Strain Gauge Bonding Techniquae

8/13/2019 Strain Gauge Bonding Techniquae

1/18

STRAIN

GAUGE

BONDING

TECHNIQUE


2/18

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


3/18

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


4/18

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.


5/18

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.


6/18

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


7/18

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.


8/18

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.


9/18


10/18

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.


11/18

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.


12/18

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


13/18

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.


14/18

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.


15/18

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.


16/18

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


17/18

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:


18/18

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.

Documents

Strain Gauge Bonding Techniquae