EN IEC 61757-8-1:2026

SLOVENSKI STANDARD
01-julij-2025
Optični senzorji - 8-1. del: Merjenje tlaka - Tlačni senzorji na podlagi
optovlakenskih Braggovih rešetk
Fibre optic sensors - Part 8-1: Pressure measurement - Pressure sensors based on fibre
Bragg gratings
Ta slovenski standard je istoveten z: prEN IEC 61757-8-1:2025
ICS:
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/1970/CDV
COMMITTEE DRAFT FOR VOTE (CDV)

PROJECT NUMBER:
IEC 61757-8-1 ED1
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2025-05-23 2025-08-15
SUPERSEDES DOCUMENTS:
86C/1955/CD, 86C/1963A/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
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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 8-1: Pressure measurement – Pressure sensors based on fibre
Bragg gratings
PROPOSED STABILITY DATE: 2029
NOTE FROM TC/SC OFFICERS:
electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions.
You may not copy or "mirror" the file or printed version of the document, or any part of it, for any other purpose without
permission in writing from IEC.

IEC CDV 61757-8-1/Ed1 © IEC 2025 – 2 – 86C/1970/CDV

1 CONTENTS
3 FOREWORD . 4
4 INTRODUCTION . 6
5 1 Scope . 7
6 2 Normative references . 7
7 3 Terms, definitions, symbols and abbreviated terms . 7
8 3.1 Terms and definitions . 7
9 3.2 Symbols . 9
10 3.3 Abbreviated terms . 9
11 4 Structure and characteristics . 9
12 4.1 Fibre Bragg grating . 9
13 4.2 FBG pressure sensor configuration . 9
14 4.3 Reference wavelength . 11
15 4.4 Stability behaviour . 11
16 4.4.1 Drift and creep . 11
17 4.4.2 Hysteresis . 12
18 4.5 Indication of the measured values . 12
19 4.6 Zero-point related measurement . 12
20 4.7 Non-zero-point related measurement . 12
21 4.8 Production set . 12
22 4.9 FBG pressure sensor standard type . 13
23 4.10 FBG pressure sensor series . 13
24 5 Features and characteristics to be measured . 13
25 5.1 Sampling and statistical evaluation . 13
26 5.1.1 Sampling . 13
27 5.1.2 Reporting the measuring result . 13
28 5.1.3 Sample conditioning . 14
29 5.1.4 Ambient test conditions . 14
30 5.1.5 Required types of tests for individual characteristics . 14
31 5.2 Bragg wavelength λ . 14
Β
32 5.2.1 General . 14
33 5.2.2 Measurement procedure . 14
34 5.2.3 Evaluation . 15
35 5.2.4 Reporting . 15
36 5.3 FBG spectral width. 15
37 5.3.1 Measurement procedure . 15
38 5.3.2 Evaluation . 15
39 5.3.3 Reporting . 15
40 5.4 FBG reflectivity . 15
41 5.4.1 Measurement procedure . 15
42 5.4.2 Evaluation . 15
43 5.4.3 Reporting . 16
44 5.5 Pressure measurement . 16
45 5.5.1 General . 16
46 5.5.2 Test setup . 16
47 5.5.3 Measurement procedure . 18

