revision and solved questions

REVISION AND SOLVED QUESTIONS (PART 1)

ByMr. Akpan Patrick U.

Department of Mechanical Engineering,UNN.

Today’s Class Objectives• Review what has been done so far?– Definition of Measurement and Measurement

systems– Standards of Measurement– Instruments types– Static characteristics of Instruments– Sensor Technologies– Temperature measurement

• Solved problems• Complete the lecture on Temperature

measurement.

Sensitivity of Measurement

Solution to Question 1

Zero Drift and Sensitivity Drift• Example 2• A spring balance is calibrated in an environment at a temperature

of 20°C and has the following deflection/load characteristic (See Table 2.2(a))

• It is then used in an environment at a temperature of 30°C and the following deflection/ load characteristic is measured (see Table 2.2 (b)).

• Determine the zero drift and sensitivity drift per °C change in ambient temperature?

Table 2.2(a))

Table 2.2(b))

Solution to Question 2• At 20°C, deflection/load characteristic is a straight line. – Sensitivity = 20 mm/kg.– Zero drift = 0 mm ( the no-load deflection)

• At 30°C, deflection/load characteristic is still a straight line. Table 2(c) below shows the actual deflections without zero drifts.

– Sensitivity = 22 mm/kg.– Bias (zero drift) = 5mm (the no-load deflection)

• Sensitivity drift = Sensitivity @ 30°C - Sensitivity @ 20°C =2 mm/kg• Sensitivity drift/°C = 2/10 = 0.2 (mm per kg)/°C• Zero drift/°C = 5/10 = 0.5 mm/°C

Load (kg) 0 1 2 3

Actual deflection without zero drift

0 22 44 66

Thermocouple Tables• Example 3 • If the e.m.f. output measured from a chromel–

constantan thermocouple is 13.419mV with the reference junction at 0oC, the appropriate column in the tables Find the temperature that corresponds to the hot junction.

• Example 4• If the measured output e.m.f. for a chromel–constantan

thermocouple (reference junction at 0oC) was 10.65 mV, tables Find the temperature that corresponds to the hot junction .

• This interpolation procedure gives an indicated hot junction temperature of 162oC.

– If the reference junction is immersed in an ice bath to maintain it at a temperature of 0oC so that thermocouple tables can be applied directly, the ice in the bath must be in a state of just melting.

– This is the only state in which ice is exactly at 0oC, and otherwise it will be either colder or hotter than this temperature. Thus, maintaining the reference junction at 0oC is not a straightforward matter, particularly if the environmental temperature around the measurement system is relatively hot.

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Non-zero reference junction temperature

Non-zero reference junction temperature

– In consequence, it is common practice in many practical applications of thermocouples to maintain the reference junction at a non-zero temperature by putting it into a controlled environment maintained by an electrical heating element.

– In order to still be able to apply thermocouple tables, correction then has to be made for this non-zero reference junction temperature using a second thermoelectric law known as the law of intermediate temperatures.

Law of Intermediate Temperatures

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Non-Zero Reference JunctionExample 5Suppose that the reference junction of a chromel–constantan thermocouple is maintained at a temperature of 80oC and the output e.m.f. measured is 40.102mV when the hot junction is immersed in a fluid. Determine the Temperature of the fluid.

Solution to Question 5

Practice Questions(1). Explain what is meant by:– (a) active instruments– (b) passive instruments.

Give examples of each and discuss the relative merits of these two classes of instruments.(2). Discuss the advantages and disadvantages of null and deflection types of measuring instrument. What are null types of instrument mainly used for and why?(3). Briefly define and explain all the static characteristics of measuring instruments.(4). Explain the difference between accuracy and precision in an instrument.

Practice Questions(5). A tungsten/5% rhenium–tungsten/26% rhenium thermocouple has an output e.m.f. as shown in the following table when its hot (measuring) junction is at the temperatures shown. Determine the sensitivity of measurement for the thermocouple in mV/°C.

(6). Define sensitivity drift and zero drift. What factors can cause sensitivity drift and zero drift in instrument characteristics?

Practice Questions

Practice Questions

Practice Questions

(9). The output e.m.f. from a chromel–alumel thermocouple (type K), with its reference junction maintained at 0°C, is 12.207 mV. What is the measured temperature?

(10). The output e.m.f. from a nicrosil–nisil thermocouple (type N), with its reference junction maintained at 0°C, is 4.21 mV. What is the measured temperature?

(11). A platinum/10% rhodium–platinum (type S) thermocouple is used to measure the temperature of a furnace. The output e.m.f., with the reference junction maintained at 50°C, is 5.975 mV. What is the temperature of the furnace?

Practice Questions• List and Explain briefly any seven (7) sensor

technologies that you know• List and Explain briefly any seven (7) type of

Temperature measurement systems.

Answers to Numerical Problems

ANY QUESTIONS?

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NEXT CLASS

• PRESSURE MEASUREMENT• Definition of types of pressure• Types of pressure measurement systems• Methods of Measuring pressure

• DYNAMIC CHARACTERISTICS OF UNSTRUMENTS• Zero order instruments• First order instruments• Second order instruments

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THANKS FOR YOUR PATIENCE

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NETWORKING TIME

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