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Applied Marine Engineering > Instrument > Flashcards

Instrument Flashcards

1
Q

What are the two types of Processs?

A
  • Self Regulating
  • Non Self Regulating
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2
Q

What are the two types of control systems?

A
  • Open loop
  • Closed loop
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3
Q

Give 3 points about Open Loop Control Systems?

A
  • Inherently stable and insensitive to disturbance
  • Simple and cheap to set-up
  • Highly inaccurate
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4
Q

Give 3 points about Closed Loop Control Systems?

A
  • Potentially unstable.
  • Complicated and expensive to set-up.
  • Very accurate.
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5
Q

What are the 3 basic elements of a closed loop control system and explain how it works?

A

Measuring Element
Control Element
Correcting Element

The measuring element determines the condition of the process. This information is compared to the desired value and an error signal is produced if the two signals do not agree. The error signal is sent to the control element.

The control element receives the error signal, conditions it and sends an operating signal to the correcting element

The correcting element then uses the operating signal to modify the process in order to eliminate the error and achieve the required result.

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6
Q

Describe the 3 types of measurement and provide an example for each one?

A

Discrete – A discrete system produces single on/off pulses or individual steps.
Example:
An on/off light switch.

Analogue – An analogue system produces an infinitely variable signal between the limits of the working range.
Example:
The output from a mercury thermometer.

Digital – A digital system produces a combination of on/off signals (binary code) that replicates a stepped analogue response.
Example:
The input to a digital computer.

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7
Q

With regards to measurement terminology, Explain Range?

A

The range of a measuring system is the
difference between the maximum and
minimum values that the system can be
used to display or output

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8
Q

With regards to measurement terminology, Explain Span?

A

The span of a measuring
system is the difference
between the extremes values
within which the system is
designed or set-up to operate.

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9
Q

With regards to measurement terminology, Explain Accuracy?

A

Accuracy is an indication of the nearness to which the output value of a measuring system matches the input value,

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10
Q

With regards to measurement terminology, Explain Error?

A

Error is an indication of the difference between the output of the measuring
system and the actual value of what it is sensing.

Error = (Output value - True value / Full range) x 100%

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11
Q

Draw and describe a Non-Linearity related to measuring systems?

A

A measuring system can be said to
maintain a linear relationship if it
produces an output that is, at all
times, directly proportional to the
input.
Non-linearity is any departure from
this linear relationship and is caused
by transmission losses within the
measuring system.

P.127 for drawing

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12
Q

Draw and describe a Hysteresis loss related to measuring systems?

A

Hysteresis loss is the non-linearity
caused by the difference in the
measuring system’s response to a
rising or falling change in the
process.

P.127 for drawing

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13
Q

Explain Insertion Error and provide an example?

A

Insertion error is the effect that the measuring system has on the process
being measured.
E.g. if a cold thermometer is put into hot liquid then the thermometer will
change the condition of the process by cooling it.

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14
Q

Explain Resolution?

A

Resolution is the smallest change in input, across the span, which can be displayed by the measuring system.

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15
Q

Explain Threshold?

A

Threshold is the smallest change in input
above the zero point of the span that can be displayed by the measuring system.
The difference between the threshold and zero is termed the ‘dead band.’

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16
Q

Explain Drift?

A

Drift is a measure of the stability of a measuring system

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17
Q

Explain Transfer function with an example?

A

Transfer function is the ratio of the output to the input of a measuring system.

The mercury in a thermometer stem moves 15mm for a 10°C increase in temperature. The transfer function is 15/10 = 1.5 mm/°C
The thermometer could also be said to have a sensitivity of 1.5mm/°C

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18
Q

What are the two ways of measuring pressure?

A
  • By measuring absolute pressure relative to a perfect vacuum. (‘Abs.’)
  • By measuring pressure relative to the ambient atmospheric pressure. (‘Gauge’)
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19
Q

With aid of a sketch, explain a U-Tube manometer?

A

A ‘U’ tube manometer is a device that is used for comparing two pressures and consists of a clear glass or plastic tube containing a fluid.
If P1 is greater than P2 then some of the fluid will be displaced from one leg of the manometer to the other creating a displacement ‘h’.

Most common fluids are water and mercury.

P.129

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20
Q

With aid of a sketch, Explain a Bourdon tube pressure sensor?

A

One end of the tube is anchored at the
gauge inlet pipe, the other end
being free to move. As the pressure
increases within the tube it tries to
change from elliptical to a round
cross section. This forces the tube
to straighten and produces a
movement in the free end. The
movement is picked up and
amplified by a linkage attached to a
gear quadrant that rotates a pinion.
A pointer or needle attached to the pinion indicates the pressure on a calibrated scale.
A light coiled return spring eliminates backlash in the gearing.

