1. ME 322: Instrumentation Lecture 7
February 1, 2016
Strain Gage Introduction, Demonstration, Wheatstone bridge, Temperature compensation

2. Announcements/Reminders
HW 2 due now (before lecture starts)
This week in Lab:
Lab 3 Pressure Transmitter Calibration
Acquire your own data and use it to calibrate the pressure transmitter you’re issued
There are only 4, two-port pressure standards which can be used by two groups at a time. There are 16 groups in lab, so please be patient (and prepared).
Please bring an electronic copy of the Excel Workbook you created for homework

3. Pressure Gage Device Pressure difference causes diaphragm deformation
Deformation can be measured using a Strain Gage

4. Strain Gauge Construction∆𝐿
Thin substrate (0.0002”)
Firmly bounded to surface
Metal or Semiconductor foil on or within substrate
Surface deformation stretches or compresses the foil and changes its resistance.
Measure DR = R - RI
However, ∆𝐿 𝐿 & ∆𝑅 𝑅 are small
Demo: Gage resistance on an aluminum beam in bending

5. Stain Gage Applications
Can measure Strain of a deformed part (or material elastic modulus)
Can be incorporated in devices to sense
Applied force or weight
Acceleration
Pressure

6. What is Strain? L δ F
Unit strain 𝜀= deformation initial length = δ L in in = cm cm =1
Microstrain 𝜇𝜀=𝜀× 𝜇in in = 𝜇m m
Strain is caused by stress
𝜎= 𝐹 𝐴
For Linear elements 𝜀= 𝜎 E
E ≡ Elastic or Young's Modulus lbf in 2 =psi 𝑁 m 2 =Pa
Material property = fn(Temperature, material)
Describes material stiffness
In Lab 5 we measure E for aluminum and steel beams

7. To use a Strain Gage, firmly bond it to a Surface
Do this in Lab 4 (next week)
The gage will experience nearly the same stain and the object’s (specimen’s) surface
eGAGE = eSURFACE
Does not measure internal stains
The gage deformation will affect the gage resistance, R
The initial (un-deformed) resistance is RI
How to predict DR = R – RI?
specimen

8. Wire Resistance Depends on length, area and material property
L, R A D
Depends on length, area and material property
R= L A 𝜌= 4𝐿𝜌 𝜋 D 2
r ≡ Electric Resistivity
Material property = fn(Temperature, Strain, …)
Stretching wire changes L, D & ρ by small amounts, and results in small changes in R

9. Effect of Small Changes in L, D & ρ on R
R= 4𝐿𝜌 𝜋 D 2
DR = R – RI
For small changes DR = dR, use the chain rule:
dR= 𝜕𝑅 𝜕𝐿 𝑑L+ 𝜕𝑅 𝜕𝐷 𝑑D+ 𝜕𝑅 𝜕𝜌 𝑑𝜌
𝑑𝑅= 4𝜌 𝜋 D 2 𝑑L+ −2 4L𝜌 𝜋 D 3 𝑑D+ 4L 𝜋 D 2 𝑑𝜌
Divide by R to get fractional change
𝑑𝑅 𝑅 = 𝑑𝐿 𝐿 −2 𝑑𝐷 𝐷 + 𝑑𝜌 𝜌
Relates fractional change in 𝑅 to fractional changes in L, D, 𝜌
How do the d( ) ( ) terms relate to strain e ?

