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T780(E)(A)T NOVEMBER EXAMINATION

NATIONAL CERTIFICATE

INSTRUMENT TRADE THEORY N3

(11040463)

23 November 2016 (X-Paper) 9:00–12:00

This question paper consists of 6 pages and 1 formula sheet.

(11040463) -2- T780(E)(N23)T


DEPARTMENT OF HIGHER EDUCATION AND TRAINING REPUBLIC OF SOUTH AFRICA

NATIONAL CERTIFICATE INSTRUMENT TRADE THEORY N3

TIME: 3 HOURS MARKS: 100

INSTRUCTIONS AND INFORMATION 1. 2. 3. 4.

Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Write neatly and legibly.


QUESTION 1 1.1

The picture above shows a truck on a weighbridge. 1.1.1 What is a weighbridge? (2) 1.1.2 Make a neat sketch to show how strain gauges are connected to

form a weighbridge.

(4) 1.1.3 Explain how the weight of the contents of the truck is going to be

determined.

(2) 1.1.4 Most weighbridges have load cells installed in them.

Explain how load cells in weighbridges work.

(2)

1.2 The piezo-electric effect was first noticed in 1880 by the brothers Pierre and

Jacques Curie. By combining their knowledge of pyro-electricity with their understanding of crystal structures and behaviour, the Curie brothers demonstrated the first piezo-electric effect by using crystals of tourmaline, quartz, cane sugar and Rochelle salt.

1.2.1 What is the piezo-electric effect? (2) 1.2.2 Give TWO applications in which piezo-electric crystals are used. (2) 1.2.3 Draw a neat, labelled sketch of a piezoelectric transducer. (4) 1.2.4 Give TWO disadvantages of the transducer in QUESTION 1.2.3. (2)


1.3 Study the diagram below and answer the questions.

1.3.1 What type of strain gauge is represented by the diagram? (1) 1.3.2 Use the diagram of the strain gauge above to explain how strain is

measured.

(2) 1.3.3 Name TWO primary considerations when selecting a strain gauge. (2) [25] QUESTION 2 2.1 The diagram shows an unbalanced wheatstone bridge.

Calculate the output voltage across points C and D and the value of R4

required to balance the bridge circuit.

(10)

2.2 Temperature can be measured by various different thermometers. 2.2.1 Name THREE different types of thermometers. (3) 2.2.2 Which unit is used to measure temperature? (1) 2.2.3 Define temperature. (1)

http://www.google.co.za/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http://pubs.usgs.gov/twri/twri8a3/&ei=kF6SVYrXKOnW7AbNnbzYAQ&bvm=bv.96783405,d.ZGU&psig=AFQjCNFrj9CgQlKTn7xr4fC-o2y1iVzL9Q&ust=1435742182323155


2.3 Below is an example of thermistors. 2.3

2.3.1 Give a definition of a thermistor. (1) 2.3.2 Name TWO applications in which a thermistor will be used. (2) 2.3.3 Explain the working principle of a thermistor. (2) 2.4 Define the following (5) Provost's theory of exchange (2) Stefan Boltzmann's law (3)

[25] QUESTION 3 3.1 Pneumatic telemetering is used when an electrical supply is not present and

where the use of electrical power is extremely dangerous.

3.1.1 Make a fully labelled drawing of a pneumatic transmitter. (4) 3.1.2 Explain how the system in QUESTION 3.1.1 works. (2) 3.1.3 The system in QUESTION 3.1.1 is not a linear system, therefore a

feedback bellows is added to assist in flapper positioning to obtain improved accuracy. Illustrate this by means of a neat sketch indicating the position of the bellows.

(4)

3.1.4 Give THREE functions of the feedback element. (3) 3.2 3.2.1 Sketch an impulse-type telemetering system that uses a bourdon

tube, a constant speed motor, a cam in the transmitter and an electromagnet and a battery in the receiver.

(6)

(11040463) -6- T780(E)(A)T


3.2.2 Explain how the instrument in QUESTION 3.2.1 works. (4)

Copyright reserved

3.3 Give ONE factor that affects the operation of electrical telemetering systems

and explain how to compensate for this effect. (2 × 1)

(2) [25] QUESTION 4 4.1 Valve maintenance is the best way of managing the efficiency of control

valves, in order to control fluid flow. Explain how you will repair a leaking valve.

(4)

4.2 The opening or closing of control valves is usually done automatically by

electrical, hydraulic or pneumatic actuators.

4.2.1 What is the use of a valve positioner? (2) 4.2.2 List THREE uses of control valves. (3) 4.2.3 Give FIVE advantages of valve positioners. (5) 4.2.4 Sketch a force-balanced-type valve positioner that incorporates the

following:

Bellows

Feedback spring

Relay

Flapper and nozzle

Valve motor

(5)

4.3 Define the following control actions and state their functions. 4.2.1 Integral Action (3) 4.2.2 Derivative Action. (3) [25] TOTAL: 100

(11040463) T780(E)(N23)T

Copyright reserved

INSTRUMENT TRADE THEORY N3 FORMULA SHEET PG = Pabs – Pa

P1 – P2 = h g

P1 – P2 = d g ( or P1 – P2 = d g (

P =

Dry calibration =

Wet calibration =

W.C. = ( ) X D.C

D.C. = ( ) X W.C.

H =

H = h

to = 9/5 t + 32o F to C = ( t + 273,15) kelvin to F = 5/9 ( t – 32)o C



NOVEMBER EXAMINATION

INSTRUMENT TRADE THEORY

23 NOVEMBER 2016

This marking guideline consists of 7 pages.

MARKING GUIDELINE

MARKING GUIDELINE -2- T780(E)(N23)T INSTRUMENT TRADE THEORY N3


QUESTION 1 1.1 1.1.1 It is an instrument used to weigh the entire rail or road vehicle

including its contents.

A scaling device mostly used to determine the weight transported by heavy duty vehicle. (Any 1 × 2)

(2)

1.1.2

Fig 1

(4)

1.1.3 By weighing the vehicle both empty and when loaded, the load

carried by the vehicle can be calculated. A simple calculation can be done to calculate the difference in weight of the truck loaded and offloaded.