IEC CDV 61757-8-1/Ed1 © IEC 2025 – 3 – 86C/1970/CDV

48 5.5.4 Calibration and evaluation . 19
49 5.6 Pressure conversion factor . 20
50 5.7 Temperature and humidity ranges . 20
51 5.7.1 General . 20
52 5.7.2 Measurement procedure . 21
53 5.7.3 Evaluation . 21
54 5.7.4 Reporting . 21
55 5.8 Durability . 21
56 5.8.1 General . 21
57 5.8.2 Measurement procedure . 21
58 5.8.3 Reporting . 21
59 6 Features and characteristics to be reported . 21
60 6.1 Construction details . 21
61 6.2 Configuration of the FBG pressure sensor . 22
62 6.3 Temperature and humidity range . 22
63 6.4 Connecting requirement . 22
64 7 Recommendations for use of FBG measuring instruments . 22
66 Figure 1 – Examples of sensor types for measuring pressure changes . 10
67 Figure 2 – Bragg wavelength changes caused by increase in pressure . 10
68 Figure 3 – Schematic diagram of pressure sensor using two FBGs . 11
69 Figure 4 – Pressure measurement test setup scheme by a dead weight tester . 16
70 Figure 5 – Schematic diagram of a pressure measurement test setup . 18
71 Figure 6 – Example of temperature dependence of the Bragg wavelengths of two FBGs . 18
72 Figure 7 – Example of pressure dependence of the Bragg wavelengths of FBG1 and
73 FBG2 . 19
75 Table 1 – Required types of tests for individual characteristics . 14
IEC CDV 61757-8-1/Ed1 © IEC 2025 – 4 – 86C/1970/CDV

78 INTERNATIONAL ELECTROTECHNICAL COMMISSION
79 ____________
81 FIBRE OPTIC SENSORS –
83 Part 8-1: Pressure measurement –
84 Pressure sensors based on fibre Bragg gratings
86 FOREWORD
87 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
88 all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
89 co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
90 in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
91 Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
92 preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
93 may participate in this preparatory work. International, governmental and non-governmental organizations liaising
94 with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
95 Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
96 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
97 consensus of opinion on the relevant subjects since each technical committee has representation from all
98 interested IEC National Committees.
99 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
100 Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
101 Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
102 misinterpretation by any end user.
103 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
104 transparently to the maximum extent possible in their national and regional publications. Any divergence between
105 any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
106 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
107 assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
108 services carried out by independent certification bodies.
109 6) All users should ensure that they have the latest edition of this publication.
110 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
111 members of its technical committees and IEC National Committees for any personal injury, property damage or
112 other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
113 expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
114 Publications.
115 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
116 indispensable for the correct application of this publication.
117 9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
118 patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
119 respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
120 may be required to implement this document. However, implementers are cautioned that this may not represent
121 the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
122 shall not be held responsible for identifying any or all such patent rights.
123 IEC 61757-7-1 has been prepared by subcommittee 86C: Fibre optic systems and active
124 devices, of IEC technical committee 86: Fibre optics. It is an International Standard.
125 The text of this International Standard is based on the following documents:
Draft Report on voting
86C/XX/FDIS 86C/XX/RVD
127 Full information on the voting for its approval can be found in the report on voting indicated in
128 the above table.
129 The language used for the development of this International Standard is English.

IEC CDV 61757-8-1/Ed1 © IEC 2025 – 5 – 86C/1970/CDV

130 This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
131 accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
132 at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
133 described in greater detail at www.iec.ch/publications.
134 A list of all parts in the IEC 61757 series, published under the general title Fibre optic sensors,
135 can be found on the IEC website.
136 The committee has decided that the contents of this document will remain unchanged until the
137 stability date indicated on the IEC website under webstore.iec.ch in the data related to the
138 specific document. At this date, the document will be
139 • reconfirmed,
140 • withdrawn,
141 • replaced by a revised edition, or
142 • amended.
IEC CDV 61757-8-1/Ed1 © IEC 2025 – 6 – 86C/1970/CDV

145 INTRODUCTION
146 This document is part of the IEC 61757 series, which is dedicated to fibre optic sensors. Generic
147 specifications for fibre optic sensors are defined in IEC 61757.
148 The individual parts of the IEC 61757 series are numbered as IEC 61757-M-T, where M denotes
149 the measurand and T the technology. The IEC 61757-8-T series is concerned with pressure
150 measurements.
IEC CDV 61757-8-1/Ed1 © IEC 2025 – 7 – 86C/1970/CDV

