strain gauge bonding techniquae
TRANSCRIPT
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STRAIN
GAUGE
BONDING
TECHNIQUE
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INSTRUCTION MANUAL
FOR
STRAIN GAUGE BONDING
1. PRINCIPLE
The change in electrical resistance of a strain
gauge is proportional to the mechanical strain to
which i t is subjected. When i t is physical ly
mounted on a test sur face and electr ical ly
connected as an arm of a Wheatstones bridgecircuit, the resulting output signal represents a
voltage analogue of the mechanical deformation of
the surface.
2. CONSTRUCTION
Strain Gauge consists of various configurations of
a resistance wire or foil bonded to paper, bakelite,
polyester, epoxy and other insulating bases. They
are available with flat and helical elements in singleor multi-axial arrays (Rossettes).
3. SELECTION
Environment, test specimen material, available
space, extent and type of deformation and the
desired life of the gauge are the factors for
consideration in the selection of strain gauges.
4. MOUNTING4.1 Preparation of mounting surface
It is recommended that a surface about twice the
size of the gauge should be prepared for mounting
the strain gauge.
Remove scales, rust, grease, or any type of
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contaminat ion from the measur ing surface.
Smoothen the protruding edges of the measuring
surface with the help of chiesel and file. Clean the
paint or rust scales with wire brush, hand grinder
or 150 to 220 gra in emery paper. Avoid polishing
of the surface to give perfect bonding. Next, clean
the surface with chemical agents to remove dust
particles, emery remains and grease, etc. Wash
the surface with Surface Cleanibg Agent on grease
free cotton wool. Hold the cotton wool withtweezers to avoid contamination. The required
surface should then be rubbed with cotton wool
soaked in Surface Cleaning Agent till no dirt
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appears on the cotton wool. Commercial benzene
or other solvents having high gasoline content must
not be used.
The cleaned surface should not be touched by hand
or else the sufface should be cleaned once again.
4.2 Preparation of Strain Gauge.
Gently remove the Strain Gauge from its packing
using a pointed forceps. Care should be taken to
avoid damage to the Strain Gauge or its leads asthese are very sensitive to mechanical forces.
Place the Strain gauges on a clean glass surface
to avoid contaminaton of the Strain Gauge surface.
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Select suitable Strain Gauge Wiring Terminals
(SGWT) to be soldered to the Strain Gauge leads.
The size of the wiring terminals is normally decided
by the distance between the lead wires of the
Strain Gauges used. The Strain Gauge WiringTerminals are normally available in sizes of 5, 7,
10 mm.
Trim each pair of the wiring terminal using a sharp
and clean pair of scissors.
Place the Strain Gauge on a glass plate and clean
its leads using a sharp knife edge to remove any
scales or remenant adhesive adhearing to the lead
surface.
Trim the Strain Gauge leads to a suitable size so
that when the Strain Gauge and its corresponding
wiring terminal is placed side by side, the lead
covers the complete diameter of the dumbel shaped
terminal on one side.
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Clean the wiring terminal copper surface with a
fine sand paper (320/600 grade) and place Strain
Gauge and the wiring terminal side by side on a
clean glass plate. Finely solder the Strain Gauge
leads to the dumbell shaped using a good quality
soldering flux preferably a non-corrosive soldering
flux. Care should be taken that the edges of the
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Strain Gauge and the wiring terminal do not overlap
each other or the gap between them should be
minimal.
4.3 Cleaning of Strain Gauges.
After all the Strain Gauges have been soldered,
place them in a clean glass bowl and pour some
Surface Cleaning Agent into the bowl. Clean both
surfaces of the Strain Gauges using a soft hair
brush and holding the Strain Gauges with a clean
forceps. Care should be taken to remove all the
remenant flux from the soldering area.
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After cleaning each Strain Gauge, place them up-
side down on a clean glass plate and allow them
to dry up. A slight heat can be applied using a
table lamp.
4.4 Placement of Strain Gauges.
Once the Strain Gauges have been soldered to the
Strain Gauge Wiring Terminal, cleaned and dried,
it is time to place the Strain Gauges at the point
where they are to be pasted.