(2)

1.1.4 The weight of the truck exerting pressure on the strain gauge

stretches or compresses it. Simple calculations can be done to calculate the effect of the applied stress on the resistance change of a strain gauge. The change in resistance is proportional to the change in weight or pressure.

(2)

1.2 1.2.1 It is the ability of certain materials to generate an electric charge

in response to applied mechanical stress.

If forces or pressure is applied to certain crystals such as quartz, Rochelle salt and barium titanate, the crystal generates an EMF proportional to the pressure or force applied to it. (Any 1 × 2)

(2)

1.2.2 Any two

- Record players√ - Ceramic microphones√ - Converting light, sound, or mechanical pressure into electrical signals√ - Quartz clock or watches√

(2)

http://www.explainthatstuff.com/light.html

http://www.explainthatstuff.com/sound.html

http://www.explainthatstuff.com/quartzclockwatch.html



1.2.3

Fig. 2

(4)

1.2.4 Affected by temperature variations

Error signals generated when long leads are used

High-impedance output signal-conditioning systems (Any 2 × 1)

(2)

1.3 1.3.1 Unbonded or nonbonded (1) 1.3.2 When pressure is applied to the diaphragm, the armature moves

and two wires stretch while the other two contract. The resulting change in electrical resistance reflects the pressure applied.

(2)

1.3.3 Operating temperature

Stability requirements

Nature of the strain to be detected (Any 2 × 1)

(2)

[25] QUESTION 2 2.1 Vc = R2/ (R1 + R2) x Vs

Vc = 120Ω/(80Ω + 120Ω) × 100 Vc = 60 V

(3)

VD = R4/ (R3 + R4) × Vs

VD = 160Ω/(480Ω + 160Ω) × 100 VD = 25 V

(3)

Vout = VC – VD

Vout = 60 – 25 Vout = 35 V

(2)

R4 = R2 R3/R1

R4 = 120Ω × 480Ω/80Ω R4 = 720Ω

(2)

http://www.google.co.za/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http://www.instrumentationtoday.com/piezoelectric-transducer/2011/07/&ei=eJuKVczbGvCd7gaO_6D4DA&psig=AFQjCNF3YFDB7pvvozTcIXO8TWF1bhzZlA&ust=1435233326264327



2.2 2.2.1 Liquid-in-glass thermometer

Digital thermometer

Bimetallic-strip thermometer

Pyrometer

Thermocouple

Thermistor

Resistance thermometer (Any three)

(3)

2.2.2 Kelvin

Celsius

Fahrenheit (Any 1 × 1)

(1)

2.2.3 The degree of hotness or coldness (1) 2.3 2.3.1 It is a temperature sensor.

It is a temperature-sensitive resistor. (Any 1 × 1)

(1) 2.3.2 - Home appliances√ e.g. ovens; refrigerator

- Digital thermometers - Temperature measurements - Microwave ovens - Circuit protectors - Recharging of batteries - Automotive industry (Any two)

(2)

2.3.3 Its resistance depends upon temperature. When the temperature

changes, the resistance of a thermistor changes.

(2) 2.4 2.4.1 Provost stated that if there are two bodies e.g. Body A and body B

and body A is warmer than body B then body A will give off energy to body B.√ This will continue until body A and body B are at the same temperature.√

(2)

2.4.2 Stefan Boltzmann stated that if two bodies of different

temperatures are placed opposite each other, there would be a transfer of heat energy from the hot body to the colder body and the amount of radiated energy per time unit is roughly proportional to the fourth power√ of the absolute temperature difference between the two bodies. √ This fourth power ratio is known as Stefan Boltzmann’s Law. √

(3) [25]



QUESTION 3 3.1 3.1.1

(4)

Fig. 3 3.1.2 If the flapper moves away from the nozzle then the transmitted or

output pressure will fall to a low value. If the flapper moves towards the nozzle then the transmitted pressure will rise to almost the supply pressure.

(2)

3.1.3

Fig 4.

(4)

3.1.4 Accurate calibration

Reduces vibration

Linear relationship between measured variable and output

(3)



3.2 3.2.1

Fig. 5

(6)

3.2.2 The transmitter is constructed from a cam which is rotated by a constant speed motor, and the cam line follower is controlled by the pressure elements. The cam followers slide over the surface of the cam and not next to the sides as usual. When the cam follower is not touching the cam, the switch is closed and the transmitting circuit operates. The time duration of the cam follower not touching the cam, keeping the switch closed, determines the time duration of the transmitter's electrical impulses.

(4)

3.3 Lead Wires To compensate for this the resistance value of the receiver is made very high.

Load resistor To compensate for this the load resistance is made much larger than the potentiometer resistance.

Supply voltage variations To compensate a regulator called a barretter is used or two voltage and two current coils are used (Any 2 × 1)

(2)

[25] QUESTION 4 4.1 Use the manufacturer's technical manual.

As soon as the leak is detected determine the cause and then apply the corrective maintenance.

Maintenance may be as simple as tightening a packing nut or gland.

A leaking flange joint may need only to have the bolts tightened or to have a new gasket inserted.

(4)


Copyright reserved

4.2 4.2.1 It is used to control the opening or closing of the actuator based on an electric or a pneumatic signal.

(2)

4.2.2 Controlling of water (irrigation)

Controlling processes (industrial use)

Residential uses (on/off and pressure control to dish washers, washing machines and taps in the home)

(3)

4.2.3 The high gain helps to overcome stem friction.

The valve stroke is closely matched to the input signal.

The input signal is dead-ended in a small chamber thereby reducing the time lag.

Split-ranging is easily obtained due to the amplification.

The positioner output is independent of the input signal permitting greater thrust.

(5)

4.2.4

Fig. 6

(5)

4.3 4.3.1 Definition: The action of a controller where the output signal is

changed at a rate(speed) depending on the magnitude(size) of the measured deviation.

Function: It is used to eliminate offset which is caused from proportional action.

