Instrumentation & Measurement Magazine 24-9 - 35

a continuous swept laser
scanning technology. The
measurement range is from
1500 nm to 1600 nm. The
trueness and accuracy of
FBGs based strain measurements
in cryogenic
conditions were evaluated
in [5] by performing tensile
tests down to 4.2 K.
Bonding Procedure
The optical fibers are
bonded to the studied material
following a specific
procedure to guarantee a
limited quantity of adhesive.
The adhesive is an
important parameter because
the deformations of
the studied material need
to be perfectly transferred
to the FBG sensors. The
Araldite™ Standard (Ultra
strong) adhesive is used,
a two-component epoxy
with long working time.
To achieve perfect bonding,
the studied material
surfaces should be roughened
with sandpaper and
degreased with a suitable
solvent. After applying the
adhesive, the optical fiber
should be pulled and
placed on the material,
and finally, covered with a
gauge tape and a soft rubber.
A pressure should be
applied on the FBGs during
curing at room temperature
for at least eight hours.
Fig. 3. Azimuthal strain measurements during one powering test of short model magnet 3 at 1.9 K. (a) Current profile
and strain profiles obtained with both optical and electrical sensors as a function of time. (b) Strain measurements as a
function of current for both optical and electrical sensors. (c) Linear regression to model agreement between electrical and
optical sensors.
Performance Testing of FBG Sensors on
Short Models
Standard RSG are sensitive to both electromagnetic fields and
low temperatures, using them with superconducting magnets
request compensation given the cryogenic and high-field
environment. The temperature and magnetic field effects compensations
for RSG are done thanks to an external compensator
with the same RSG in the same configuration. This compensator
is bonded on a thin titanium plate and attached on top of
the RSG measuring grids and oriented in the same direction.
It is located as close as possible to the active strain measuring
RSG so that they are both in the same magnetic flux. Nonetheless,
the FBG strain sensor only needs to be compensated
December 2021
thermally which is done thanks to a calibration study realized
in the past and depending on the material where it is bonded.
RSG has been the instrumentation most used to monitor the
strain profile on superconducting structures along with compensation
of temperature and magnetic field. The materials of
low-temperature superconducting (LTS) magnets are sensitive
to strain. During assembly and training of the LTS magnets, excessive
strain could damage the properties of the magnets. The
C-series RSG from HBK® provide a wide temperature range
(from 4 to 523 K) that makes it an ideal choice for strain measurements
on LTS magnets [6]. The supplier specifies that the
C-series RSG can measure only down to 4 K due to their testing
limitations. Practically, several studies in the accelerator field
were performed and demonstrated the successful operation
IEEE Instrumentation & Measurement Magazine
35

Instrumentation & Measurement Magazine 24-9

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