ROCTEST FOS Series User manual

INSTRUCTION MANUAL
SENSOPTIC FIBER-OPTIC SENSORS
FABRY PEROT STRAIN GAUGE
FOS Series
Roctest Limited, 2000. All rights reserved.
This product should be installed and operated only by qualified personnel. Its misuse is potentially dangerous. The Company makes no
warranty as to the information furnished in this manual and assumes no liability for damages resulting from the installation or use of this
product. The information herein is subject to change without notification.
Tel.: [phone] • 1.877.ROCTEST (Canada, USA) • 33.1.64.06.40.80 (France) • 41.91.610.1800 (Switzerland)
www.roctest.com
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TABLE OF CONTENTS
1PRODUCT...............................................................................................................1
1.1 Introduction.......................................................................................................1
1.2 Description........................................................................................................1
1.3 Specifications....................................................................................................3
2INSTALLATION.......................................................................................................4
2.1 Gauge installation .............................................................................................4
2.1.1 Metal surface preparation...........................................................................5
2.1.2 Bonding method.........................................................................................6
2.1.3 Rebar installation.....................................................................................11
2.1.4 Installation of sensors with composite laminates on concrete structures..11
2.2 Mating gauges to the readout unit...................................................................13
3DATA READINGS AND ANALYSIS......................................................................14
4MISCELLANEOUS................................................................................................16
4.1 Conversion factors..........................................................................................16

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1 PRODUCT
1.1 INTRODUCTION
The SENSOPTIC line of fiber optic sensors are specially developed instruments that can
be used in a variety of applications where their small size, high accuracy, broad
measurement range and complete immunity to EMI / RFI / Lightning are of paramount
importance. In addition, they have an excellent dynamic response, which allows
combined static and dynamic measurements, according to the specific needs of the
investigated structure.
The FOS can be surface-mounted on a variety of materials including steel, concrete and
composite materials using suitable industrial quality adhesives. The gauge is intended for
long-term, precise strain measurements and can be read by all ROCTEST's fiber-optic
readout units which displays readings directly in units of microstrains (see separate
instruction manual for operation of the readout unit).
1.2 DESCRIPTION
The gauge, namely a Fabry-Perot strain gauge, is based on a white-light interferometric
extrinsic principle that uses a common multimode fiber.
The patentedprinciple consists in assessing the length of a Fabry-Perot cavity
contained in the strain gauge by means of a Fizeau interferometer located in the readout
unit, that optically reproduces the length of the Fabry-Perot cavity and allows to digitize
that length on a high density linear photodiode array attached along one side of the
interferometer (Figure 1).
The Fabry-Perot cavity is made of two 125 microns diameter fibers facing each other
and fused in a 200 microns diameter glass micro-capillary, with a semi-reflective mirror
coating on each fiber’s tip. Then, the strain variations transferred to the gauge are
converted into cavity length variations.
The length of the Fabry-Perot cavity, as compared to the distance between the fused
welding spots on the fibers, defines the range of the strain gauge, whereas the
sensitivity of the gauge is defined by the density of the photodiode array used in the
readout unit.
The FOS-B model consists of an FOS sensor embedded in a composite material blade
ready to be bonded to concrete and other surfaces.
U.S. Patent 5,202,939

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Figure 1 Fabry-Perot strain gauge schematic principle

