Semester 2

Question 1

What is the relationship between precision and significant figures?

Answer 1

The higher the significant figures, the higher the precision.

Question 2

What does precision refer to?

Answer 2

The limit of error in a measurement or variable.

Question 3

What does accuracy refer to?

Answer 3

Closeness to the true value of a variable.

Question 4

How is precision/uncertainty preserved in a calculation?

Answer 4

When multiplying, dividing or using other functions, the answer should have the same number of significant figures as the smallest in the calculation.

Question 5

How is precision/uncertainty preserved when adding or subtracting?

Answer 5

The answer should have the same number of decimal places as the smallest used in the calculation.

Question 6

Ideally, what is the relationship between the input and output of and an instrumentation system?

Answer 6

Linear and is defined by the sensitivities.

Question 7

How do you find sensitivity, k, from a output range/input range graph?

Answer 7

It is the slope of the graph:

k = slope = Δo/ΔI

Question 8

What is sensitivity in terms of the output?

Answer 8

k(I - Imin) + Omin = O

O = output

Question 9

What “other inputs” can affect a system?

Answer 9

Modifying input and interfering input

Question 10

What does modifying input (Im) do?

Answer 10

Modified the linear sensitivity I.e temperature in strain gauges.

Question 11

What does interfering input (Ii) do?

Answer 11

Interferes with the zero condition, therefore changing the static reference.

Question 12

What does an error band do?

Answer 12

Defines for a particular input, the output will be within +- ΔO

Question 13

What combines to give an overall statistical behaviour?

Answer 13

Random effects in I, Im and Ii.

Question 14

The random fluctuations in Im, Ii and I (overall statistical behaviour) can be assumed to have what kind of function?

Answer 14

Gaussian probability density function around the mean value

Question 15

How is accuracy of a measurement system quantified?

Answer 15

Using measurement error E

E = measured value - true value

Question 16

What does the error band centred around the ideal output represent?

Answer 16

The accuracy of the system

Question 17

When does a bridge become more linear?

Answer 17

When r&raquo_space; 1

Question 18

What Is the bridge ratio when maximum sensitivity occurs?

Answer 18

r = 1

Question 19

How can bridges be linearised?

Answer 19

A combination of sensors (with equal characteristics) and negative feedback.

Question 20

What is the force given by Newtons 2nd law, Hooke’s law and resistive damping?

Answer 20

F = ma +rv + ky

Question 21

How can non-linearity be removed in a bridge?

Answer 21

• 2 strain gauges to be used in an adjacent arm (R1 and R2)

- R1 in tensile (+) and R2 in compressive (-) strains.

Question 22

What is stress?

Answer 22

σ = force/area

Question 23

What is the effect of stress called?

Answer 23

Strain, which is defined as the fractional change in length.

ε = Δl/l

Question 24

What is Youngs modulus (E)?

Answer 24

E = σ/ε

Question 25

When a material extends in length due to strain, there is a decrease in cross sectional area, what does this result in?

Answer 25

Two opposing strains are produced; longitudinal and transversal, which are related by Poissons ratio.

Question 26

What damping ratio gives optimum response and settling times?

Answer 26

0.7

Question 27

What is a major limitation of a hydrophone?

Answer 27

It has a very high output impedance, effectively making it a current source.5

Question 28

How should the strain gauges in a bridge be arranged to eliminate interfering inputs?

Answer 28

The strain gauges under longitudinal strain in opposite arms and similarly for compressive strain.

Question 29

How do you determine the static error in a measurement system?

Answer 29

Assume all of the statistical variations may be combined as a linear sum of the component variances. In addition the variances must be scaled using the partial derivatives of the model equations.

Question 30

How is an error band use to analyse a system?

Answer 30

The statistical variation takes into account any modifying, interfering, offset, non-linearities, hysteresis, parameter variations and defines the behaviour via a derivation from the linear relationship.

Question 31

How do you find the conditions on k so that the closed loop system is stable?

Answer 31

Using the Routh table.