APPENDIX II

QUESTIONS
LEARNING UNIT II: TRANSMITTERS & TELEMETERING

2.1 The light gathering capability of the optical fibre is expressed in terms
of numerical aperture (NA). Determine the NA if half the angle of the
acceptance cone for the optical is = {a} degrees.
Please make use of the following values for {a}:
a = 15.5
a = 43.2
a = 20.6
a = 27.2
a = 27.9
a = 34.1
a = 28.9
a = 45.4
a = 18.9
a = 22.4
2.2 Determine the numerical aperture (NA) if the core material n1 = {b} and
the surrounding cladding material n2 = {c}.
Please make use of the following values for {b} & {c}:
b = 0.010 & c = 0.0009
b = 0.006 & c = 0.0007
b = 0.007 & c = 0.0003
b = 0.001 & c = 0.0001
b = 0.005 & c = 0.0001
b = 0.010 & c = 0.0009
b = 0.001 & c = 0.0005
2.3 Name the following symbols that are used in optic transmission.
 n?
 v?
 c?
 n1  ?
 n2  ?
 NA  ?
2.4 Make a sketch of a Current to Air signal converter that is used for
telemetering purposes.
2.5 Make a sketch of a Pneumatic to Current signal converter that is used for
telemetering purposes.
2.6 Make a sketch of a Current to Voltage signal converter that is used for
telemetering purposes.
2.7 Make a sketch of a Voltage to Current signal converter that is used for
telemetering purposes.
2.8 Make a sketch of a Millivolt to Current signal converter that is used for
telemetering purposes.
2.9 Make a sketch of a Resistance to Current signal converter that is used for
telemetering purposes.
2.10 Make a sketch of a Frequency to Current signal converter that is used
for telemetering purposes.
2.11 Sketch and describe the working principles of a 4-Wire Current
Transmission System.
APPENDIX II LEARNING UNIT II QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
2.12 At low frequencies, the distance over which the current signal can be
transmitted, is limited by the available (a) _____ at the transmitter. With (b)
_____ at the transmission, the signal can be transmitted over several
kilometres.
2.13 What is the advantage of using a 4-20 mA signal, instead of a 0-20 mA?
2.14 Sketch and describe the working principles of a 2-Wire Current
Transmission System.
2.15 A transmitter is calibrated for a pressure measurement of 0-300 kPa.
Make use of a two-wire current transmission system and determine the
transmission current and output voltage if the measured pressure is equal too
100 kPa.
2.16 Name and describe the two parity checking methods used in Digital
Transmission Systems.
2.17 Name the three techniques for the transmission of information in the
form of pulses that are generally used.
2.18 Digital information is transmitted over transmission lines as (a) _____
serial or (b) _____ parallel data.
2.19 When one refers to Digital information transmission over long
distances, what type of communication method is preferred?
2.20 Explain the term “Parity Checking”.
2.21 List some of the outstanding feature one may expect of fibre optic
transmission if it is used for transmitting process measurement data.
2.22 Make a sketch that shows the light energy transmission through optical
fibre.
2.23 Explain the term “Telemetering”.
2.24 Name the four elements of a basic telemetering system.
2.25 Name the four components of a basic circuit for an optic transmission
system.
2.26 Make a sketch of a basic optic transmission system.
2.27 Name two light sources that can be used for fibre optic transmission.
2.28 What does the abbreviation, LED, stand for?
2.29 What does the abbreviation, ILD, stand for?
2.30 List the factors one should keep in mind when selecting a light source
for optic transmission.
2.31 Construct a table that shows the comparison between ILD and LED
sources.
2.32 List {X} factors one should consider when selecting suitable connectors
for an optic transmission system.
2.33 What materials are used for the fibre of the optic fibre cables?
2.34 List 3 cable packaging methods for fibre optic cables.
2.35 Construct a table that shows the comparison between the different
types of optic fibre cables that can be used in optic fibre transmission.
2.36 Make a sketch of a fibre optic cross section and explain the principle
of operation with respect to refraction.