Ordinary Cello tape 3/4 inches wide is used to place
the Strain Gauge in position. Take about 3 times
long Cello tape and fold about 5 mm. at one end
to hide the sticking portion. This will be used tolift the Cello tape when required. Place the prepared
Strain Gauge in the middle of this Cello tape such
that the terminal end is towards the folded end of
the tape and the soldered side is on the sticking
side of the tape. Thus the bonding side of the
Strain Gauge and the Wiring terminal is now
exposed.
Use a clean forceps to place the Strain Gauge on
the tape. Once the Strain Gauges have been
thoroughly cleaned, they should not be handled
with hands as this contaminates their clean
surface.
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Bring very viscous adhesives to the required
viscosity by adding recommended solvents. The
milk like precipitation on the adhesive surface in
monsoons or winters is prevented by heating the
adhesive to approximately 30-40C using a tablelamp or a hair dr ier. Once the precipitat ion
dissappears, the adhesive is ready for use.
4.6 Adhesive application.
One the adhesive has been prepared and the Strain
Gauges have been positioned at their location, it
is time to apply the adhesive to the Strain Gauges.
Gently lift the Strain Gauge adhearing tape from
its folded side till the full surface of thr Strain Gaugeand wir ing terminal is exposed. dur ing this
operation some of the adhesive of the tape is left
behind on the test surface.
This residual tape adhesive has to be thoroughly
cleaned before Strain Gauge bonding. Make a
small cotton ball and hold it with a forceps. Dip it
in surface cleaning agent and scrub the surface
below the Strain Gauge to remove all the residual
tape adhesive. Repeat if required till a clean surfaceis obtained.
Now apply a very thin layer of the adhesive to the
complete exposed back side of the Strain Gauge
and wiring terminal. Single component adhesives
can be directly applied while two component
adhesives should be applied by soft hair brushes.
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Now place back the Strain Gauge tape at its desired
location and cover the Strain Gauge area with a
fine PVC sheet. Apply a slight thumb pressure on
the Strain Gauge area to push out the excess
adhesive if any.
Place a 2-3 mm thick Neoprene Rubber sheet over
the Strain Gauge area and apply 1-2 kg/cm 2
pressure over it for desired period as applicable
for a particular adhesive.
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Quick setting adhesives like cyno-acrylate, Acrylic,
Ceramic, etc., require 1-5 minutes of pressure
application while others may require 6-24 hours
for curing. Room temperature setting adhesivescure without the need for heating, while thermo-
setting adhesives require ovens for curing at
desired temperatures.
Once the adhesive is fully cured, remove the
weight, cushioning rubber sheet, and PVC sheet.
Now gently lift the Cello tape that is holding the
Strain Gauge and the wiring terminal to the test
specimen. The soldering side of the wiring terminal
and the Strain Gauge are now exposed. Check the
resistance of the gauge which should tally with
the resistance marked on the packet. In general a
slight difference may be found. In case of zero or
infinite resistance, the gauge has become short
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circuited or open circuited during the cementing
procedure. Measure the resistance between the
leads and the metal body which should be more
than 2000 megohms.
The colour of the gauge should be uniform and nospots seen which other-wise indicates air bubbles
or dust specs and incorrect cementing.
4.7 Cable connections.
Before making cable connections to the bonded
Strain Gauge, layout the cables for al l the Strain
Gauges right upto the measuring instruments. Hold
the cables at suitable locations using adhesive
tapes or suitable anchors. Care should be takento avoid hanging cables. Provide suitable supports
below the cable bunch where required.
Low resistance cables with multi-strand silvered
copper wires should invariably be used for Strain
Gauge installations. Flat twin cables can be used
for short distances of upto 1-2 meters while single
core shielded cables should be used when the
distance between the Strain Gauge and the
instrumentation is more. This minimises stray
signal pickup from noisy electrical lines etc.
Once all cables have been laid, finely solder the
two ends of the cables to the other two ends of
the dumbel shaped Strain Gauge Wiring Terminals.