(2) (1)

4.3.2 Definition: It is the action of a controller where the magnitude (size)

of the output signal is dependent on the rate (speed) at which the measured deviation changes.

Function: It is used to speed up the recovery time.

(2) (1) [25]

TOTAL: 100

Copyright reserved

T850(E)(A5)T

APRIL EXAMINATION



(11040463)

5 April 2016 (X-Paper) 09:00–12:00

Calculators may be used


(11040463) -2- T850(E)(A5)T







(11040463) -3- T850(E)(A5)T


SECTION A QUESTION 1 Various options are given as possible answers to the following questions. Choose the answer and write only the letter (A–D) next to the question number (1.1–1.5) in the ANSWER BOOK.

1.1 Butterfly valves are generally favoured because … A

B C D

they are low in cost and light in weight. they have very good shut-off capabilities. they have reduced bore and full bore. the flow is controlled by raising or lowering the valving element.

1.2 A wire strain gauge is made by which component? A

B C D

Inductor Capacitor Resistor Diode

1.3 An instrumentation system consists mainly of three sections which are used

together for measuring and controlling. Which ONE below is not part of the sections?

A

B C D

Input Controlling Process Output

1.4 Which ONE of the following is not a disadvantage of a pneumatic system? A

B C D

Large time lag Pressure drop Leakages None of the above-mentioned

1.5 Detecting element is … A

B C D

a primary device in direct contact with the process. an element that receives a signal from the detecting element. an element which provides a control signal for transmission to the correcting element. part of the correcting unit which adjusts the correcting element in response from the controller output signal.

(11040463) -4- T850(E)(A5)T


1.6 Pressure that is less than the atmospheric pressure A

B C D

Vacuum. Gauge pressure Absolute pressure None of the above.

1.7 Capacitive transducer cannot be used to measure: A

B C D

Light intensity Pressure Level All of the above

1.8 What is the degree of boiling water in Fahrenheit ?

A B C D

212 100 373 460

1.9 Which factor does not influence capacitance in capacitive transducers A

B C D

The area of the plates The distance between the plates Linear output The insulating material between the plates

1.10 Area of application for strain gauges are: A

B C D

Torque Force All of the above None of the above

(10 x 1)

[10]

(11040463) -5- T850(E)(A5)T


QUESTION 2 Choose an item from COLUMN B that matches a description in COLUMN A. Write only the letter (A–G) next to the question number (2.1–2.5) in the ANSWER BOOK.

COLUMN A COLUMN B

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10

A device with a moveable feature that opens and closes a passageway in order to allow, prevent or control the flow of fluids With this type the wire is not cemented but rather wound around the structure A device that measures a potential difference which is unknown by comparing it with a known potential difference Compensation for long-term drift The device embedded inside electric irons and street lights that triggers them to switch on and off A very interesting principle of the piezo-electric effect Instruments used to measure inductance Transducers that work on a physical condition causing the resistance of the device to change Device that is in direct contact with the process-thermocouple The pyrometer is so designed that the necessity of focusing the instrument with an eyepiece is eliminated

A

B

C

D

E

F

G

H I J

K

L

M

N

strain gauge precaution when using resistance thermometer precaution when using pyrometers potentiometer non-bonded strain gauges bonded strain gauges valves Detecting element LVDT Wedge type Resistance Crystal Rare Earth Mirror type

(10 x 1) [10] TOTAL SECTION A: 20

(11040463) -6- T850(E)(A5)T


SECTION B QUESTION 3 3.1

FIGURE 1 3.1.1 What is the function of the instrument represented by the picture

above?

(1) 3.1.2 The instrument above (FIGURE 1) has load cells inside.

Explain how a load cell works.

(3)

3.1.3 What is calibration? (3) 3.1.4 How would you calibrate the instrument above (FIGURE 1)? (2) 3.2 A load cell makes use of a copper-nickel alloy strain gauge which has a

nominal resistance of 120 Ω and a gauge factor of 2.The load cell is then subjected to a mass which results in a compression of stress of 27 MPa. Calculate the change in resistance of the strain gauge if the module of elasticity of the load cell beam (to which the strain gauge is attached) is 120 GPa

(8)

3.3 State THREE advantages of a bonded strain gauge. (3)

[20]

(11040463) -7- T850(E)(A5)T


QUESTION 4 4.1 Thermistors have a negative temperature coefficient; this means that with an

increase in temperature the thermistor's resistance decreases and vice versa.

4.1.1 Give TWO applications of thermistors (2) 4.1.2 Explain how thermistors are constructed. (5) 4.1.3 State THREE disadvantages of thermistors. (3) 4.2 Optical pyrometers are nothing more than a photometer using monochromatic

light which compares the intensity of radiation from a standard source with the intensity of radiation emitted by the body whose temperature is to be measured. Make a neat, labelled sketch illustrating a disappearing filament, optical pyrometer

(7)

4.3 State Stefan Boltzmann's law. (3)

[20] QUESTION 5 5.1 Pneumatic telemetering, also known as transmission telemetering uses a

flapper and nozzle to vary pressure and is used in the absence of electrical supply.

5.1.1 Draw a neat, labelled sketch of a flapper-and-nozzle system that is

used to vary the pressure of the output signal which is proportional to the measured variable.

5.1.2 Explain how the system in QUESTION 5.1.1 works.

(2 x 5)

(10) 5.2 Draw a non-bleed type relay valve and explain its working principle. (2 x 5) (10)

[20]

(11040463) -8- T850(E)(A5)T

Copyright reserved

QUESTION 6 6.1 A valve positioner is a device installed between the controller and the valve

actuator.

6.1.1 Draw a labelled sketch for a motion-balanced type valve positioner. (7) 6.1.2 Give FIVE factors to consider when choosing a valve. (5) 6.1.3 State THREE reasons for using valve positioners. (3) 6.2

FIGURE 2 6.2.1 Considering that there are THREE main control processes in use,

what does the system in the above sketch represent?