2.37 Describe what optical fibre is, and what one should consider in all
application when making use of optical fibre? (Include equation/s in your
answer)
2.38 Sketch and describe the light gathering capability of optical fibre.
(Include equation/s in your answer)

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APPENDIX II LEARNING UNIT II QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
2.39 Describe the minimum angle that supports internal reflection if one
refers to optical fibre. (Include equation/s in your answer)
2.40 Name the two categories in which detectors for optic transmission
falls into.
2.41 For what applications is hybrid sensors/detectors used for in optic
transmission?
2.42 For what applications is fibre optic sensors/detectors used for in optic
transmission?
2.43 Make a sketch of a fibre-optic digital link.
2.44 Name three different formats in which the data in optical fibre
transmission can be transmitted.
2.45 List the five important factors one should consider when designing an
optical transmission system.
2.46 Sketch and describe the operating principles of a fibre-optic analogue
link.
2.47 List the advantages of optical fibre transmission.
2.48 Name the most general mediums for telemetering.
2.49 The choice of a transmitting medium is dependant on many factors.
List the general guidelines to follow when selecting a transmitting medium.
2.50 Sketch and describe the operating principles of a typical pneumatic
transmission system.
2.51 Explain briefly why there is a time delay in pneumatic transmission
systems.
2.52 Name the factors which determine the duration of the delay in the
case of a rise in the output of the transmitter if one considers time delay in a
pneumatic transmission system.
2.53 Make a sketch that shows the time delay in response to a step change,
of pressure applied to a 6 mm and 9 mm diameter pipe from 2 to 100 meters
when considering pneumatic transmission.
2.54 Describe how one can approximately determine the time delay of a
pneumatic system.
2.55 Name the advantages of pneumatic transmission.
2.56 Name the disadvantages of pneumatic transmission.
2.57 Electrical transmission is classified according to the nature of the
transmitted signal. The type of transmission which will be used depends on the
application. List six of the most commonly used types of electrical
transmission.
2.58 Describe the operating principles of voltage transmission.
2.59 Discuss signal cables if one considers current transmission.
2.60 When should one opt for frequency transmission?
2.61 Briefly describe the basic operation principles of frequency
transmission.
2.62 Sketch and describe the operating principles of a capacitance level
measuring system which is used in frequency transmission.
2.63 Sketch and describe the basic operating principle of a frequency
transmission system.
2.64 Explain for what reason, is the conversion of current to frequency
especially suited for in frequency transmission?

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APPENDIX II LEARNING UNIT II QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
2.65 Sketch and describe the operating principle of a frequency transmitter
that is used to drive counters.
2.66 Make a sketch of a frequency transmitter that is used to drive
counters.
2.67 Under what circumstances, is it preferable to transmit data in a digital
format? Also include an example of a digital transmission system.
2.68 Name the three different techniques for the transmission of digital
information in the form of pulses.
2.69 Explain the term “Pulse Duration Telemetering”.
2.70 Explain the term “Pulse Count Telemetering”.
2.71 Describe the operating principles of Pulse Code Telemetering.
2.72 Name the two methods of multiplexing that are generally used
telemetering.
2.73 What does the abbreviation, FDM, stand for?
2.74 What does the abbreviation, TDM, stand for?
2.75 Describe the operating principles of Frequency Division Multiplexing.
2.76 Describe the operating principles of Time Division Multiplexing.
2.77 Name three different cables that may be used to reduce the effect of
inductance and capacitance noise in voltage transmission.
2.78 Sketch and describe the operating principles of a Current to Air signal
converter that is used for telemetering purposes.
2.79 What is the other name for a Current to Air signal converter and what
is it used for?
2.80 Sketch and describe the operating principles of a Pneumatic to Current
signal converter that is used for telemetering purposes.
2.81 Sketch and describe the operating principles of a Current to Voltage
signal converter that is used for telemetering purposes.
2.82 What is a current to voltage converter used for in telemetering?
2.83 Sketch and describe the operating principles of a Voltage to Current
signal converter that is used for telemetering purposes.
2.84 Sketch and describe the operating principles of a Millivolt to Current
signal converter that is used for telemetering purposes.
2.85 What is a millivolt to current converter used for in telemetering?
2.86 What is a resistance to current converter used for in telemetering?
2.87 Sketch and describe the operating principles of a Resistance to
Current signal converter that is used for telemetering purposes.
2.88 Sketch and describe the operating principles of a Frequency to Current
signal converter that is used for telemetering purposes.