152 FIBRE OPTIC SENSORS –
154 Part 8-1: Pressure measurement –
155 Pressure sensors based on fibre Bragg gratings
159 1 Scope
160 This part of IEC 61757 defines the terminology, structure, and measurement methods of optical
161 pressure sensors for gases or liquids based on a diaphragm in combination with fibre Bragg
162 gratings (FBGs) as the sensing element. This document also specifies the most important
163 features and characteristics of these fibre optic pressure sensors and defines procedures for
164 measuring these features and characteristics.
165 2 Normative references
166 The following documents are referred to in the text in such a way that some or all of their content
167 constitutes requirements of this document. For dated references, only the edition cited applies.
168 For undated references, the latest edition of the referenced document (including any
169 amendments) applies.
170 IEC 60068-2 (all parts), Environmental testing – Part 2-X: Tests
171 IEC 61300-2 (all parts), Fibre optic interconnecting devices and passive components – Basic
172 test and measurement procedures – Part 2-X: Tests
173 IEC 61754 (all parts), Fibre optic interconnecting devices and passive components – Fibre optic
174 connector interfaces
175 IEC 61757, Fibre optic sensors – Generic specification
176 IEC 61757-1-1:2020, Fibre optic sensors – Part 1-1: Strain measurement – Strain sensors
177 based on fibre Bragg gratings
178 IEC 62129-1, Calibration of wavelength/optical frequency measurement instruments – Part 1:
179 Optical spectrum analyzers
180 IEC 62129-2, Calibration of wavelength/optical frequency measurement instruments – Part 2:
181 Michelson interferometer single wavelength meters
182 IEC 62129-3, Calibration of wavelength/optical frequency measurement instruments – Part 3:
183 Optical frequency meters internally referenced to a frequency comb
184 ISO/IEC GUIDE 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
185 uncertainty in measurement (GUM:1995)
186 3 Terms, definitions, symbols and abbreviated terms
187 3.1 Terms and definitions
188 For the purposes of this document, the terms and definitions given in IEC 61757, IEC 61757-1-
189 1, and the following apply.
190 ISO and IEC maintain terminology databases for use in standardization at the following
191 addresses:
192 • IEC Electropedia: available at https://www.electropedia.org/
193 • ISO Online browsing platform: available at https://www.iso.org/obp

IEC CDV 61757-8-1/Ed1 © IEC 2025 – 8 – 86C/1970/CDV

194 3.1.1
195 pressure
196 p
197 amount of force applied perpendicular to the surface of an object per unit area
198 Note 1 to entry: Pressure is calculated as
F
p=
A
199 where
200 F is the magnitude of the normal force, expressed in N;
201 A is the area of the contact surface, expressed in m .
202 Note 2 to entry: This definition addresses measurement methods of optical pressure sensors for gases or liquids
203 based on fibre Bragg gratings in combination with a diaphragm. IEV 113-03-65 provides a broader definition of
204 pressure.
205 3.1.2
206 FBG pressure sensor
207 fibre optic sensor using one or more fibre Bragg gratings as a sensing element for pressure
208 measurement of gases or liquids
209 3.1.3
210 pressure conversion factor
211 κ
p
212 ratio of the relative change in wavelength to a pressure change introduced to an FBG pressure
213 sensor
214 Note 1 to entry: The pressure conversion factor κ is expressed in m /N and calculated as
p
Δλλ
( )
κ =
p
Δp
215 where
216 Δλ/λ is the relative change in wavelength;
217 Δp is the pressure change.
218 Note 2 to entry: The pressure conversion factor κ is commonly used by manufacturers to characterize the pressure
p
219 response of their products.
220 Note 3 to entry: The conversion factor κ for an FBG pressure sensor assumes a linear relation between wavelength
p
221 change and pressure. Considering the whole measurement system (sensor, device, and cabling), it can be separately
222 defined for the various components of the measurement system. It is only valid for defined conditions. In the case of
223 a non-linear characteristic, the relation between wavelength change and pressure change is considered to be linear
224 within a defined permissible measurement error.
225 Note 4 to entry: The term pressure sensitivity, expressed for example in (pm/kPa), is used by some manufacturers
226 to characterize the pressure response of their products.
227 3.1.4
228 temperature compensation constant
229 C
230 constant for correcting the influence of temperature changes when the pressure is obtained
231 from wavelength changes
232 Note 1 to entry: The temperature compensation constant is usually provided by the manufacturer.
233 Note 2 to entry: The term temperature sensitivity, expressed for example in (pm/°C), is used by some manufacturers
234 to characterize the influence of temperature changes in their products.