10. Evaluate Terms dL L = 𝜀 a axial strain (assuming 𝜀 a = eSpeciman)
This is what we are tying to measure!
dD D ≡ Transverse strain = 𝜀 t =−υ 𝜀 a
υ ≡ Gage material Poison’s ratio
~ 0 – 0.5
~ 0.3 for metals
d𝜌 𝜌 =Crystalline realignment effect= C strain ε a
Cstrain ≡ Strain Coefficient of Resistivity
Material property = fn(Temperature, …)
Can be large, and > or < 0 for semiconductors

11. Combined Effects dR 𝑅 = dL L −2 dD D + d𝜌 𝜌
dR R = ε a −2 −υ 𝜀 𝑎 + C strain ε a
dR R = 1+2υ+ C strain 𝜀 𝑎 = 𝑆 𝜀 𝑎
S=1+2υ+ C strain =𝑆𝑡𝑟𝑎𝑖𝑛 𝐺𝑎𝑔𝑒 𝐹𝑎𝑐𝑡𝑜𝑟
Dimensionless
Metal Foils: S = to 4 (typical 2.07)
Semiconductors: S = -140 to 175
dR R =S 𝜀 𝑎
S= dR R ε = dR R dL L

12. Example Apply 1000 lbf to diameter D = 0.25” steel rod.
E = 207 GPa = 30x106 psi
For a strain gage with: RI = 120 Ω, S = 2.07
Find final resistance: R = RI + ∆R
Solution
∆𝑅 𝑅 =𝑆 𝜀
𝜀= (𝜎) 𝐸 = 𝐹 𝐸(𝐴) = 4𝐹 𝜋𝐸 𝐷 2 = 4(1000 𝑙𝑏𝑓) 𝜋(30,000,000 𝑙𝑏𝑓 𝑖𝑛 2 ) (0.25𝑖𝑛) 2 = 𝑖𝑛 𝑖𝑛 =679 𝜇𝑖𝑛 𝑖𝑛 =0.068%
∆𝑅 𝑅 =𝑆 𝜀 = = Ω Ω
R = ( ) = W
Very small fractional change!

13. Undesired Temperature Sensitivity
Gauge resistivity 𝜌 is affected by temperature
Thermal expansion of specimen and gage may be different
This can stain the gage
Resistance is determined by measuring the voltage across the gage while passing a current though it, which can heat the gage!
Temperature factor ST:
dR R =S 𝜀 𝑎 S T ∆T
Can a circuit “automatically” compensate?
Desired measurand
Gage Temperature Change = undesirable sensitivity

14. Wheatstone Bridge Circuit
Two voltage dividers
𝑉 0 = 𝑉 DC − 𝑉 BC = 𝑅 3 V 𝑠 𝑅 2 + 𝑅 3 − 𝑅 4 𝑉 𝑠 𝑅 1 + 𝑅 4
𝑉 0 𝑉 𝑠 = 𝑅 3 𝑅 1 + 𝑅 4 − 𝑅 4 𝑅 2 + 𝑅 𝑅 2 + 𝑅 3 𝑅 1 + 𝑅 4
= 𝑅 1 𝑅 3 − 𝑅 2 𝑅 4 𝑅 2 + 𝑅 3 𝑅 1 + 𝑅 4 R3
Use strain gages for some or all of these resisters
If R1R3 ~ R2R4, then the output voltage will be close to 𝑉 0 ≈0

15. 𝑉 0 𝑉 𝑠 = 𝑅 1 𝑅 3 − 𝑅 2 𝑅 4 𝑅 2 + 𝑅 3 𝑅 1 + 𝑅 4 Initial StateR3 - +
𝑉 0 𝑉 𝑠 = 𝑅 1 𝑅 3 − 𝑅 2 𝑅 4 𝑅 2 + 𝑅 3 𝑅 1 + 𝑅 4 - +
Choose initial resistances so that initial VO,I ~ 0
R1R3~ R2R4 (For example, all could be ~equal)
Small changes in each Ri will cause a relatively large fractional change in VO (compared to VO,I ~ 0)
To increase 𝑉 0
Increase 𝑅 1 or 𝑅 3 and/or decrease 𝑅 2 or 𝑅 4