2.89 What is a frequency to current converter used for in telemetering?
2.90 Name the reasons why signal converters are needed for telemetering.
2.91 What is the most common current signal format, which is used for
telemetering?
2.92 Show by means of an equation how one determines the minimum angle
that supports internal reflection by an optical.
2.93 Show by means of an equation how one determines the index of
refraction. Also describe each variable within this equation.
2.94 Show by means of an equation how one determines the light gathering
capability of optical fibre when it is expressed in terms of numerical aperture
(NA).

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APPENDIX II LEARNING UNIT II QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
2.95 What is the transmission element that is used in a pneumatic system?
2.96 In process instrumentation it is general practice to control a plant
from a central control centre with the aid of…?
2.97 A transmitter is calibrated for a pressure measurement of 0-{a} kPa.
Make use of a two-wire current transmission system and determine the output
voltage at the receiving end if the pressure measured at the transmitter is
equal too {b} kPa.
Please make use of the following values for {a} & {a}:
b = 150 & a = 300
b = 18 & a = 300
b = 29 & a = 300
b = 160 & a = 300
b = 180 & a = 300
b = 283 & a = 300
b = 497 & a = 500
b = 101 & a = 500
b = 260 & a = 500
b = 420 & a = 500
b = 191 & a = 500
b = 377 & a = 500
2.98 State whether the following statements are True or False.
(a) The use of a 0-20 mA, has the advantage that an open circuit can immediately be
observed.
(b) It is recommended that current transmission signal cables be placed in separate
cable racks, away from other cables.
(c) When information has to be transmitted over a very long distance, analogue signals
are inclined to be inaccurate. This is the result of noise and undesired induction on the
signal cables. Under these circumstances, it is preferable to convert the signal from an
analogue signal to a digital signal, transmit the signal in digital form, and to switch back to
analogue at the receiving end.
(d) When information has to be transmitted over a very long distance, digital signals
are inclined to be inaccurate. This is the result of noise and undesired induction on the
signal cables. Under these circumstances, it is preferable to convert the signal from
a digital signal to an analogue signal, transmit the signal in analogue form, and to switch
back to digital at the receiving end.
(e) Another example of digital transmission is a telegraph system where letters and
numbers are depicted by a system of coded pulses.
(f) In frequency system, the number of cycles per second is looked into, while in the pulse
count method, the number of pulses is looked into.
(g) In the pulse code method, the analogue value of a signal is converted to a
succession of pulses which represents a analogue number.
(h) Digital information is transmitted over transmission lines as serial or parallel data.
(i) Serial communication is preferred when the distance is very short.
(j) Not all the light that enters the cone is received at the receiving end, when one refers
to optic transmission. Some of the light is lost in the cladding. The angle at which the
light enters the core, will determine whether it will be transmitted or lost.
(k) The light gathering capability of the optical fibre is expressed in terms of numerical
aperture (NA). This is shown by the angle, theta, in Fig. 2.? below.
(l) The minimum angle depends on the selected cable and the indexes of refraction of the
cladding and the cone. Rays striding the cone/cladding interface at angles less than min will
be lost in the cladding.
(m) Optic transmission can only be used for transmission of digital signals.
2.99 State whether the following statements are True or False.
(a) Laser-diode sources are generally not economically feasible for industrial systems.
LED’s are typically used in these cases.

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APPENDIX II LEARNING UNIT II QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
(b) LED sources are generally not economically feasible for industrial systems. Laser-
diode sources are typically used in these cases.
(c) Materials used for the optic fibre are: glass, plastic, or a combination of the two.
(d) Cable packing can only be in fibre bundles of plastic flat-ribbon cable construction,
of hybrid construction where metallic or other strengthening material is used with the
optical fibre.
(e) Plastic fibres and plastic fibre bundles are suitable for long distances and both offer
good durability with a low degree of flexibility.
(f) Fibre bundles have the advantage that its performance is not influenced much by the
breakage of a single strand.
(g) The format of the signal can be TTL logic, RS232, RS422 or any other type, as long
as the interface circuitry is designed to accept the format as its input.
(h) An analogue signal can be transmitted directly through variation in light intensity.