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21
Q

Explain a Bourdon tube pressure gauge with Sensing, Transacting and Conditioning?

A
  • Sensing a pressure change within the Bourdon tube, relative to the atmospheric pressure surrounding the tube.
  • Transducing the change into a mechanical movement by using the resultant flexing of the tube
  • Conditioning the mechanical movement by the linkage and gears to produce an amplified rotary movement of the needle – the output.
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22
Q

What temperature is absolute zero?

A

-273 Degrees C

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23
Q
A
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24
Q

List the different types of Temperature sensors?

A
  • Thermometer
  • Gas Filled Bulb Thermometer
  • Bi-Metal Strip
  • Resistance Thermometer (PT100)
  • Thermocouple
  • Pyrometers
  • Thermistors
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25
Q

Explain a Bi-Metal strip?

A

Two metals joined together, as the different metals have different temperature coefficients of expansion, when heated one will expand or contract quicker than the other causing the metal to bend.

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26
Q

With aid of a sketch, explain a Resistance Thermometer?

A

A conductor of electricity will change its resistance as the
temperature changes.

Sensor has a thin piece of platinum at the end inside a metal sleeve like a probe. The amount of platinum is calculated to produce 100Ω at 0°C.

It can be used in junction with a Wheatstone bridge.

Rt=R0(1 + at)

P.139

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27
Q

Draw a Wheatstone bridge and explain why PT100s typically have 3 or 4 wires to connect them to the temperature indicating device?

A

The are connect in bridge Circuits as the device requires signal conditioning and the resistance of the leads need to be compensated. A 4th wire is used over long distances

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28
Q

Draw and explain the theory behind the workings of a thermocouple?

A

Two different wire materials connected via a junction. As the temperatures increase a current will flow around the circuit.

p.141

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29
Q

Use a diagram to explain how the theory is applied to produce a practical thermocouple temperature measuring device?

A

A third metal, copper is added to connect to a voltmeter, Amplifier and a junction where a set temperature can be referred to. The EMF is proportional to the difference between the hot junction and the reference of the cold junction.

P.142

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30
Q

How are thermocouples manufactured to cater for different temperature ranges?

A

By using different combinations of metals

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31
Q

Provide some examples of different Thermocouple combinations?

A

Type T
- Copper-Constantan
– Temperature range –270 – 400C

Type J
- Iron-Constantan
– Temperature range –210 -
1200°C

Type K
- Chromel-Alumel
- Temperature range –270 - 1320°C

Type B
- Platinum-Rhodium
– Temperature range 0 –1820°C

32
Q

Explain Pyrometers with an aid of a sketch?

A

Using the principle of black body radiation, it determines the surface temperature from the spectrum of radiation that the hot surface emits.

The three types are Optical, Radiation and two colour.

P.143

33
Q

List the different types of Flow sensors?

A
  • Change in pressure
  • Vane or gear flow meter
  • Turbine flow meter
  • Rotameter
34
Q

Explain how a change in pressure can be used to measure flow?

A

By having a restricted orifice in the pipeline, fluid can be monitored by using the first law of thermodynamics. Flow must be equal upstream as downstream of the reducer. If not the velocity energy of the fluid increases and the pressure energy will drop. This giving us High Pressure at a Low Velocity or a Low Pressure at a High Velocity.

35
Q

Explain how a Vane or gear can be used as a flow meter?

A

As the fluid passes over the vanes or gears, this sends a rotational signal to a dedicated counter. Used on fuel bunker line.

36
Q

Explain how a turbine flow meter can be used?

A

A rotor is free to spin against air or gas. A magnet acted to the rotor sends a signal ever revolution to a mounted detector. This is then calculated depending on the area and angle of the blades to provide a flow rate.

37
Q

Explain how a Rotameter works for flow measuring?

A

Is a tapered glass tube mounted vertically in parallel with the pipework. As flow is increased the float, which has a density greater than the fluid, Will be lifted until a balance against the flow is found. Providing a flow reading.

38
Q

List the different methods of level sensors?

A
  • Gauge Glass
  • Floatation Devices
  • Hydrostatic Pressure
  • The Bubbler Gauge
  • Ultrasonic sensor
  • Radar Sensor
  • Magnetic Float Switch
    -Capacitance Sensor
  • Pizzo Electric
39
Q

Explain a Gauge Glass?

A

It works in parallel with the tank and basically forms a U-Tube. They are fragile and need shut off cocks incase of an emergency.

40
Q

Explain the three types of floatation level sensors?

A

There are three types, the Mobray, a Linear Variable Differential Transformer (LVDT) and a Discrete.

