Installing a Strain Gage

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Installing a Strain Gage
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for students attending Louisiana Tech University.
The author of this information does not make
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information or methods presented. The
procedures demonstrated here are potentially
dangerous and could result in injury or damage.
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things you may need . . .
MEEN 382
strain gage glue
sanitizing wipes
sand paper (220 and 400 grit)
conditioner
neutralizer
dial caliper
maskingtape
tape measure
clear tape
safety glasses
probe
solder flux
strain gages
soldering iron
mulitmeter
gloves
wire strippers
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cut aluminum bar using pliers
length 1-foot width 1/4 thickness 1/8
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condition the aluminum bar

1. apply a few drops of conditioner
2. sand the bar with the coarser grit sandpaper wet
   sanding with conditioner will prepare the metal
   surface for bonding
3. wipe off the conditioner with a sterile cloth
4. repeat steps 1 through 3 with the finer grit
   sandpaper

wet with a few drops of conditioner
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sand in neutralizer

1. apply a few drops of neutralizer
2. sand the bar with the finer grit sandpaper wet
   sanding with neutralizer will cleanse the surface
   of any active ingredients
3. wipe off the neutralizer with a sterile cloth
   (wipe in only one direction)

example of a well-prepared surface
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lay strain gage on the bar, shiny side up

• scoot it around with something other than your
  finger (could add oil grit)
• be sure not to let anything dirty touch the bottom

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get the gage on a piece of clear tape

• obtain a strip of clear tape about 3-inches long
• bring the tape down on top of the strain gage
  which should be lying freely on top of the
  aluminum bar
• push down on the strain gage to be sure it is
  attached to the tape

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tape the strain gage onto the beam

• tape it where you want it to be after bonding

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expose the bottom of the strain gage

• pull up the tape so you can get glue under the
  strain gage

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.
curl the tap over to prepare for gluing
a 1/4 gap between the bottom of the gage and the
aluminum bar is desired
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apply the catalyst

• you need a very thin layer of catalyst on the
  stain gage
• dab it 10 times on the rim of the catalyst jar to
  remove excess liquid
• wipe the bottom of the strain gage (the exposed
  side) until it is all coated once(dont apply
  too much)

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wait one minute before applying glue
move thumb this way
add a drop of glue at base of tape, then move
thumb over tape and hold pressure over gage for
one minute
sliding thumb while holding pressure will evenly
distribute glue under tape
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remove the tape

• pull the tape back at a very sharp angle, pulling
  it over a corner first
• work it slowly, minimizing lift on the gage

keep a high angle to avoid pulling up the strain
gage
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prepare lead wires
separate two of the wires, cutting off about 2/3
of the strands on each of the two wires to be
joined
tin the wires crop wires
evenly (about 3/16)
separate the strands together as shown
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tin the solder tabs on the strain gage

1. Clean and tin the tip of the soldering iron
2. Lay a thin strand of solder over the solder tabs
3. Touch each tab for no longer than one second to
   tin the tabs

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tape wires over solder pads with masking tape
tape wire into position and apply flux to the tabs
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solder wires to strain gage

1. Use a probe or small screwdriver to lightly force
   the wire into the tab
2. Touch the solder tip to the top of the tinned
   wire for about one second this should bond the
   wire to the gage
3. Be careful about tugging on the wires since the
   solder tabs will pull off, forcing you to start
   all over

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check the resistance of your strain gage

• The strain gages you are using are 120O
• If the resistance between the black and white (or
  black and red) leads is not around 120O, then
  something is wrong
• maybe the solder didnt bond
• the solder could be shorted between leads
• the solder tab may be pulled away from the gage

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Strain Gage Circuits
 
strain is computed as
 
for a uniaxial state of stress
 
 
 
 
 
 
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Wheatstone bridge circuit
we will use specific values for the resistors and
voltage source below
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redrawing the circuit
find the voltage at points B and D using Ohms law
R1 120W
R2 120W
B
D
Rg 120W
R4 120W
find the current through ABC
find the current through ADC
 
 
find the voltage at point B
find the voltage at point D
 
 
 
voltage drop between D and B
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equations for Wheatstone bridge circuit
 
what if RgR1R4R2?
what happens when the resistance of the strain
gage changes?
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summary