(i) The light gathering capability of the optical fibre is expressed in terms of numerical
aperture (NA), and is the sine of the full angle of the acceptance cone for the optical.
(j) When altering current signals have to be transmitted over very long distances, it is
very difficult to obtain the desired accuracy, with current or voltage transmission.
Frequency transmission overcomes this problem.
(k) Frequency transmission is not that effective to measure variables such as
capacitance, reactance and transformer effects.
(l) In another frequency transmission design system, the pulses are counted at the
receiving end for a certain period of time to provide a frequency measurement. To ensure
a high resolution, it is necessary to count the signal for a reasonable period of time. If
however the signal is a slow changing one, this method of transmission will not provide
satisfactory results.
(m) The conversion of current to frequency is especially suited for the driving of
counters, where for example the total volume of flow is to be determined for a certain
period of time.
(6.5)
2.100 State whether the following statements are True or False.
(a) By using multiplexing, it is possible to transmit more than one signal over the same
transmission line.
(b) Time division multiplexing divides the transmission into a series of frequency slots,
and then certain times are allocated for the transmission of a certain signal. For TDM, a de-
multiplexer is needed for separation of the various signals.
(c) Voltage signals can either be direct current of altering current signals if one refers to
voltage transmission.
(d) Low voltage alternating current signals are generally used as a result of problems
experienced with noise such as induction and capacitance. This effect also restricts the
distance over which the signal can be transmitted.
(e) The most general mediums for telemetering are wi-fi or electrical.
(f) Electrical signals have the disadvantage that interferences, for instance induction, can
occur, unless the system is designed, by using captyre cables for instance, to prevent it.
With pneumatic systems, these problems do not occur.
(g) With modern day trends to use computerised instrumentation, pneumatic
instrumentation has fallen into disuses to a great extent.
(h) Pneumatic instrumentation is not intrinsic safe.
(i) If power loss occurs in a pneumatic transmission system, stored compressed air can
still supply the pneumatic control instrumentation, to keep control over the plant.
(j) In pneumatic transmission systems conversion of signals is needed from electrical to
pneumatic to operate the final control elements such as for instance pneumatic control
valves.
(k) Pneumatic instrumentation is not easily adapted to digital instrumentation.
(l) No time delays are present in pneumatic instrumentation.
(m) Any air can be used for pneumatic instruments.
(6.5)
2.101 State whether the following statements are True or False.
(a) A change in the measured value is detected by the transmitter and the output of
the transmitter changes accordingly. If there is a distance between the transmitter and
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APPENDIX II LEARNING UNIT II QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
receiver, (which can be a controller or a recorder) there will be no time delay before the
change is observed by the receiver.
(b) A frequency to current converter is used to convert digital inputs from equipment
such as turbine type flow-meters, tachometers, etc. to current signals of 4-20 mA.
(c) A current to air converter is also known as a Pneumatic-Electro Converter, and is used
to convert an input signal of 4-20 mA to a pneumatic signal of 20-100 kPa.
(d) The current to voltage converter is used to convert a current input signal, 4-24
mA, to a voltage signal, 1-5 V.
(e) The millivolt to current converter is used to convert the e.m.f. generated by a
RTD's, to a current signal of 4-20 mA.
(f) The pneumatic to current converter provides an indirect current output signal,
proportional to the pneumatic input signal.
(g) For a pneumatic to current converter: a variable capacitor which varies when the
input pressure changes is included in the circuitry. There is a moveable plate within the
capacitor that will be moved by the pressure capsule.
(h) The resistance to current converter is used to convert the resistance signal
measured by a resistance thermometer, to a current signal of 4-20 mA.
(i) The voltage to current converter is used to convert a voltage input signal, 0-5V, to
a current signal, 4-20 mA.
(j) Laser-diode sources are generally economically feasible for industrial systems. LED’s
are typically used in these cases.
(k) With modern day trends to use computerised instrumentation, pneumatic
instrumentation has fallen into disuses to a great extent.
(l) In another frequency transmission system design, the pulses are counted at the
receiving end for a certain period of time to provide a frequency measurement. To ensure
a high resolution, it is necessary to count the signal for a reasonable period of time. If
however the signal is a fast changing one, this method of transmission will not provide
satisfactory results.
2.102 Add any new questions?