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1.3 SPECIFICATIONS
Gauge type:
FOS: Standard fiber-optic strain gauge
FOS-N: Non temperature compensated FOS
FOS-C: Temperature compensated FOS
FOS-B: FOS encapsulated in a blade
FOS-R: FOS encapsulated in a rod
Stain range (microstrains):
±1 000, ±2 000, ±3 000, ±5 000
0 to 2 000, 0 to 3 000, 0 to 5 000
minus 2 000 to 0, minus 3 000 to 0, minus 5 000 to 0,
(other available on request)
Resolution:
0.01% of full scale
Precision:
Range dependent
Transverse sensitivity:
0.1% of full scale
Response time:
Readout unit/data logger dependent
Operating temperature:
Minus 55°C to +350°C; operating temperature is fiber-
optic cable dependent
EMI/RFI susceptibility:
Intrinsic immunity
Combatibility::
All ROCTEST's fiber-optic readout units and data loggers
Cable length:
1.5 meter length. Custom length up to 5 km available
Gauge dimensions:
310 microns diameter
Fiber-Optic Cable:
4.0 mm O.D. polyurethane jacket Kevlarreinforced
(minus 55°C to +85°C), or 1 mm O.D. braided fiberglass
cable (minus 55°C to +350°C)
Gauge material:
Glass
Connector:
ST Connector

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2 INSTALLATION
2.1 GAUGE INSTALLATION
The steps for bonding the fiber optic strain gauges depend upon the adhesive used.
Epoxy and ceramic adhesives and even metal arc spraying can be used to bond
gauges. As with conventional strain gauges, high residual compression strains occur
when adhesives are heat cured at elevated temperatures. When gauges adhere to the
pieces at elevated temperatures, compressive strains are generated in the gauges as
the pieces cool down. Residual compression strains can be controlled by curing
adhesives while the pieces are put into tension prior to cooling, or consideration may be
given to this behavior while ordering the gauges. Contact the Manufacturer for
information.
The Manufacturer suggests using the M-BOND AE-10 or AE-15 epoxy adhesives from
MEASUREMENTS GROUP (Micro-Measurements Division) for low temperature
applications (up to 95°C). For high temperature applications, the use of M-BOND 600
(up to 370°C short term and 260°C long term) is recommended. Mixing instructions and
adhesive characteristics are given by the adhesive manufacturer. Slowly mixing the
resin and hardener is recommended to prevent excessive amount of air (bubbles). The
basic steps for bonding strain gauges with AE-10 or AE-15 epoxy from Micro-
Measurements are given in the next paragraphs. Remember: the key for successful
bonding of strain gauges is to have a good contact between the gauge and the
specimen to be measured. Here are some accessories and products that you will need
for installation of the FOS strain gauge:
CSM-1A Degreaser (from Measurements Group) or Isopropyl Alcohol;
MJG-2 Mylar Tape (from Measurements Group) or "Scotch" Tape;
M-Prep Conditioner A (from Measurements Group) or mild phosphoric acid
compound (10% concentration in distilled water);
M-Prep Neutralizer 5A (from Measurements Group) or ammonia-based neutralizer
compound made of isopropanol (15% conc.), Ammonium Hydroxide (5% conc.), and
Distilled Water (80% conc.);
Five Minute type epoxy;
Mylar or high temperature adhesive tape such as Teflon or Kapton adhesive tape;
Silicon-Carbide Abrading Paper (Grit 220 or 320 and grit 400);
Low lint wipers (Kimwipes wipers);

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Cotton tipped applicators (Cotton Swabs);
Waterproof tape (Aquaseal);
Nitrile and butyl rubber sealants;
M-Coat Protective Coating (from Measurements Group) - Optional.
2.1.1 METAL SURFACE PREPARATION
The surface preparation technique is the same basic procedure as for conventional
strain gauges. The initial step is to clean the surface in order to remove oil, grease,
organic contaminants and chemical residues. Solvents such as CSM-1A from
MEASUREMENTS GROUP (Micro-Measurements Division) or GC-6 Isopropyl Alcohol
or solvent 755 from Loctite can be used. The substitution of Isopropyl Alcohol as a
degreasing agent should be considered for materials that may be sensitive to strong
solvents. Any degreasing should be done with clean solvents. Thus the use of a "one-
way" container, such as the aerosol can, is highly advisable.
Next, the surface must be abraded to remove any loosely bonded adherents like scale,
rust or paint. Dry-abrade the gauge area with silicon-carbide paper of the appropriate grit
(generally 220 or 320). For rough or coarse surfaces, a grinder or file can be used to
start abrading. Apply M-Prep Conditioner A (or the equivalent compound) and wet-
abrade the gauge area. Keep the surface wet while abrading. Remove the residue and
Conditioner by slowly wiping through the gauge area with clean low lint wipers
(Kimwipes wipers). The wet-abrade and wiping procedure should then be repeated with
Note: The manufacturer can provide a complete Gauge Installation Kit which
includes the adhesive with all the accessories and products mentioned
above. For more information, please contact the manufacturer.
HANDLING PRECAUTIONS: Epoxy resins and hardeners may cause dermatitis
or other allergic reactions, particularly in sensitive persons. The user is cautioned
to: (1) avoid contact with either the resin or hardener; (2) avoid prolonged or
repeated breathing of the vapors; and (3) use these materials only in well-
ventilated areas. If skin contact occurs, thoroughly wash the contaminated area
with soap and water immediately. In case of eye contact, flush immediately and
secure medical attention. Rubber gloves and aprons are recommended, and care
should be taken not to contaminate working surfaces, tools, container handles,
etc. Spills should be cleaned up immediately.

