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Leading tab to module number:) XE103
SCHOOL OF ENGINEERING
Honours Degree Modular Course
LEVEL 1
SEMESTER ONE
2000 - 2001
Measurements
Examiners: Dr. G.J. Awcock & Mr. R. Fletcher
Answer BOTH questions fromSECTION A.Time allowed TWO hours
AND one further question as follows:
Electrical Engineering students answer question 3 from SECTION E
Mechanical Engineering students answer question 4 fromSECTION M
Special requirements:None
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Leading tab to module number:) XE103
SECTION A; - ALL Students
1(a)Figure Q1.1 shows the scale of an analogue voltmeter instrument. It is also known that this instrument is of accuracy class 0.5.
(i)Define the term ‘effective range’. Clearly identify the methods that are commonly used by instrument manufacturers to indicate the extent of the effective range. Hence, or otherwise, identify the effective range of the particular instrument illustrated in Figure Q1.1. (4)
(ii)Define the term ‘fiducial value’. State the rules that are commonly applied to determining the fiducial value, depending upon the position of the zero point with respect to the scale of the instrument. Hence, or otherwise, identify the fiducial value of the particular instrument illustrated in Figure Q1.1. (4)
(iii)Define the term ‘accuracy class’.(1)
(iv)Determine the ‘limit of error’ in ANY reading, and also determine the ‘percentage limit of error’ in a reading of 92.6 volts, with reference to the particular instrument illustrated in Figure Q1.1. (4)
Figure Q1.1: A Voltmeter Scale
Question 1 is continued over the page…
Question 1 continued…
1(b)The ‘decade inductance box’ in Figure Q1.2 is made up of the following components, which each have an associated component tolerance:
10 Inductors each of 0.1 10%
10 Inductors each of 10 mH 5%
10 Inductors each of 1 mH 2%
Given this information, determine both in millihenrys and as a percentage, the limit of uncertainty in a setting of 0.785 henrys. (12)
Figure Q1.2: A Three Decade Inductance Box Set to 0.785 henrys
2(a)A moving coil meter movement is supplied with a full-scale sensitivity of 50 A and an internal resistance of 1000 . A special instrumentation task requires that this one movement can be applied to the measurement of voltages up to a maximum of 500 V DC, and, under separate circumstances, currents up to a maximum of 100 mA.
(i)Calculate the value of the scaling component(s) that will be required to convert the meter movement for the voltmeter function. Draw a diagram to show clearly how the scaling component(s) should be connected. (5)
(ii)Calculate the value of the scaling component(s) that will be required to convert the meter movement for the ammeter function. Draw a diagram to show clearly how the scaling component(s) should be connected. (5)
(iii)In the case of Question 2(a) part (i), above, what is the sensitivity of the voltmeter that you have created, expressed in terms of ‘Ohms per Volt’? (1)
(iv)If the voltmeter circuit using this moving coil meter was suitably adapted to allow it to measure alternating voltages, what precautions would the user have to take in order to ensure that readings obtained were accurate. (2)
(b)Conductors carrying signals are vulnerable to interference from external sources. Intereference may be ‘coupled’ to the signal conductors by ‘electromagnetic coupling’, or ‘electrostatic coupling’ effects.
Clearly explain the ‘electromagnetic coupling’ and ‘electrostatic coupling’ phenomena in the context of unwanted pick-up of noise by signal conductors. In each case identify common methods that are used to minimise the effect of these pheneomena. Take care to highlight the strengths and weaknesses of the interference rejection methods that you describe. (8)
Extra marks will be awarded for describing an interference minimisation strategy that involves changing the way that the signal is transmitted in order to ensure that it can be received without superimposed interference. (4)
SECTION E; - Students of Electrical Engineering, and Related Pathways, ONLY
3This question is concerned with the operation of the Cathode Ray Oscilloscope (CRO, or oscilloscope)
(a)Draw the block diagram of a simple Cathode Ray Oscilloscope(2)
(b)Explain the operation of the ‘Timebase’ of the CRO. Take care to explain the main features of this section of the instrument. (5)
Explain the meaning of ‘free-running’ in the context of timebase operation. (2)
How can a free-running timebase be ‘locked’ to a signal under investigation? Use diagrams to illustrate your answer thoroughly. What are the advantages of doing this? (9)
(c)Briefly explain the difference between ‘Alternate’ and ‘Chop’ modes of operation of a ‘dual-trace’ oscilloscope. What are the advantages and disadvantages of each mode? (7)
SECTION M; - Students of Mechanical Engineering, and Related Pathways, ONLY
4One method of measuring a fluid flow velocity is the use of a Pitot Tube.
(a)Explain the principle employed with reference to Bernoulli’s principle.(8)
(b)Some Pitot Tubes have some small holes around the stem of the tube leading to a different pressure tapping (Figure Q4.1) Explain their function? (9)
(c)Using the equation Pt = P0 + u02/2
Where:Pt is pressure at the pitot tube head
P0 is the system pressure
is the fluid density
u0 is the fluid velocity
find the value of Pt when u0=10ms-1: P0=0.05Mpa: =103 kgm-3(8)
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