16. Effect of small resistance changes on VO
𝑉 0 = 𝑅 1 𝑅 3 − 𝑅 2 𝑅 4 𝑅 2 + 𝑅 3 𝑅 1 + 𝑅 4 𝑉 𝑠
Chain Rule:
𝑑 𝑉 0 = 𝜕 𝑉 0 𝜕 𝑅 1 𝑑 𝑅 1 + 𝜕 𝑉 0 𝜕 𝑅 2 𝑑 𝑅 2 + 𝜕 𝑉 0 𝜕 𝑅 3 𝑑 𝑅 3 + 𝜕 𝑉 0 𝜕 𝑅 4 𝑑 𝑅 4
Find all four partial derivatives 𝜕 𝑉 0 𝜕 𝑅 𝑖 and plug in
use R1R3= R2R4 and 𝑑 𝑉 0 = 𝑉 0 − 𝑉 0𝐼 = V 0 − 0 = V 0 ,
Simply ...
𝑉 0 = 𝑉 𝑆 𝑅 2 𝑅 𝑅 2 + 𝑅 𝑑 𝑅 1 𝑅 1 − 𝑑 𝑅 2 𝑅 𝑑 𝑅 3 𝑅 3 − 𝑑 𝑅 4 𝑅 4
If all four resistances start off roughly the same (satisfies 𝑅 1 𝑅 3 ≈ 𝑅 2 𝑅 4 )
𝑅 1 = 𝑅 2 = 𝑅 3 = 𝑅 4 and 𝑅 2 𝑅 𝑅 2 + 𝑅 = 𝑅 𝑅 2 = 1 4
𝑉 0 𝑉 𝑆 = 𝑑 𝑅 1 𝑅 1 − 𝑑 𝑅 2 𝑅 𝑑 𝑅 3 𝑅 3 − 𝑑 𝑅 4 𝑅 4
For precision resistors, 𝑑 𝑅 𝑖 𝑅 𝑖 =0, but not for stain gages

17. Incorporate gages into some bridge legs
𝑉 0 𝑉 𝑆 = 𝑑 𝑅 1 𝑅 1 − 𝑑 𝑅 2 𝑅 𝑑 𝑅 3 𝑅 3 − 𝑑 𝑅 4 𝑅 4
To increase 𝑉 0
Increase 𝑅 1 and 𝑅 3 and/or decrease 𝑅 2 and 𝑅 4
Install stain gages in some or all legs
𝑑 𝑅 𝑖 𝑅 𝑖 = 𝑆 𝑖 𝜀 𝑖 + 𝑆 𝑇𝑖 ∆ 𝑇 𝑖 =𝑆 𝜀 𝑖 + 𝑆 𝑇 ∆ 𝑇 𝑖
Use gages with the same characteristics
Ri=R, Si =S, and STi = ST
If identical gages are installed in all four legs, then
𝑑 𝑉 0 𝑉 𝑠 = 1 4 𝑆 𝜀 1 − 𝜀 2 + 𝜀 3 − 𝜀 4 + 𝑆 𝑇 ∆ 𝑇 1 −∆ 𝑇 2 +∆ 𝑇 3 −∆ 𝑇 4 R3

18. Quarter Bridge In general For quarter bridge with a gage only at 3+ - - +
In general
𝑉 0 𝑉 𝑆 = 𝑑 𝑅 1 𝑅 1 − 𝑑 𝑅 2 𝑅 𝑑 𝑅 3 𝑅 3 − 𝑑 𝑅 4 𝑅 4
For quarter bridge with a gage only at 3
𝑑 𝑅 1 =𝑑 𝑅 2 =𝑑 𝑅 4 =0
𝑑 𝑅 3 𝑅 3 =𝑆 𝜀 3 + 𝑆 𝑇 ∆ 𝑇 undesired sensitivity
𝑉 0 𝑉 𝑆 = 1 4 𝑆 𝜀 3 + 𝑆 𝑇 ∆ 𝑇 3