(2) 6.2.2 Give the definition for the system above. (3)

[20] TOTAL SECTION B:

GRAND TOTAL: 80

100

(11040463) -9- T850(E)(A5)T



FORMULA SHEET

PG = Pabs – Pa

P1 - P2 = hg

P1 – P2 =

1

2

1

A

Agd OR

1

1

221

A

AgdPP

A

Mgp

2

1

2

1

hk

hk

Q

Q

Dry calibration = g

PP

21

Wet calibration = g

PP

)( 12

21

W.C. =

12

2

D.C.

D.C. =

2

12

W.C.

g

PH

2

1

1

2

hH

11

2

2

1

1

2

2Hh

P

PH

hhH

FtCt 325

9

),tCt 15273( kelvin

CtFt )32(9

5

MARKING GUIDELINE -1- T...(E)(A…)T INSTRUMENT TRADE THEORY N3



APRIL EXAMINATION


5 APRIL 2016


= 1 Mark

=

Mark

MARKING GUIDELINE

MARKING GUIDELINE -2- T850(E)(A5)T INSTRUMENT TRADE THEORY N3


SECTION A QUESTION 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10

A C B D A A A A C C

(10 x 1)

[10] QUESTION 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10

G E D B A L I K H N

(10 x 1)

[10] TOTAL SECTION A: 20 SECTION B QUESTION 3 3.1 3.1.1 To measure weight/mass. (1) 3.1.2 Load cell is a special type of a spring that works on a principle of

deforming in proportion to the weight placed on the load-receiving end and the weight of the object placed would be given digitally on the screen.

(3)

3.1.3 Calibration is the set of operations that establishunder specified

conditions, the relationship between the values of quantities indicated by a measuring instrument and the corresponding values realised by reference standard.

(3)

3.1.4 The scale can be calibrated by weighing the known weight on a

scale and noting discrepancies (if any) on the display.

(2)



3.2

GIVEN R = 120 Ω K = 2 STRESS (σ) = 27 MPa ΔR =? E = 120 GPa

E =

ε =

ε =

ε =

(4)

K =

= K x ε

ΔR = K x ε x R

ΔR = 2 x x 120 ΔR = 0.054 Ω (4)

(8) 3.3 They have a slightly higher gauge factor.

They can easily be made waterproof.

They follow the contour of the surface onto which they are bonded more accurately.

Hysteresis and creep effect is smaller.

Special patterns are easily produced.

Special forms of the element can be easily manufactured as required.

Because of high contact area, foil gauges dissipate heat easily. (Any 3 x 1)

(3) [20] QUESTION 4 4.1 4.1.1 Can be used to measure temperature (alarm).

Can be used as a current suppressor (current limiter). (2 x 1)

(2)

4.1.2 They are made from metal oxides.

The oxides used are that of cobalt, copper, iron, tin and zinc.

The oxides in powder form are pressed into a shape and then heated forming a ceramic body.

Two terminals are also added before the heating process.

There are a few shapes such as beads, rods, disc and washers.

(5 x 1)

(5)

4.1.3 They are not all manufactured exactly the same.

Interchangeability can be a problem.

Non-linear temperature vs resistance curve. (3 x 1)

(3)



4.2

(7)

4.3 If two bodies of different temperatures are placed opposite each other, there

is a transfer of heat energy from the hot body to the colder body and the amount of radiated energy per time unit is roughly proportional to the fourth power of the absolute temperature difference between the two bodies. This fourth power ratio is known as Stefan Boltzmann's law.

(3) [20]


(5)

5.1.2 When the flapper is completely away from the nozzle we have a

high flow-out of the nozzle creating a low pressure at the output receiver. When the flapper seals the nozzle the output pressure builds up to supply pressure which is then a small flow from the nozzle and a high pressure to the output receiver.The reason for the restriction is that when the flapper is away from the nozzle there is a large pressure drop. If the flapper is nearest to the nozzle then there is a small pressure drop across the restrictor.

(5)



5.2

(5)

No air bleeds out if the nozzle back-pressure is lower than the diaphragm

forces. With only the supply pressure on the relay no air will bleed to atmosphere. The input pressure area is bigger than the feedback area. When the back pressure increases the supply port will be opened and therefore cause the output pressure to increase. Air will now bleed to the atmosphere.

(5) [20]

QUESTION 6

6.1 6.1.1

(7) 6.1.2 Valve size

Type of plug

Valve characteristics

Type of connection to process lines

Type of material and physical strength of valve (5 x 1)

(5)



6.1.3 To move the actuator stem in accordance with the value of

the input signal

To increase the accuracy and speed

To overcome friction

(3)

6.2 6.2.1 Closed-loop control / Feedback system (2) 6.2.2 A closed-loop system is a system which has feedback from the

measuring element back to the correcting element. The measured variable is compared to the set-point of the controller, and the difference between these two signals is sent to the correcting element.

(3) [20]

TOTAL SECTION B:

GRAND TOTAL: 80

100


T850(E)(A5)T

AUGUST EXAMINATION NATIONAL CERTIFICATE


(11040463)

5 August 2015 (Y-Paper) 13:00–16:00


(11040463) -2- T850(E)(A5)T





INSTRUCTIONS AND INFORMATION 1. 2. 3. 4. 5.

Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Label ALL the sketches and drawings. Write neatly and legibly.

(11040463) -3- T850(E)(A5)T


SECTION A QUESTION 1 1.1 Various options are provided as possible answers to the following questions.

Choose the answer and write only the letter (A–D) next to the question number (1.1.1–1.1.5) in the ANSWER BOOK.

1.1.1 The mathematical expression or formula for a gauge factor is: A K = ΔR ÷ R / ΔL ÷ L

B R = Ɩ / a

C E = ò / ɛ

D ɛ = ΔL / L

1.1.2 What is a resistance thermometer? A

B C D

An instrument that measures temperature An instrument that measures temperature but is calibrated in temperature units An instrument that measures temperature then converts readings to resistance An instrument that measures resistance

1.1.3 The … type of optical pyrometer is widely used. A

B C D

Morse wedge mirror photo-electric

1.1.4 Which ONE of the following is NOT a bonded strain gauge? A

B C D

Grid type Rosette type Foil type Welded type

1.1.5 Which ONE of the following is NOT applicable to strain gauges? A

B C D

They are easily made waterproof Special patterns are easily produced They have a high gauge factor Their speed of response is fast

(5 × 1)

(5)

(11040463) -4- T850(E)(A5)T


1.2 Indicate whether the following statements are TRUE or FALSE. Choose the

answer and write only 'true' or 'false' next to the question number (1.2.1–1.2.5) in the ANSWER BOOK.

