailed description or manufacturer, model, and serial number of the thermal enclosures;
383 – operating mode of the DTS device (single-ended or loop configuration as shown in Figure 1
384 and Figure 2, or channel(s) tested in case of a multi-channel system using the same
385 hardware);
386 – wavelength(s) of the launched signals (operating wavelength(s));
387 – manufacturer, model, and serial number of the DTS device;
388 – manufacturer, model, and length of the optical fibres in the test setup;
IEC CDV 61757-2-2 © IEC 2026
389 – optical loss (one-way in dB) of the optical setup to the end of the sensor (L );
Z
390 – wavelength used to measure the loss to end of the sensor ( L );
Z
391 – distance measurement range of the DTS device;
392 – sample spacing used for all measurements;
393 – spatial resolution setting used for all measurements;
394 – measurement time used during DTS device calibration (if relevant);
395 – lengths of fibre coils L , L , L and L and spool(s) L ;
A B C D W
396 – nominal temperatures of thermal enclosures TE and TE in °C;
C ABC
397 – manufacturer, model, and serial number of the reference temperature sensor;
398 – maximum (hot) and minimum (cold) operating temperature limits (°C) of the DTS device.
399 The general required information for the tested DTS device and the associated test setup shall
400 be recorded along with the calculated measurement specifications.
401 5 Measurement procedures for performance parameters
402 5.1 Temperature measurement error
403 5.1.1 Test procedure and conditions
404 The following steps shall be performed:
405 a) Use a general test setup as shown in Clause 4.
406 b) Adjust the total fibre lengthL equal to or greater than the distance measurement range
Z
407 quoted for the DTS device being tested.
408 c) Place the DTS device in a thermal enclosure TE and stabilize it thermally at operating
DEV
409 temperature. Unless otherwise specified or agreed, the operating temperature T shall be
op
410 (21 ± 2) °C with a stability of ± 0,5 °C. Give the DTS device enough warm-up time before
411 performing the following steps to reach thermal equilibrium with the environment in
412 accordance with the manufacturer's recommendations.
413 d) Calibrate the DTS device according to manufacturer's recommendations over a specified
414 fibre optic sensor temperature range. The sensor temperature range shall be agreed by the
415 manufacturer and the customer, unless otherwise specified.
416 NOTE 1 Typically the fibre optic sensor temperature range corresponds to the intended application.
417 e) Stabilize the TE temperature within ± 0,5 °C of fibre lengths L at three temperatures T ,
D D low
418 T , T within the specified fibre optic sensor temperature range. Unless otherwise
mid high
419 specified or agreed, T shall be the lowest, T , the middle and T the highest
low mid high
420 temperature of the temperature range.
421 f) Collect 20 temperature traces at three measurement times provided by DTS device: shortest
422 time t , middle time t and longest possible time t for each fibre optic sensor
min mid max
423 temperature T , T , T . Record the true fibre optic sensor temperature T by a
low mid high ref
424 reference temperature sensor.
425 NOTE 2 The same data sets can be used for the performance evaluation of spatial resolution, spatial temperature
426 uncertainty, and temperature repeatability.
427 5.1.2 Parameter calculation
428 For calculation of the temperature measurement error, the following steps shall be performed
429 (see Figure 3 and Figure 4):
IEC CDV 61757-2-2 © IEC 2026
430 a) Compute the average of all 20 temperature traces for each temperature sample point over
431 the entire location.
432 b) Calculate the smoothed average by computing a centred and uniformly weighted moving
433 average over 51 of the averaged DTS device temperature data.
434 c) Compute the average error for each sample point by subtracting the smoothed average from
435 the true fibre temperature T . See Figure 4.
ref
436 d) Calculate the absolute average error by taking the absolute value of each average error for
437 each temperature sample point.
438 e) The temperature measurement error for that set of test conditions is the maximum value of
439 all absolute average error values that correspond to measurements collected from the fibre
440 length L that was clearly inside the thermal enclosure TE . Temperature data of fibre
D D
441 lengths outside the stabilized thermal chamber (e. g. end or lead in fibre lengths, L length
W
442 of fibre) shall not be used for computation of the temperature measurement error. See Figure
443 5.
444 f) Repeat calculation steps a) through e) for all other sets of test conditions (a total of 3
445 temperatures T , T , T exist for each measurement timet , t , t ).
low mid high min mid max
446 g) Record the test parameters and all 9 measured values for the temperature measurement
447 error.
449 Figure 3 – Temperature measurement error calculation: step a)
IEC CDV 61757-2-2 © IEC 2026
451 Figure 4 – Temperature measurement error calculation: steps b) and c)
453 Figure 5 – Temperature measurement error calculation: steps d) and e)
454 5.2 Spatial resolution
455 5.2.1 Test procedure and conditions
456 The following steps shall be performed:
457 a) Repeat steps a) to d) of 5.1.1.
458 b) Adjust the fibre lengths in thermal enclosure TE as follows:
ABC
IEC CDV 61757-2-2 © IEC 2026
459 1) Length L shall be less than the claimed DTS device spatial resolution, such that no
A
460 single temperature point will show more than 75 % of the reference temperature T in
ref
461 thermal enclosure TE .
ABC
462 2) Length L shall be equal to the claimed spatial resolution, such that one data point shows
B
463 a temperature value increase > 95 % of the reference temperature.
464 3) Length L shall be greater than 4 times the claimed spatial resolution, such that at least
C
465 three data points show a temperature value equal to the reference temperature within
466 the measurement uncertainty.
467 c) Adjust the sample spacing to less than or equal to one-half the claimed spatial resolution.
468 d) Stabilize the TE temperature of fibre lengths L at (20 ± 0,5) °C. Generate a temperature
C C
469 step of 20 °C by changing the temperature of TE (increasing by heating or decreasing
ABC
470 by cooling, e. g. by using a crushed ice bath).
471 e) Collect 20 temperature traces at measurement timet provided by DTS device unless
mid
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