solved students evaluation in measurement and instrumentations
- 1. 1. a) Describe the functional elements of an instrument with its block diagram (5marks) b) What are the types errors in the Instrument (3marks) c) Explain the different types of measurement standards (3marks) d) List only four where the measurement is needed (4marks) e) Differentiante the direct and indirect measurements and give an example for each. (4marks) f) Differentiate the Static and Dynamic characteristic of an instrument and list only four characteristics for each (6marks) 2. a) The expected value of the voltage to measured is 150V. The measurement gives a value of 149V. Calculate: i) absolute error, ii) percentage error, iii) relative accuracy iv) percentage accuracy and v) error expressed as percentage of full scale reading, if the scale range is 0-200V. (5marks) b) The two wattmeter method of measuring total power is used to measure the power used by an unbalanced three phase load. If W1 reads 4.5 kW after the reversing switch is closed and W2 reads 13 kW, determine the total power in the load. (2marks) 3. In the Maxwell bridge shown in Figure below, let the fixed value bridge components have the following values: R3=5Ω ; C=1 mF. i) Calculate the value of the unknown impedance (Lu, Ru) if R1=159 Ω and R2=10 Ω at balance. (5marks) ii) Calculate the Q factor for the unknown impedance at a supply frequency of 50 Hz. (3marks)
- 2. 1.a. Describe the functional elements of an instrument with its block diagram (5marks) The various elements can be grouped as: • Primary sensing element: Any element of an instrument which makes first, the contact with the quantity to be measured is called primary sensing element • Data conditioning element divided into two parts: Variable conversion element: The output of primary sensing element is in electrical form such as voltage, frequency or any other electrical parameter. Such an output may not be suitable for the actual measurement system. Variable manipulation element: manipulates the signal preserving the original nature of the signal. • Data Transmission Element: When the elements of the system are physically separated, it is necessary to transmit the data from one stage to other • Data presentation element: used for monitoring, controlling or analyzing purposes. Thus the person handling the instrument must get the information in proper form, according to the purpose for which is intended. The block diagram showing the functional elements of an instrument
- 3. 1.b.What are the types errors in the Instrument (3marks) Gross error: largely human errors, among them misreading of instruments, incorrect adjustment and improper application of instruments, and computational mistakes. Systematic Errors: These are the errors that remain constant or change according to a definite law on repeated measurement of the given quantity. Random error: These errors are of variable magnitude and sign. The presence of random errors becomes evident when different results are obtained on repeated measurements of one and the same quantity. 1.c. Explain the different types of measurement standards (3marks) • International standards: are devices designed and constructed to the specifications of an international forum. They represent the units of measurements of various physical quantities to the highest accuracy that is attainable by the use of advanced techniques of production and measurement technology. • Primary standards: are devices maintained by standards organizations/national laboratories in different part of the world. These are fundamental and derived quantities and are calibred by absolute quantities. • Secondary standards: are basic reference standards employed by industrial measurement laboratories. The important functions of an industrial laboratory is the maintenance and periodic calibration of secondary standards against primary standard of the national standard laboratory/organization. • Working standards: These are high accuracy devices that are commercially available and are duly checked and certified against either the primary or secondary standards.
- 4. 1.d. List only four where the measurement is needed (4marks) Need of Measurement Establish standard Interchangeability Customer Satisfaction Validate the design Physical parameter into meaningful number True dimension Evaluate the Performance 1.e. Differentiante the direct and indirect measurements and give an example for each. (4marks) Direct measurement – measured quantity is registered directly from the instruments display. Measuring voltage with voltmeter Measuring length with ruler Indirect measurement – result is calculated (using formula) from the values obtained from direct measurements Example: Finding work done by current: U – voltmeter I – ammeter t – clock A=U*I*t
- 5. 1.f. Differentiate the Static and Dynamic characteristic of an instrument and list only four characteristics for each (6marks) Static Characteristics: the measurement of quantities that are either constant or vary very slowly with time i.e do not vary with time. • Accuracy • Precision • Sensitivity • Tolerance • Range or span • Linearity • Threshold • Resolution • Sensitivity to disturbance Dynamic characteristic of an instrument: The relationship between the system input and output when the measured quantity (measurand) is varying rapidly with time. (i) Speed of response (ii) (ii) Lag iii) Fidelity (iii) (iv) Dynamic error (iv) (v) Steady state periodic (v) (vi) Transient • Dead space • Hysteresis effect • True value • Drift or Zero drift • Repeatability • Reproducibility • Zero stability: • Backlash • Bias
- 6. 2. a) The expected value of the voltage to measured is 150V. The measurement gives a value of 149V. Calculate: i) absolute error, ii) percentage error, iii) relative accuracy iv) percentage accuracy and v) error expressed as percentage of full scale reading, if the scale range is 0-200V. (5marks)
- 7. 2. b) The two wattmeter method of measuring total power is used to measure the power used by an unbalanced three phase load. If W1 reads 4.5 kW after the reversing switch is closed and W2 reads 13 kW, determine the total power in the load. (2marks)
- 8. 3. In the Maxwell bridge shown in Figure below, let the fixed value bridge components have the following values: R3=5Ω ; C=1 mF. i) Calculate the value of the unknown impedance (Lu, Ru) if R1=159 Ω and R2=10 Ω at balance. (5marks) ii) Calculate the Q factor for the unknown impedance at a supply frequency of 50 Hz. (3marks) (3marks) (5marks)