IEC CDV 61757-8-1/Ed1 © IEC 2025 – 9 – 86C/1970/CDV

235 3.2 Symbols
236 For the purposes of this document, the following symbols apply:
237 R reflectivity of the FBG
FBG
238 n effective refractive index of the FBG
eff
239 Δp pressure change
240 ΔT temperature change
241 Λ FBG period
242 λ Bragg wavelength
B
243 λ reference wavelength
244 3.3 Abbreviated terms
245 FBG fibre Bragg grating
246 FWHM full width at half maximum
247 SNR signal-to-noise ratio
248 UV ultraviolet
249 4 Structure and characteristics
250 4.1 Fibre Bragg grating
251 Fibre Bragg gratings (FBGs) are phase diffraction gratings inscribed into optical waveguides.
252 They are frequently produced using ultraviolet (UV) light (e.g. from an excimer laser at 248 nm).
253 The fibre is exposed to an interference pattern of this UV radiation. UV photosensitive
254 processes then produce changes in the refractive index of the fibre core, which is susceptible
255 to this UV light. The interference pattern is imaged onto the fibre core to permanently change
256 the refractive index of the fibre core, so that the refractive index varies periodically along the
257 fibre. Incident and transported light is reflected by these periodic refractive index changes along
258 the fibre. At a certain wavelength, the reflected light is additively superimposed (through
259 constructive interference); this spectral part of the incident light is reflected back to the input
260 port of the fibre. In the transmitted light, this wavelength (denoted Bragg wavelength λ ) is
Β
261 attenuated accordingly, due to the reflectance in the FBG.
262 The value of the reflected Bragg wavelength λ is determined by the Bragg condition shown in
Β
263 Formula (1).
λ = 2 n Λ
(1)
B eff
264 where
265 n is the effective refractive index of the FBG;
eff
266 Λ is the FBG period, expressed for example in nm.
267 According to Formula (1), the Bragg wavelength depends on the effective refractive index and
268 the period of the FBG. The spectral width of the Bragg wavelength peak is determined by the
269 number of grating periods and the magnitude of the refractive index modulation (for more details
270 see IEC 61757-1-1:2020, 5.1).
271 4.2 FBG pressure sensor configuration
272 The FBG pressure sensor can be manufactured from various materials and in various forms
273 (using one or more FBGs as sensing elements). The FBG pressure sensor is typically used to
274 monitor the pressure of fluids, such as liquids or gases. Typical applications include water level
275 measurement in rivers, drainage status measurement for excavations, and water pressure
276 measurement within pressure pipes, banks and perforations.

