Part 1

Question 1

How do you calculate measurement error?

Answer 1

E = Measured Value - True Value

Question 2

Define accuracy

Answer 2

How close the measured value is to the true value

Question 3

Define resolution

Answer 3

Smallest change in the input that produces an observable change in the output

Question 4

What is the gauge factor equation linking resistance and length?

Answer 4

ΔR/R = Gf ΔL/L = Gf ε

Question 5

What is a typical value of Gauge Factor?

Answer 5

Chromium-nickle alloys have a Gf around 2, platinum around 5 and then a semiconductor can be up to 70-135

Question 6

How do you calculate the gauge factor itself?

Answer 6

Gf = P + 1 = 2v
P = piezoresistive coefficient
v = poissons ratio

Question 7

What are the two possible causes of a strain gauge resistance changing?

Answer 7

• Ann induced mechanical strain ε which is the true variable to be measured
• A change in temperature -> this would be measurement error

Question 8

How are two ways in which strain gauge circuits are adapted to minimise the error?

Answer 8

• They are made of special alloys with a very low temp coefficient
• Since the circuit could also heat up (P=I²R), I should be minimised

Question 9

What does a strain gauge actually do in a circuit?

Answer 9

It converts changes in resistance to voltage that can be easily measured