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Soldering should be proper as any dry soldering
will result in inaccurate measurements.
Avoid acidi fied soldering pastes as they cause
errors in Strain Gauge measurements. Do not use
a very hot soldering iron or keep it for long on theleads rendering the strain gauge inserviceable.
Measure resistance at the instrument end of the
cable.This would be slightly higher than the Strain
Gauge resistance as the resistance of the cable
is now included.
After checking Strain Gauge resistance at both the
ends of the cables, thoroughly clean the soldered
area on the wiring terminal using a soft hair brushand surface cleaning agent till all the residual flux
is removed. Heat the junction slightly using a table
lamp or a hair drier to remove all moisture.
4.8 Moisture proofing.
Humidity has a deterimental effect on the operation
of Strain Gauges. To protect them from moisture,a sui table water proof ing compound should
invariably be spread completely over the Strain
Gauge and the wiring terminal area after thorough
cleaning and drying.
Rohits Water Proofing compound sticks can be
used for short term measurement for 1-2 weeks.
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Rohits Liquid Water Proofing compound can be
used for long term un-submerged conditions while
Rohits Silicon based Water Proofing Compound
should invariably be used for splash & submerged
applications.
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4.9 Application of compensating gauge.
In case of quarter bridge applications a dummyStrain Gauge should invar iably be used to
compensate for the temperature drift of the Strain
Gauges. The two most important factors that are
affected by temperature are the co-efficient of
resistance and the co-efficient of linear expansion.
To compensate for the former, a Strain Gauge from
the same lot should be used as a dummy Strain
Gauge, while for the latter, the dummy Strain
Gauge should be mounted on the same material
as that of the active Strain Gauge and should be
located as near to the test specimen as possible
so that both undergo the same temperature
changes.
Apply the dummy compensating gauge to the
surface undergoing no mechanical change and
place it in the vicinity of the test specimen. Care
should also be taken that the cable lengths of theActive and the Dummy Strain Gauges should be
similar as the cable resistance also poses the
same problems.
4.10 Checking installation.
To ensure proper installation of Strain Gauges,
make two checks. The first after installing the
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Strain Gauge and second after connect ing it with
the test circuit.
Resistance between the gauge and the mounting
surface should be of the order of 2000 Megohms.
if it is less, then it may be due to moisture orincomplete drying of the adhesive which can be
removed with hot air dryer. If drying does not
correct the condition replace the gauge.
Resistance of the installed Strain Gauge at the
instrument end should be nearly the same as
mentioned on the Strain Gauge wrapper plus the
resistance of the interconnecting cable.
To check the integrity of the bond, connect theStrain Gauge to the measuring instrument. Gently
probe the Strain Gauge with a thumb and observe
the readout system indication. A true bonded Strain
Gauge wi l l not show any f luctuat ion in the
instrument reading. A Variation in the instrument
reading signifies defective Strain Gauge bonding.
The sources of error could be :
a . Uncured adhesive.b. Presence of dust part icle under the Strain
Gauge measuring grid.
c . Dry soldering.
d. Entrapment of moisture around the mounted
Strain Gauge.
In such cases scrap the mounted Strain Gauge
and bond a fresh one keeping in view the facts
stated above
5. TEMPERATURE COMPENSATION
Since strain gauges are temperature sensitive,
more accurate measurement will be obtained with
temperature compensation. Some methods for
accomplishing this are:
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a. Twin gauges (active and dummy): Place the
gauges side by side but bond only the ac tive one
to the test surface. Mount the dummy one on a
similar but unstrained material located near the
test specimen. Then connect them electrically asadjacent arms of a Wheatstone Bridge.
b. Gauge selection: Choose gauge material of
se lec ted tempera ture coef f ic ien t o f l inear
expansion that will nullify the resistance change
due to the differential expansion between gauge
material and specimen.
c . C ab les : Use high qual it y mu lt i- st rand ,
silvered copper wire cables for wiring to minimisecable resistance.
d. Cable length : Use equal lengths of cables
for active and dummy Strain Gauges.