IEC CDV 61757-8-1/Ed1 © IEC 2025 – 10 – 86C/1970/CDV

277 The method used to convert a pressure change into a change of the Bragg wavelength of an
278 FBG depends on the manufacturer of the pressure sensor. There are a variety of methods, but
279 a comprehensive description of these methods is outside the scope of this document.
280 The principle of fibre optic pressure measurement is based on a base body that deforms under
281 pressure in a controlled manner. This body often has an intentionally weakened (thin) surface,
282 the diaphragm, as shown in Figure 1. The diaphragm should be strong and elastic enough to
283 withstand the external pressure. The amount of deformation of the diaphragm under pressure
284 is measured with an FBG (see FBG2 in Figure 1). If the diaphragm bulges under pressure, the
285 FBG will be strained or, if pre-strained, compressed accordingly. This change in strain in the
286 FBG then changes the Bragg wavelength reflected from this FBG, as shown schematically in
287 Figure 2. Therefore, the pressure can be determined by measuring the reflected Bragg
288 wavelength of the FBG.
a) axial directional force applied to FBG b) lateral force applied to FBG
289 Figure 1 – Examples of sensor types for measuring pressure changes
290 Figure 1 a) shows a structure in which the central part of the diaphragm moves to the left as
291 the external pressure increases on the right side of the diaphragm, so that the tensile strength
292 acting on FBG2 weakens and its grating period decreases. As a result, the wavelength reflected
293 from FBG2 decreases, according to Formula (1). In Figure 1 a), FBG2 is attached to the
294 diaphragm in a pre-stretched state, so it should be assembled with care. In Figure 1 b), on the
295 other hand, the grating period of FBG2 increases with increasing external pressure, so that the
296 wavelength reflected from FBG2 increases with external pressure. In this case, adhesion of
297 FBG2 to the diaphragm is important, because the diaphragm can repeatedly expand and
298 contract as the external pressure varies.

a) with axial directional force applied to FBG b) with lateral force applied to FBG
299 Figure 2 – Bragg wavelength changes caused by increase in pressure
300 A broadband light source and an optical spectrometer can be used to measure the change in
301 the Bragg wavelength of an FBG. The light source and the spectrometer are typically connected
302 to the pressure sensing FBG via an optical circulator, as shown schematically in Figure 3.
303 In Figure 1 a) and b), an additional FBG (denoted FBG1) is inserted near the pressure sensing
304 FBG (denoted FBG2) to allow for compensation of the temperature dependence of FBG2 (as
305 described in 5.5.3). The additional FBG1 measures only temperature changes, whereas FBG2
306 measures pressure and temperature changes. FBG1 and FBG2 can be connected in series, as
307 shown in the schematic diagrams of Figure 3, or alternatively in parallel.

IEC CDV 61757-8-1/Ed1 © IEC 2025 – 11 – 86C/1970/CDV

a) with axial directional force applied to FBG

b) with lateral force applied to FBG
308 Figure 3 – Schematic diagram of pressure sensor using two FBGs
309 4.3 Reference wavelength
310 The Bragg wavelength measured with a given FBG can depend on the evaluation method and,
311 more importantly, on the specific installation of the FBG. In the context of this document, the
312 wavelength measured after installation of the FBG in the pressure sensor is denoted as the
313 reference wavelength λ .
314 The reference wavelength is not necessarily the same as the Bragg wavelength specified by
315 the manufacturer of the FBG. If the FBG is pre-strained, for example, there is a difference
316 between the reference wavelength and the manufacturer’s Bragg wavelength. If the FBG is not
317 pre-strained, the difference between the reference wavelength and the manufacturer’s Bragg
318 wavelength is usually very small, so that both wavelength values can be used interchangeably
319 without significant error.
320 If the reference wavelength is measured when the measurement cycle is started, this
321 wavelength measurement can be considered as the zero-point measurement value.
322 4.4 Stability behaviour
323 4.4.1 Drift and creep
324 Stability, in general, is the ability of a measurement system to maintain its metrological
325 characteristics and to meet other specifications over the intended time of operation. In the
326 context of this document, stability describes the property of the applied FBG pressure sensor
327 to maintain its optical characteristics over the time period of use, which is determined by the
328 application, or to show only small permissible deviations.
329 Variations in the measured value can occur:
330 – when the materials concerned are subject to long-term stress (creep);
331 – without loading stress applied (zero-point drift).
332 Creep and zero-point drift can result from slowly progressing chemical or physical degradation
333 of the materials used in the sensor (e.g. from ageing), or from changes of the initial
334 environmental conditions (e.g. either temperature or humidity, or both).

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