1.2.1 Bellows are used for temperature measurement. 1.2.2 The output signal of instrument circuits is normally 4–20 mA. 1.2.3 Load cells are used for measuring weights. 1.2.4 The on/off controllers are normally used for high loads. 1.2.5 A flapper nozzle is used in a pneumatic controller.

(5 × 1)

(5) [10]

QUESTION 2 Choose an item from COLUMN B that matches a description in COLUMN A. Write only the letter (A–G) next to the question number (2.1–2.5) in the ANSWER BOOK.

COLUMN A COLUMN B

2.1 2.2 2.3 2.4 2.5

When a body is heated up, a change in color takes place and an experienced operator would know the temperature of the substance If there are two bodies, body X and body Y, and body X is warmer than body Y then body X would give off energy to body Y. Body X would keep on giving off energy until both body temperatures are the same A device put into a valve to ensure that it is at a correct position of opening as per control signal The device that compares a control signal to a valve actuator and moves the actuator accordingly An electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit

A

B

C

D

E

F

G

Wheatstone bridge positioner Stefan Boltzmann's law Provost's theory potentiometer black body condition valve positioner

(5 × 1) [5]

TOTAL SECTION A: 15

(11040463) -5- T850(E)(A5)T


SECTION B QUESTION 3 3.1 In the olden days sand bags were placed on the wings of an aeroplane to

determine the shear stress limits, the bending moment and their strength. Nowadays hydraulic actuators are used in conjunction with 'certain instruments' that are cemented to the wings to perform such tests.

3.1.1 Name the instruments that are used to measure the bending

moment and stress.

(1)

3.1.2 Name ONE type of adhesive used to cement these instruments to the wings of the aeroplane to be tested.

(1)

3.1.3 Explain how the instruments mentioned in QUESTION 3.1.1 are

used for measurements.

(3) 3.2 Show, by means of a sketch, how the instruments mentioned in

QUESTION 3.1.1 are connected in a weighbridge.

(5) 3.3 Transducers are widely used for converting one form of energy to the other. 3.3.1 Draw a diagram of a rare-earth transducer.

3.3.2 State FIVE advantages of these transducers. (2 × 5)

(10) [20]

QUESTION 4 4.1 Expansion thermometers indicate temperature by means of change in volume

or pressure of the fillings inside them and are classified according to these filling materials.

Name FOUR fillings of the thermometers known to you. (4) 4.2 A Wheatstone bridge has a supply voltage of 10 V and four resistors.

R1 = 20 Ω, R2 = 20 Ω, R3 = 20 Ω and R4 is a resistance thermometer.

The resistance thermometer is exposed to an operating temperature of

300 °C. If at 25 °C a reading of 15 Ω is obtained and α = 0,004 Ω/Ω °C. Calculate the galvanometer reading if resistance of lead wires are not taken into account.

(9)

4.3 What is the principle of a black body according to Kirchoff? (3)

(11040463) -6- T850(E)(A5)T

Copyright reserved

4.4 Optical pyrometers require an operator to be present to balance the

instrument and can thus not record the temperatures.

4.4.1 How would you overcome the disadvantage of the pyrometer

mentioned in QUESTION 4.4 above?

(3) 4.4.2 Give the name of the pyrometer mentioned in QUESTION 4.4.1. (1) 4.4.3 Draw the pyrometer mentioned in QUESTION 4.4.2. (5)

[25] QUESTION 5 5.1 Complete the following sentences/paragraph(s) by giving the missing word(s).

Write only the word(s) next to the question number (5.1.1–5.1.5) in the ANSWER BOOK.

Pneumatic telemetering is used when a/an 5.1.1 … is not present. It is used

where the use of 5.1.2 … is extremely dangerous. The transmitter, also known as the 5.1.3 …, changes movement from a measured variable into a single 5.1.4 … which is proportional to the measured variable. The receiver is a pressure gauge or recorder which displays the 5.1.5 … (5 × 1)

(5)

5.2 5.3

Draw a flapper and nozzle system with feedback bellows. Explain the operation of the instrument mentioned in QUESTION 5.2.

(5) (5)

5.4 Make a sketch of a current telemetering system. (5) [20] QUESTION 6 6.1 Name THREE main control processes in use today. (3) 6.2 Make a sketch and describe the basic operation of a closed-loop system. Sketch (6) + Description (3) (9) 6.3 Make a drawing for a proportional, integral and derivative position balance

controller.

(6) 6.4 Draw a characteristics curve for a quick opening valve. (2)

[20]

TOTAL SECTION B: 85 GRAND TOTAL: 100

(11040463) T850(E)(A5)T

Copyright reserved

INSTRUMENT TRADE THEORY N3 FORMULA SHEET PG = Pabs – Pa

P1 – P2 = h g

P1 – P2 = d g ( or P1 – P2 = d g (

P =

Dry calibration =

Wet calibration =

W.C. = ( ) X D.C

D.C. = ( ) X W.C.

H =

H = h

to = 9/5 t + 32o F to C = ( t + 273,15) kelvin to F = 5/9 ( t – 32)o C



AUGUST EXAMINATION


5 AUGUST 2015


MARKING GUIDELINE



= 1 Mark = ½ Mark SECTION A QUESTION 1 1.1 1.1.1 A 1.1.2 B 1.1.3 A 1.1.4 D 1.1.5 D

(5 × 1) (5) 1.2 1.2.1 False 1.2.2 True 1.2.3 True 1.2.4 False 1.2.5 True

(5 × 1) (5) [10]

QUESTION 2 2.1 F 2.2 D 2.3 B 2.4 G 2.5 A

(5 × 1) [5]

TOTAL SECTION A: 15



SECTION B QUESTION 3 3.1 3.1.1 Strain gauges (1) 3.1.2 Fendhals or Araldite (1) 3.1.3 This is done by means of cementing a compensating gauge next

to the measuring gauge or active gauge. Both are exposed to the same temperature changes and therefore the effect of temperature can be compensated for.