19. Half Bridge - + For this half bridge
𝑉 0 𝑉 𝑆 = 𝑑 𝑅 1 𝑅 1 − 𝑑 𝑅 2 𝑅 𝑑 𝑅 3 𝑅 3 − 𝑑 𝑅 4 𝑅 4
For this half bridge
𝑑 𝑅 1 =𝑑 𝑅 4 =0
𝑑 𝑅 2 𝑅 2 =𝑆 𝜀 2 + 𝑆 𝑇 ∆ 𝑇 2 ; 𝑑 𝑅 3 𝑅 3 =𝑆 𝜀 3 + 𝑆 𝑇 ∆ 𝑇 3
𝑉 0 𝑉 𝑆 = 1 4 − 𝑆 𝜀 2 + 𝑆 𝑇 ∆ 𝑇 2 + 𝑆 𝜀 3 + 𝑆 𝑇 ∆ 𝑇 3
If Gage 3 is placed on a deformed specimen ( 𝜀 3 , ∆ 𝑇 3 ) and Gage 2 is placed on an identical but un-deformed specimen, then
∆ 𝑇 2 =∆ 𝑇 3 , 𝜀 2 =0
𝑉 0 𝑉 𝑆 = 1 4 𝑆 𝜀 3 (Automatic Temperature Compensation)

20. Beam in Bending: Half Bridgeε3
ε2 = -ε3
Place gage 2 on the side opposite of gage 3, so ε2 = -ε3
𝑉 0 𝑉 𝑆 = 1 4 𝑆 𝜀 3 − 𝜀 2 + 𝑆 𝑇 ∆ 𝑇 3 −∆ 𝑇 2 = 2 4 𝑆 𝜀 3
𝑉 0 𝑉 𝑆 = 1 2 𝑆 𝜀 3
Twice the output amplitude of a quarter bridge, and with temperature compensation
ε2 = -ε3

21. Full Bridge R3 + -
𝑉 0 𝑉 𝑆 = 𝑑 𝑅 1 𝑅 1 − 𝑑 𝑅 2 𝑅 𝑑 𝑅 3 𝑅 3 − 𝑑 𝑅 4 𝑅 4
All four legs are identical stain gages
𝑑 𝑉 0 𝑉 𝑠 = 1 4 𝑆 𝜀 1 − 𝜀 2 + 𝜀 3 − 𝜀 4 + 𝑆 𝑇 ∆ 𝑇 1 −∆ 𝑇 2 +∆ 𝑇 3 −∆ 𝑇 4
In bridge, opposite legs (1, 3) and (2, 4) reinforce
Adjacent legs (1, 2) and (3, 4) oppose

22. Beam in Bending: Full Bridge + - - +
𝑉 0 𝑉 𝑠 = 1 4 𝑆 𝜀 1 − 𝜀 2 + 𝜀 3 − 𝜀 4 + 𝑆 𝑇 ∆ 𝑇 1 −∆ 𝑇 2 +∆ 𝑇 3 −∆ 𝑇 4
𝑉 0 𝑉 𝑠 = 1 4 𝑆 4 𝜀 3 + 𝑆 𝑇 0 =𝑆 𝜀 3
V0 is 4 times larger than quarter bridge
And has temperature compensation.
= e3
= -e3
= -e3
= DT3
= DT3
= DT3

23. Tension R3 2 3 4 1
ε4=-υ ε3 ε2=-υ ε3 ε1=ε3

25. General Guidelines for HWs
Use your Course ID numbers (which you can find in MyNevada), not your name or student ID number, when submitting your HWs
Units and significant digits always!
No hand-drawn plots! Starting from HW3, whenever you are asked to plot your data, plot them using computer software, i.e. Excel, Matlab, Mathcad etc.
Include labels for both axes with the units. If necessary, include legends too. Make it look good!
Show you work! Do not skip the steps and just write your final answer. Whenever applicable, list your assumptions, write out your formulas and work through to your final answer. If you use a Table (or graph) on your solutions, give reference to that table in your book, i.e from Table 6.3, z=-1.28.
Be clear with your solutions and work neatly! If the grader needs to spend more than 3 minutes to figure out what you write, you may not get even partial credit.