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Cork Institute of Technology
Higher Certificate in Engineering in Mechanical Engineering- Stage 1
(NFQ – Level 6)
Summer 2006
Electrotechnology & Instrumentation
(Time: 3 Hours)
Answer FIVE questions, at least TWO from each Section.
All formulae and calculations must be shown.
Examiners: Mr. R. Simpson Mr. J. Connolly Mr. S. Cotter
Section A ( Electrotechnology )
- (a) Define or explain the terms: resistivity and temperature coefficient of resistance. ( 8 marks )
(b) A platinum resistance thermometer indicates 105.00 Ω at a room temperature of 18 o^ C. Calculate its resistance at (i) 0 o^ C, and at (ii) 50 o^ C. Temperature coefficient of resistance for platinum = 3.93 x 10-3^ K -1^. (12 Marks)
- (a) State Kirchoff’s Circuit Laws ( 6 marks )
(b) Three resistors of 2 Ω, 5 Ω and 4 Ω are connected in parallel and they in turn are connected in series with a resistor of 3 Ω and powered by a 24 V d.c. power supply. Draw the circuit diagram. What is the equivalent resistance of the parallel branch? What is the external resistance of the circuit? What is the circuit current and the current in the 4 Ω resistor? ( Marks)
- (a) Describe how a galvanometer may be converted into an ammeter, a voltmeter and an ohmmeter. (10 marks)
(b) A galvanometer with full-scale deflection (f.s.d.) of 2.00 mA and coil resistance 70 Ω is to be used as a ammeter to read currents up to 5 A. What maximum value of shunt resistance is required? If the range of the ammeter is to be increased to 10 A, what maximum value of shunt resistance is required? (10 marks)
- (a) Describe the construction and operation of an industrial version of the Wheatstone bridge circuit as an accurate method for the measurement of electrical resistance. (7 marks)
(b) How would you use the Wheatstone Bridge as a signal converter in measurements (give an example with diagrams)? ( 7 marks )
(c) A Wheatstone bridge is used to measure the value of an unknown resistance. The value of the standard resistor is 10.000 ohms and the bridge was balanced when the uniform resistance wire of R 3 was 60.0 cm and R 4 was 40.0 cm. Calculate the value of the unknown resistance. ( 6 marks )
Section B ( Measurement & Instrumentation )
- (a) Define or explain, with the aid of diagrams and examples, the meaning of the following measurement terms: Accuracy, Precision, Range, Span, and Resolution. ( 8 marks )
(b) Define or explain, with the aid of diagrams and examples, the meaning of the following calibration-type errors: Zero error, Proportionality error, maximum Non-linearity error, and Hysteresis error. ( 12 marks )
- (a) Draw a block diagram of the elements in a general measurement system. Explain the function of each element and give one example of each. ( 4 marks )
(b) Describe the construction and operation of a capacitive type sensor element for the measurement of pressure. ( 6 marks )
(c) A metal wire strain gauge has a nominal resistance of 120.00 Ω and a gauge factor of 2.1. What will be the change in resistance produced if the gauge is subjected to a strain of 0.20 %? ( 10 marks )
- Describe fully, with the aid of diagrams, a laboratory experiment that you have performed on the calibration of either a Variable-Area flowmeter or a load cell. ( 20 marks )
8. (a) Most errors associated with measuring instruments can be grouped under the
following headings: Construction, Operating, Environmental, Ageing, and Insertion errors. Explain each type and give one example of each. ( 10 marks )
(b) State the main advantages and disadvantages of the following signal transmitters: mechanical, electrical, hydraulic, pneumatic, electromagnetic, and fibre optic. ( 10 marks )