(3)

3.2

(Marks for drawing: 10 × ½)

(5)

3.3 3.3.1


(5)

3.3.2 Negligible hysteresis effects

They can withstand high pressures and overload

Temperature effects are very small

They have very good frequency response

High accuracy and repeatability (5 × 1)

(5)

[20]



QUESTION 4 4.1 Liquid

Vapour

Gas

Mercury

(4)

4.2 Rt = Ro (1 + αt)

Ro = Rt / 1 + αt Ro = 15 / 1 + (0,004 x 25) Ro = 13,64 Ω Rt = 13,64 (1 + (0,004 x 300) Rt = 30,008 Ω Vm = V1 – V2

Vm = [R1/(R1 + R2)]Vt – [R3/(R3 + Rt)]Vt

Vm = (20/40) 10 – (20/50,008) 10 Vm = 5 – 4 Vm = 1 V

(9)

4.3 It is a body that absorbs all the radiation falling upon it and it radiates energy

at all the wavelengths of the spectrum at a maximum rate, corresponding to its temperature.

(3)

4.4 4.4.1 This may be overcome by constructing a pyrometer that permits

the radiation from the hot body to control the flow of current in a circuit. The controlled current from the photocell is then amplified and used to actuate a strip chart recorder.

(3)

4.4.2 Photo-electric pyrometer (1) 4.4.3


(5) [25]




5.1.2 5.1.3 5.1.4 5.1.5

Electrical supply Electrical power Primary element Air pressure Air pressure

(5 × 1)

(5)

5.2


(5)

5.3 If the flapper is connected to a primary measuring device, C-tube, then when

the flapper moves closer to the nozzle, the back pressure of the nozzle increases expanding the bellows and moving the flapper away from the nozzle. This reduces pressure in the nozzle until equilibrium is reached. When the nozzle pressure and output pressures are equalised, then this is an indication of the measured variable which is recorded at the output. Accurate positioning and stability between measured variable and output pressure are thus obtained.

(5)

5.4


(5) [20]


QUESTION 6 6.1 Pneumatic

Electrical Electronic

(3)

6.2 Sketch


(6)

Description

The setpoint (desired value) is adjusted to 50% level. The measured variable is compared to the setpoint. An output signal is now sent to the valve (correcting element) which will ensure that the level in the tank remains at the desired value.

(3)

6.3

(Marks for sketch or drawing; 12 × ½)

(6)



Copyright reserved

6.4

Marks for sketch or drawing; 4 × ½) (2)

[20]

TOTAL SECTION B: 85 GRAND TOTAL: 100


T830(E)(N13)T




(11040463)

13 November 2014 (Y-Paper) 13:00–16:00

Calculators may be used.

This question paper consists of 4 pages and a diagram sheet.

(11040463) -2- T830(E)(N13)T







(11040463) -3- T830(E)(N13)T


QUESTION 1 1.1 Briefly explain the construction of the following: 1.1.1 Unbonded-type strain gauge 1.1.2 Grid-type strain gauge 1.1.3 Spiral-type strain gauge 1.1.4 Welded-type strain gauge

(4 × 4)

(16) 1.2 Two strain gauges are usually mounted close together at a measuring point. 1.2.1 Explain for what reason this is done. (4) 1.2.2 Show how these two gauges would be connected in a Wheatstone

bridge circuit.

(5) [25]

QUESTION 2 2.1 FIGURE 1 on the attached DIAGRAM SHEET shows a potentiometer circuit

which has automatic cold junction compensation and a supply voltage standardisation. The following data is supplied: Recorder measuring range: 200 °C to 600 °C Supply voltage: 1,5 V Standard cell voltage: 1,019 V Slide wire resistance: 25 ohms Thermocouple outputs: 8,1 mV at 200 °C and 41,3 mV at 600 °C Cold junction resistor: 5,77 ohms at 0 °C and 6,1 ohms at 25 °C Current in each branch: 5 mA Ambient temperature: 25 °C Calculate the values of all the following resistors:

2.1.1 R1 (3) 2.1.2 R2 (1) 2.1.3 R3 (3) 2.1.4 RK (6) 2.1.5 R4 (3)

(11040463) -4- T830(E)(N13)T


2.2 Sketch a total radiation pyrometer that uses mirror systems. (5) 2.3 Sketch a basic current telemetering system. (4)

[25] QUESTION 3 3.1 Sketch and describe the operation of an inductive transducer (10) 3.2 Show, with the aid of TWO sketches, the transmitter only, of a ratiometer

telemetering system which contains TWO diodes (rectifiers). The first sketch must show the output signal at 50% displacement and the second sketch must show the output signal at 100% displacement for liquid level in a tank.

(10)

3.3 Provide a fully-labelled block diagram of a frequency telemetering system. (5)

[25] QUESTION 4 4.1 A pneumatic controller can be used to control a plant.

Sketch such a control system for a pneumatic, position balance controller which is equipped with proportional, integral and derivative action.

(6)

4.2 With the aid of a fully-labelled sketch, explain the operation of a pneumatic,

motion-balance type, valve positioner.

(10) 4.3 Sketch a block diagram of a closed-loop control system. (6) 4.4 Briefly explain the difference between a closed-loop and an open-loop control

system.

(3) [25]

TOTAL: 100

(11040463) -5- T830(E)(N13)T


DIAGRAM SHEET

FIGURE 1





13 NOVEMBER 2014


____= 1 Mark _= ½ Mark

MARKING GUIDELINE



QUESTION 1 1.1 1.1.1 Unbonded type

It consists of a stationary frame, which supports a movable

armature. The strain wire is wound between pins on both the

frame and armature.When the armature is moved, the strain wire

is lengthened and its resistance increases.