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400-grit silicon-carbide paper. Rewet the surface with Conditioner A (or the equivalent
compound) and scrub with cotton tipped applicators until a clean applicator is no longer
discolored by the scrubbing. Remove the residue and Conditioner by slowly wiping
through the gauge area with clean Kimwipes.
The final step is to apply a liberal amount of M-Prep Neutralizer 5A from Micro-
Measurements to the gauge area. Keeping the surface wet, scrub with cotton tipped
applicators. Do not allow evaporation of the cleaning material since this would leave a
thin, unwanted film between the adhesive and the surface. Remove the neutralizer by
slowly wiping through the gauge area, allowing the clean Kimwipes to absorb the
Neutralizer. Do not wipe back and forth over the gauge area since this may allow
contaminants to be redeposited on the cleaned area.
2.1.2 BONDING METHOD
A) With a lead pencil, make two marks (A) along the projected bond line,
approximately 50 to 60 mm apart.
A
A
50 to 60 mm

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B) With a flashbreaker or an electrical tape, create a channel into which the fiber
sensor will be installed. The width of this channel must be approximately 2 mm.
C) Place the gauge between the channel of electrical tape and hold it in place by
applying tape ("scotch" tape may also be used) on the incoming fiber-optic 25
mm away from the micro-capillary. Position the gauge on the surface and tack
the tape to maintain proper alignment. The sensitive area is collinear to the axis
of the fiber. The location of the sensitive area of the gauge is shown on the box
cover that contain the fiber-optic gauge.
A
A
2 mm
Note: The gauge micro-capillary is made of glass and doesn't have any
protective coating so it should be handled with care prior to bonding the
gauge. Any scratches made to the glass micro-capillary may impair the
efficiency and the lifetime of the gauge.
Incoming fiber optic Electrical tape
Micro-capillary Fiber optic cable
Gage sensitive region

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D) The incoming optical fiber must be secured on the specimen with a very small
drop (less than 1 mm) of 5-minutes epoxy about 3 mm away from the micro
capillary. To ensure that the 5-minutes epoxy will not flow to the micro-capillary,
we recommend letting the epoxy cure slightly before applying it on the fiber in
order to increase viscosity.
E) Once the 5-minutes epoxy is cured, mix the adhesive by following the mixing
instructions of the adhesive manufacturer. Slowly mix the resin and hardener to
prevent excessive amount of air bubbles using a clean stick (or brush) and apply
the adhesive slowly with a linear motion parallel to the gauge orientation. Make
sure the entire gauge is covered from the tip to the 5-minutes epoxy.
Reminder: To avoid surface contamination, do not touch the micro-capillary of
the Fiber-Optic Strain Gauge with your hands. The gauge micro-
capillary must be free of grease, dust, etc. If necessary, use a
Duster can or clean dry air to blow away dust, dirt and grit on the
micro-capillary.
5-minutes epoxy
3 mm