Installing a Strain Gage

1. MEEN 382 Installing a Strain Gage

2. living with the lab The content of this presentation is for informational purposes only and is intended only for students attending Louisiana Tech University. The author of this information does not make any claims as to the validity or accuracy of the information or methods presented. The procedures demonstrated here are potentially dangerous and could result in injury or damage. Louisiana Tech University and the State of Louisiana, their officers, employees, agents or volunteers, are not liable or responsible for any injuries, illness, damage or losses which may result from your using the materials or ideas, or from your performing the experiments or procedures depicted in this presentation. If you do not agree, then do not view this content. DISCLAIMER

3. 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

4. MEEN 382 cut aluminum bar using pliers length = 1-foot width = 1/4” thickness = 1/8”

5. MEEN 382 condition the aluminum bar apply a few drops of conditioner sand the bar with the coarser grit sandpaper; wet sanding with conditioner will prepare the metal surface for bonding wipe off the conditioner with a sterile cloth repeat steps 1 through 3 with the finer grit sandpaper wet with a few drops of conditioner

6. living with the lab sand in neutralizer apply a few drops of neutralizer sand the bar with the finer grit sandpaper; wet sanding with neutralizer will cleanse the surface of any active ingredients wipe off the neutralizer with a sterile cloth (wipe in only one direction) example of a well-prepared surface

7. living with the lab 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

8. living with the lab 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

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

10. living with the lab 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

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

12. living with the lab 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)

13. living with the lab 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

14. living with the lab 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

15. living with the lab 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

16. living with the lab tin the solder tabs on the strain gage Clean and tin the tip of the soldering iron Lay a thin strand of solder over the solder tabs Touch each tab for no longer than one second to tin the tabs

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

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

19. living with the lab check the resistance of your strain gage • The strain gages you are using are 120Ω • 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

20. MEEN 382 Strain Gage Circuits strain is computed as for a uniaxial state of stress 00 (given)

21. living with the lab Wheatstone bridge circuit we will use specific values for the resistors and voltage source below A R2 ≈ 120W R1 ≈ 120W + - Vo - + Vs= 5V D B Rg ≈ 120W R4 ≈ 120W C

22. living with the lab redrawing the circuit find the voltage at points B and D using Ohm’s law A A R1 ≈ 120W R2 ≈ 120W - + Vo D B Rg ≈ 120W R4 ≈ 120W + - Vs= 5V C C 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:

23. MEEN 382 equations for Wheatstone bridge circuit A R2 ≈ 120W R1 ≈ 120W + - Vo - + Vs= 5V D B Rg ≈ 120W R4 ≈ 120W C what if Rg=R1=R4=R2? or the bridge is balanced an imbalance occurs, and we can relate this imbalance to what happens when the resistance of the strain gage changes?

24. MEEN 382 equations in terms of A R2 ≈ 120W R1 ≈ 120W + - Vo - + Vs= 5V D B Rg ≈ 120W R4 ≈ 120W C if Rg=R1=R4=R2 before strain is applied and Rgbecomes Rgo+ after strain is applied, then . . .

25. MEEN 382 summary A R2 ≈ 120W R1 ≈ 120W + - Vo - + Vs= 5V D B Rg ≈ 120W R4 ≈ 120W C 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).