1.1.2 Grid type

It consists of a length of very thin wire which is formed into a grid

pattern and bonded to a suitable backing material. This material

must be able to withstand high temperatures.The grid pattern is

then covered with a layer of insulating material.The ends of the

wires have tags soldered on them to form terminals.

1.1.3 Spiral type

It is manufactured by printed circuit methods.The spiral pattern

circuit is printed onto a flexible backing material.The soldering

tags and the gauge proper are one continuous structure.

The pattern may be stamped or etched out of thin metal foil and

mounted on a thin plastic sheet base.

1.1.4 Welded type

The strain wire is fitted inside a metal tube, but forms one piece

from terminal to terminal.The central part of the strain wire is

made thinner.These gauges are spot welded to the surface

being measured.This surface may be flat or curved.

(4 × 4)

(16)

1.2 1.2.1 To eliminate the effect of temperature a second gauge

(compensating) is mounted near the measuring (active) gauge.

Only the active gauge is stressed.

Changes in ambient temperature affect both gauges equally.

With the active and temperature compensating gauges connected in opposite arms of the Wheatstone bridge, ambient temperature

effects cancel each other.Indication on the output meter will only

be that of the stress or strain on the active gauge.

(4)



1.2.2

WHEATSTONE BRIDGE CIRCUIT DIAGRAM (5) [25]

QUESTION 2 2.1 2.1.1 R1 = Rs Vsc/Is

= 1,019 / 0,005

= 203,8

(3)

2.1.2 R2 = Rcj = 6,1 (1)

2.1.3 R3 = Rz = Vmin / Is + Rcj

= 0.0081 /0,005 + 6,1

= 1,62 + 6,1

= 7,72

(3)

2.1.4 RK = Rshunt Rsw × Rt Rt= Vmax - Vmin

Rsw – Rt Is

25 * 6.64 = 0,0413 – 0,0081

25 – 6,64 0,005

= 9,041 = 6,64

(6)

2.1.5 R4 = Ru = Vsc – Vmax

Is

= 1,019 – 0,0413

0,005

= 195,54

(3)



2.2

PYROMETER USING MIRROR SYSTEMS (5)

2.3

CURRENT TELEMETERING SYSTEM (4) [25]



QUESTION 3

3.1

INDUCTIVE TRANSDUCER (5) The pressure in the bellows determines the position of the

magnetic core.This varies the inductance of the coil,and thus

the current in the coil and also the voltage across the coil.The

coil is connected to an AC inductance bridge circuit.The change

in inductance is proportional to the applied pressure.

(5)

3.2

(6)

RATIOMETER – TRANSMITTER ONLY

Output signal at 50% Output signal at 100%

(10)

(4)



3.3

BLOCK DIAGRAM OF A FREQUENCY TELEMETERING SYSTEM (5) [25]

QUESTION 4

4.1

POSITION BALANCE CONTROLLER – PROPORTIONAL, INTEGRAL AND DERIVATIVE ACTION

(6)



4.2

MOTION-BALANCE TYPE VALVE POSITIONER (6) The output signal from the controller is applied to the bellows.Assume an

increase in pressure. The bellows expands and the flapper moves towards

the nozzle. Nozzle pressure increases.It is amplified by the relay and

transmitted to the diaphragm of the control valve.The diaphragm pushes

the valve stem downwards, moving the flapper away from the nozzle. A position of equilibrium is eventually reached where the valve opening

corresponds to the value of the controller signal.

(4)

4.3

BLOCK DIAGRAM: CLOSED-LOOP CONTROL SYSTEM (1/2 mark per label)

(6)

4.4 A closed loop has a feedback loopand hence the output is controlled.

An open loop has no feedback control and hence the output is not

controlled.

(3) [25]

TOTAL: 100


T830(E)(J23)T

AUGUST EXAMINATION



(11040463)

23 July 2014 (Y-Paper) 13:00–16:00

Calculators may be used.

This question paper consists of 4 pages and 1 diagram sheet.

(11040463) -2- T830(E)(J23)T







(11040463) -3- T830(E)(J23)T


QUESTION 1 1.1 Make a neat, fully labelled sketch of the following types of strain gauges: 1.1.1 Spiral pattern foil strain gauge (4) 1.1.2 Rosette type strain gauge (4) 1.2 Briefly explain the construction and operating principle of the spiral pattern foil strain

gauge.

(5) 1.3 State FIVE advantages of foil strain gauges over wire strain gauges. (5) 1.4 Explain what is meant by the term bonded strain gauge. (2) 1.5 State FIVE properties of good strain gauge wire. (5)

[25]

QUESTION 2 2.1 The resistance of a platinum resistance bulb for different temperatures are given

below: Temperature Resistance

0C 30 ohms

100C 40,2 ohms

544,6C 61,3 ohms

2.1.1 Calculate the zero temperature coefficient of resistance. (3) 2.1.2 What will the resistance of the bulb be, when heated to a temperature of

85C?

(3) 2.1.3 What is the temperature of the bulb, when the bulb resistance is 35,7

ohms?

(3) 2.2 Refer to FIGURE 1 on the attached DIAGRAM SHEET and calculate the voltage

displayed on 'Vm' when the bridge is out of balance due to a change in resistance in one of the legs.

(8) 2.3 Draw the IEC symbol of an 'NTC' thermistor. (1) 2.4 Draw the characteristic curve of an 'NTC' thermistor. (2) 2.5 Make a sketch of a fully labelled disappearing filament type pyrometer. (5)

[25]

(11040463) -4- T830(E)(J23)T


QUESTION 3 3.1 A linear variable differential transformer can be used to convert an applied pressure

to an electrical signal.

3.1.1 Sketch such a transformer circuit. (6) 3.1.2 Explain the operation of the LVDT circuit. (4) 3.2 Conductive type telemetering: 3.2.1 Make a sketch of a conductive type telemetering system for transmitting

and measuring a signal which represents a differential pressure.

(6) 3.2.2 State FOUR methods of counteracting the effects of a supply voltage

variation.

