Installing a Strain Gage MEEN 382

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MEEN soldering iron sand paper (220 and 400 grit) gloves wire strippers mulitmeter dial caliper strain gage glue sanitizing wipes neutralizer conditioner safety glasses strain gages solder & flux probe clear tape masking tape tape measure things you may need...

MEEN cut aluminum bar using pliers length = 1-foot width = 1/4” thickness = 1/8”

MEEN 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

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) 6 living with the lab example of a well-prepared surface

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 7 living with the lab

get the gage on a piece of clear tape 8 living with the lab 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

tape the strain gage onto the beam 9 living with the lab tape it where you want it to be after bonding

expose the bottom of the strain gage 10 living with the lab keep a low angle to avoid damaging the strain gage pull up the tape so you can get glue under the strain gage

curl the tap over to prepare for gluing. 11 living with the lab a 1/4“ gap between the bottom of the gage and the aluminum bar is desired

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) 12 living with the lab

wait one minute before applying glue 13 living with the lab add a drop of glue at base of tape, then move thumb over tape and hold pressure over gage for one minute move thumb this way sliding thumb while holding pressure will evenly distribute glue under tape

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 14 living with the lab keep a high angle to avoid pulling up the strain gage

prepare lead wires 15 living with the lab 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”)

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 16 living with the lab

tape wires over solder pads with masking tape tape wire into position and apply flux to the tabs 17 living with the lab

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 living with the lab 18

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 living with the lab 19

Strain Gage Circuits MEEN 382 strain is computed asfor a uniaxial state of stress

Wheatstone bridge circuit we will use specific values for the resistors and voltage source below 21 living with the lab VoVo V s = 5V +-+- R 1 ≈ 120  R 4 ≈ 120  R 2 ≈ 120  R g ≈ 120  - + A B C D

redrawing the circuit find the voltage at points B and D using Ohm’s law 22 living with the lab find the current through ABC: find the voltage at point B: find the current through ADC: find the voltage at point D: V s = 5V +-+- R 1 ≈ 120  R 4 ≈ 120  R 2 ≈ 120  R g ≈ 120  B D voltage drop between D and B: A A C C VoVo - +

MEEN equations for Wheatstone bridge circuit VoVo V s = 5V +-+- R 1 ≈ 120  R 4 ≈ 120  R 2 ≈ 120  R g ≈ 120  - + A B C D what happens when the resistance of the strain gage changes? what if R g =R 1 =R 4 =R 2 ?

MEEN VoVo V s = 5V +-+- R 1 ≈ 120  R 4 ≈ 120  R 2 ≈ 120  R g ≈ 120  - + A B C D

MEEN summary VoVo V s = 5V +-+- R 1 ≈ 120  R 4 ≈ 120  R 2 ≈ 120  R g ≈ 120  - + A B C D 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).