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MEEN 382
Installing a Strain Gage
living with the lab
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MEEN 382
strain gage glue
things you may need . . .
sanitizing wipes
neutralizer
dial
caliper
masking
tape
sand paper
(220 and 400 grit)
conditioner
tape
measure
clear tape
safety
glasses
probe
solder & flux
strain gages
soldering iron
mulitmeter
wire strippers
gloves
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MEEN 382
cut aluminum bar using pliers
length = 1-foot
width = 1/4”
thickness = 1/8”
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MEEN 382
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
keep a low angle to avoid
damaging 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
(don’t apply too much)
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wait one minute before applying glue
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
separate the strands
together as shown
tin the wires
crop wires evenly (about 3/16”)
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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
1. The strain gages you are using are 120Ω
2. If the resistance between the black and white (or black and red) leads is not around
120Ω, then something is wrong:
• maybe the solder didn’t bond
• the solder could be shorted between leads
• the solder tab may be pulled away from the gage
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MEEN 382
Strain Gage Circuits
Δ𝑅
𝑅
strain is computed as 𝜖 = 𝑜
𝐺𝐹
𝜖 = 𝑛𝑜𝑟𝑚𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑖𝑛 𝑔𝑎𝑔𝑒 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛
∆𝑅 = 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝑠𝑡𝑟𝑎𝑖𝑛 𝑔𝑎𝑔𝑒
𝑅𝑜 = 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑡𝑟𝑎𝑖𝑛 𝑔𝑎𝑔𝑒
𝐺𝐹 = 𝑔𝑎𝑔𝑒 𝑓𝑎𝑐𝑡𝑜𝑟 𝑓𝑜𝑟 𝑡ℎ𝑒 𝑠𝑡𝑟𝑎𝑖𝑛 𝑔𝑎𝑔𝑒 (given)
for a uniaxial state of stress 𝜎 = 𝐸 ∙ 𝜖
𝐸 = 𝑒𝑙𝑎𝑠𝑡𝑖𝑐 𝑚𝑜𝑑𝑢𝑙𝑢𝑠 𝑜𝑓 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑡𝑜 𝑤ℎ𝑖𝑐ℎ 𝑔𝑎𝑔𝑒 𝑖𝑠 𝑏𝑜𝑛𝑑𝑒𝑑
𝜎 = 𝑛𝑜𝑟𝑚𝑎𝑙 𝑠𝑡𝑟𝑒𝑠𝑠 𝑢𝑛𝑑𝑒𝑟 𝑠𝑡𝑟𝑎𝑖𝑛 𝑔𝑎𝑔𝑒 (𝑎𝑠𝑠𝑢𝑚𝑖𝑛𝑔 𝑢𝑛𝑖𝑎𝑥𝑖𝑎𝑙)00
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Wheatstone bridge circuit
we will use specific values for the resistors and voltage source below
A
R2 ≈ 120W
R1 ≈ 120W
+
Vs = 5V
-
B
- Vo +
D
Rg ≈ 120W
R4 ≈ 120W
C
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redrawing the circuit
find the voltage at points B and D using Ohm’s law
A
A
Vs = 5V
R1 ≈ 120W
+
-
R2 ≈ 120W
- V +
o
B
D
Rg ≈ 120W
R4 ≈ 120W
C
C
find the current through ABC:
𝐼𝐴𝐵𝐶 =
𝑉𝑠
𝑅1 + 𝑅4
𝐼𝐴𝐷𝐶 =
find the voltage at point B:
𝑉𝐵 = 𝐼𝐴𝐵𝐶 ∙ 𝑅4 =
find the current through ADC:
𝑉𝑠 ∙ 𝑅4
𝑅1 + 𝑅4
voltage drop between D and B:
𝑉𝑠
𝑅2 + 𝑅𝑔
find the voltage at point D:
𝑉𝐷 = 𝐼𝐴𝐷𝐶 ∙ 𝑅𝑔 =
𝑉𝑜 = 𝑉𝐷 − 𝑉𝐵 =
𝑉𝑠 ∙ 𝑅𝑔
𝑅2 + 𝑅𝑔
𝑉𝑠 ∙ 𝑅𝑔
𝑅𝑔 𝑅1 − 𝑅4 𝑅2
𝑉𝑠 ∙ 𝑅4
−
= 𝑉𝑠 ∙
𝑅2 + 𝑅𝑔 𝑅1 + 𝑅4
(𝑅2 +𝑅𝑔 )(𝑅1 + 𝑅4 )
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MEEN 382
equations for Wheatstone bridge circuit
A
R2 ≈ 120W
R1 ≈ 120W
+
Vs = 5V
-
B
- Vo +
D
𝑉𝑜 = 𝑉𝑠 ∙
𝑅𝑔 𝑅1 − 𝑅4 𝑅2
(𝑅2 +𝑅𝑔 )(𝑅1 + 𝑅4 )
Rg ≈ 120W
R4 ≈ 120W
C
what if Rg=R1=R4=R2?
𝑉𝑜 = 0 or the bridge is balanced
what happens when the resistance of the strain gage changes?
an imbalance occurs, and we can relate this imbalance to ∆𝑅𝑔
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MEEN 382
equations in terms of ∆𝑅
A
R2 ≈ 120W
R1 ≈ 120W
+
Vs = 5V
-
B
- Vo +
D
𝑉𝑜 = 𝑉𝑠 ∙
𝑅𝑔 𝑅1 − 𝑅4 𝑅2
(𝑅2 +𝑅𝑔 )(𝑅1 + 𝑅4 )
Rg ≈ 120W
R4 ≈ 120W
C
if Rg=R1=R4=R2 before strain is applied and Rg becomes Rgo+∆𝑹𝒈 after strain is applied, then . . .
𝑉𝑜 = 𝑉𝑠 ∙
𝑅𝑔 + ∆𝑅𝑔 𝑅1 − 𝑅4 𝑅2
𝑉𝑠 ∙ ∆𝑅𝑔 ∙ 𝑅1
≈
(𝑅2 +𝑅𝑔𝑜 + ∆𝑅𝑔 )(𝑅1 + 𝑅4 ) (𝑅2 +𝑅𝑔𝑜 )(𝑅1 + 𝑅4 )
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MEEN 382
summary
A
R2 ≈ 120W
R1 ≈ 120W
+
Vs = 5V
-
- Vo +
B
D
𝑉𝑜 = 𝑉𝑠 ∙
𝑅𝑔 𝑅1 − 𝑅4 𝑅2
(𝑅2 +𝑅𝑔 )(𝑅1 + 𝑅4 )
Rg ≈ 120W
R4 ≈ 120W
C
𝑉𝑠 ∙ ∆𝑅𝑔 ∙ 𝑅1
𝑉𝑜 ≈
(𝑅2 +𝑅𝑔𝑜 )(𝑅1 + 𝑅4 )
Δ𝑅𝑔
𝑅𝑔𝑜
𝜖=
𝐺𝐹
𝜎 =𝐸∙𝜖
some initial imbalance of the bridge can be tolerated, but if it gets too
large, it should be balanced by adding more accurate resistors or by adding
a trimmer (a variable resistor that can be adjusted with a screw to balance
the bridge).
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