BS/EN 61757-1-1-2017 pdf download.Fibre optic sensors Part 1-1 : Strain measurement – Strain sensors based on fibre Bragg gratings.
5.7 Stability behaviour
5.7.1 Drift and creep
Stability, in general, is the ability of a measurement system to maintain its metrological characteristics and meet other specifications over the intended time of operation. Stability, in the context of this standard, describes the property of the applied FBG strain sensor to keep its optical characteristics constant over a period of use determined by the objectives, or to show only a small permissible deviation.
Variations in the measured value might occur:
— when the materials concerned are subject to long-term stress (creep);
— without loading stress (zero point drift).
This may be caused by the slow progress of chemical or physical degradation within the materials used (e.g. ageing), or by a change in the initial physical conditions (e.g. temperature or humidity).
Creep is a quantity that depends on the materials employed, the set-up of the sensor and the type of operation, and can only be determined experimentally. According to current experience, the error contribution as a consequence of creep remains irrelevant within the scope of the given uncertainty of measurement for the gauge factor k, when the bonding material prescribed by the manufacturer is used.
Drift is a slow change of the metrological characteristics of the measurement system. The drift error of an FBG strain sensor is negligibly small, according to the state of the art; hence for this standard, no further specification is required. However, if drifts are generated by for example, a modified production process or inadequate recoating material, the drift should be stated.
5.7.2 Shape stability of the Bragg grating peak
For correct operation, no significant variations in the shape of the spectral response should occur. The spectral response and stability of an FBG element depend on the manufacturing process and subsequent treatment of the grating. During further processing of the FBG into an FBG strain gauge, or an FBG strain sensor, variations can occur in the spectral response, which may lead to deterioration of the required stability characteristics in consequence. A spectrum is acceptable when the side-lobe maxima are at least 5 dB below the main peak. The specification of the FBG applies to the condition of the FBG strain sensor on delivery.
5.7.3 Hysteresis
Hysteresis in material science describes a particular material behaviour whereby the material does not return to its original state, or does so following a time delay, once the input load has been removed. This means that the output value for an elasto-plastic deformation behaviour does not depend only on the input value but also on rate-dependent processes.
When the strain (or temperature) changes, the silica-based FBG’s peak commonly shifts without showing hysteresis effect. Coatings of silica-based FGB strain sensors, or protective material in which FBG strain sensors are embedded and constitutes a transition zone between the sensor element and the object of measurement (according to 5.2), may cause hysteresis effects. If hysteresis occurs for repeated or cyclic conditions within the specified operation range of the sensor, the amount of hysteresis should be described.
5.8 Test specimen
Here, flexural beams, plates or other objects are designated as test specimens upon which the FBG strain sensors are installed in order to determine and verify their properties.
The geometrical data for length, width, height and distance of the sensitive element from the object of measurement, as well as the relevant dimensions for assembly, shall be reported by the manufacturer,
6.2 Configuration of the FBG strain sensor
The configuration of the FBG strain sensor according to 5.2 shall be reported by the manufacturer. If more than one configuration is reported, features and characteristics measured according to Clause 7 shall be given for every configuration.
6.3 Temperature and humidity range
The manufacturer shall report the temperature and humidity ranges for storage, installation and operation.
6.4 Connecting requirement
It shall be indicated whether or not the sensor is supplied with an optical connector. If a connector is used, the type shall be indicated according to IEC 60874-1. The smallest radius permitted for laying the connecting leads shall be stated. When the sensor is connected to the leading cable, compatibility regarding the diameter of the mode field shall be ensured. Splice losses occur through faulty matching. If there are splice losses, the producer shall Inform about the fibre parameters and this additional attenuation. If the FBG strain sensor can be operated from one side only, the manufacturer shall mark the side to be used for the connection. This can be the case when FBG arrays with a high reflectance are employed.
7 Features and characteristics to be measured
7.1 Sampling and statistical evaluation
7.1.1 Sampling
The following sampling methods shall be used according to the intended scope of testing:
— random sampling;
— type testing;
— series testing:
— individual sample testing.
Many of the FBG strain sensor properties can only be determined on an installed sensor. A statistical evaluation shall be performed in this case. The number of sample sensors as well as the date of the evaluation should be noted.BS/EN 61757-1-1-2017 pdf download.
BS/EN 61757-1-1-2017 pdf download
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