(4) 3.3 State FIVE advantages of an electrical telemetering system. (5) [25]

QUESTION 4 4.1 A pneumatic controller can be used to control a plant. Make a sketch of such a control system for a pneumatic position balance controller,

which is equipped with proportional and derivative action.

(5) 4.2 Draw the circuit diagram of a simple TWO-TERM (on-off) electronic controller. (5) 4.3 With the aid of a fully labelled diagram, describe the operation of a pneumatic force

balance type valve positioner.

(8) 4.4 State THREE disadvantages of a flapper and nozzle system, with feedback bellows,

of a pneumatic controller.

(3) 4.5 Draw a diagram of a butterfly valve in the open position. (4) [25]

TOTAL: 100

(11040463) -5- T830(E)(J23)T


DIAGRAM SHEET

FIGURE 1 VT = 80 V R1 =18 Ω R2 =12 Ω R3 =10 Ω R4 =22 Ω



AUGUST EXAMINATION


23 JULY 2014


____= 1 Mark

____= ½ Mark

MARKING GUIDELINE

MARKING GUIDELINE -2- T850(E)(J23)T INSTRUMENT TRADE THEORY N3


QUESTION 1 1.1 1.1.1 Spiral pattern foil strain gauge

(4) 1.1.2 Rosette Type strain gauge

(4) 1.2 This gauge is manufactured by printed circuit method.

The spiral pattern is printed onto a flexible backing material. Its terminals are soldered tags. It works on the principle that the resistance path is produced by etching thin metal foil and mounting on a non-conducting backing material. Measures overall expansion of an object, where the circumference of the gauge increases and hence its resistance.

(5) 1.3 Special patterns are easily produced.

It is easily water proofed. They have a high gauge factor. The ratio of element contact area to volume is very high. The transmission of the strain from the specimen to the foil is performed to a high degree of efficiency. The soldering tags and the gauge proper are one continuous structure. They follow the surface to which it is cemented very well. Large surface area allows for good heat dissipation. They have relatively small 'creep' and hysteresis effects. They can be prepared within a few minutes with the proper cement and equipment. (Any 5 x 1)

(5)



1.4 The resistance wire is glued onto the elastic backing material .

OR The gauge is glued onto a surface whose strain has to be monitored.

(2)

1.5 Maximum strain gauge factor (K)

Maximum resistance per unit length Maximum melting point Minimum resistance-temperature coefficient Optimum flexibility Minimum coefficient of linear expansion Minimum thermo-electric tendency (unlikely thermocouple) Must be easily soldered (Any 5 x 1)

(5) [25]

QUESTION 2 2.1 2.1.1 Rt = Ro (1 + t)

= Rt/Ro -1 t

= 40.2/30 -1 100

= 0,0034 /C

(3) 2.1.2 Rt = Ro (1 + t)

= 30 (1 + 0, 0034*85)

= 38.67

(3) 2.1.3 Rt = Ro (1 + t)

t = Rt/Ro – 1

= 35,7/30 –1 0,0034

= 55,88 C

(3) 2.2 V meter = Vcb - Vca

Vcb = Vt * R4/R4+R3 Vca = Vt * R1/R1+R2 = 80 * 22/22+10 = 80 * 18/18+12 = 55 V (3) = 48 V (3) Vm = Vcb - Vca Vm = 55V – 48 V = 7 V (2)

(8)



2.3

(1) 2.4

(2) 2.5 Disappearing filament pyrometer

(5) [25]



QUESTION 3 3.1 3.1.1 Linear variable differential transformer

Rv

Amplifier

Core in centre

E1=E2

(6) 3.1.2 Operation of LVDT

AC is supplied to the primary coil and the magnetic flux generated by this coil is transferred to both the secondary coils by the movable armature. The secondary coils are connected in series opposition. Core in centre position: E1 = E2, therefore the output is zero volts.

Core moves up: E1 > E2, this unbalances the measuring circuit and

a voltage output will appear at the amplifier output which will be a function of the applied pressure.

(4) 3.2 3.2.1 Conductive type telemetering

(6)



3.2.2 Counteracting the effects of supply voltage variations

By using a barretter lamp to regulate the current and hence keep the voltage constant

By using a voltage source which is stabilized with the aid of a zener diode

By using a ratiometer instrument to compensate for supply voltage variations

By using a variable resistor in line with the power supply to standardise the supply

By using two voltage and two current coils to counteract changes in the supply voltage (Any 4 x 1)

(4) 3.3 Advantages of electrical telemetering systems

Better for long distance transmission

No time lag

Easier and cheaper to install

No pressure drops

Linear output

Not sensitive to vibration

Unlimited range

Easier to fault find Electrical energy is readily available (Any 5 x 1)

(5) [25]

QUESTION 4 4.1 Position balance controller – proportional and derivative action

(5)



4.2 Two-term electronic controller circuit

(5) 4.3 Force balance type valve positioner (Diagram = 5 & exp. =3)

The output signal from the controller is applied to the bellows. Assume an increase in pressure. The bellows expands and the flapper moves away from the nozzle. The supply to the valve motor decreases and the valve stem moves upwards, moving the flapper towards the nozzle. A position of equilibrium is eventually reached where the valve opening corresponds to the value of the controller signal.

(8) 4.4 Disadvantages of flapper & nozzle system

Air supply has to be held very constant for good results.

Relationship between flapper travel and nozzle pressure is non-linear.

Slight changes in the position of the flapper cause friction and backlash and this affects nozzle pressure.

The restrictor and nozzle can become blocked due to its small diameter.

The flapper-nozzle system can only supply small volumes of the output pressure.

Flapper movement is very small and sensitive to vibration. The air leaving the nozzle acts as a force on the flapper and this may

restrict the element driving the flapper. (Any 3 x 1)

(3)



4.5 Butterfly valve – open position

(1 mark for showing open position)

(4) [25]

TOTAL: 100

Documents

Email: [email protected] questions trade theory n2 ......1.3.1 What type of strain gauge is represented by the diagram? (1) 1.3.2 Use the diagram of the strain gauge above