The Mobray works on a float being attached to a bottom of a rod. As this transmits mechanical movement a series of on/ off signals are used from agents and proximity switches.

LVDT works the same but can provide continuous remote monitoring due to its variable electrical analogue signals.

Discrete, like the Mobary works on just a high and low float. As the fluid rises and moves to float a fixed magnet will cause another magnet in the fixed body to send a set of electrical signals showing on/ off.

41
Q

With aid of drawing, explain a flapper and nozzle pneumatic-mechanic signal?

A

Known as a Motion Balance Transmitter. Air is fed into the nozzle at a constant pressure. The air leaks from the nozzle at just above atmospheric pressure when at maximum. As the flapper moves closer to the nozzle, the pressure of the air inside the nozzle increases till the nozzle and flapper are flush, equalling 1.0 Bar.

But the system is too sensitive, the relationship between the input and output is only linear over a small part.

By fitting a feedback below this is eliminated as the increase in pressure will expand the bellows and move the flapper away.

This will only work in the 0.3-0.6 bar range so a pressure booster rely is fitted to get it around the 0-1.0 bar range. Air from the nozzle increases and pushes on a diaphragm which pushes on the spool valve to allow more air to flow through to the output signal.

p.164

42
Q

Explain how a Bubbler Gauge works?

A

A dip tube is fitted in the tank and air is supplied constantly then escapes from the bottom. The air pressure need to overcome the hydrostatic pressure is then pre calculated to give a reading of the level of the tank.

43
Q

Explain how a Ultrasonic sensor works?

A

An emitter & receiver module is mounted on the top of the tank. The time taken for the sound to reflect back to the receiver is calibrated to give a direct reading of tank level/content quantity.

44
Q

Explain how a Radar sensor works?

A

An emitter & receiver module is mounted on the top of the tank. The time taken for the sound to reflect back to the receiver is calibrated to give a direct reading of tank level/content quantity.

45
Q

Explain how a Magnetic Float Switch works?

A

A float with a magnet operates a series of read switches in a sealed tube, the switches change the resistance of the circuit which is converted into a level signal

46
Q

Explain how a Capacitance Sensor works?

A

The change in tank level will alter the dielectric between a probe & the side of the tank. This is translated into a level reading.

47
Q

Explain how a Pizzo Electric Sensor works?

A

The sensor is supplied with high frequency impulses, the amount of static pressure on the sensor will modify the amplitude of the return signal, giving an indication of level.

48
Q

What are the three basic ways of measuring systems to transmit a signal and explain each?

A

Discrete – A discrete system produces single on/off pulses or
individual steps.
Example: An on/off light switch.

Analogue – An analogue system produces an infinitely variable signal between the limits of the working range.
Example: The output from a mercury thermometer.

Digital – A digital system produces a combination of on/off signals
(binary code) that replicates a stepped analogue response.
Example: The input to a digital computer.

49
Q

What are the 7 basic methods of transmission?

A
  • Mechanical.
  • Pneumatic.
  • Hydraulic.
  • Electrical.
  • Light.
  • Sound.
  • Radio wave.
50
Q

List the pros and cons of Mechanical Transmission and the measuring systems used to transmit a signal?

A

Advantages:
* Robust.
* Long life span if properly maintained.
* Can transmit and amplify extremely powerful signals over short distances.
* Can have a wide temperature tolerance.
* Can be easily converted into other forms of transmission.

Disadvantages:
* Not practical for long distance transmission (above, say, a
metre).
* Prone to wear and corrosion.
* Can have high impedance losses caused by inertia and resistance.
* Prone to vibrational disturbance.

Can be only be used for discrete or analogue transmission

51
Q

List the pros and cons of Pneumatic Transmission and the measuring systems used to transmit a signal?

A

Advantages:
* The supply of the operating medium – if air – is unlimited.
* Relatively safe and can be used in most hazardous areas with little danger.
* Mechanically simple and robust.
* If air used, then no return to storage needed. Air can be allowed to leak away.
* Not usually sensitive to extremes of heat.

Disadvantages:
* Requires large pressure storage containers.
* Air needs careful cleaning and drying before use.
* Signal transmission by air is comparatively slow due to high
resistance and capacitive losses

Can only be used for discrete or analogue transmission, but can
replicate logic gates.

52
Q

List the pros and cons of Hydraulic Transmission and the measuring systems used to transmit a signal?

A

Advantages:
* Very high power to weight ratio.
* Mechanically simple and robust.
* Capable of transmitting a signal over long distances with little loss of power.
* Infinitely variable control.
* Depending on operating medium not too sensitive to temperature variation.
* When oil is used as the transmitting medium, then self-lubricating.

Disadvantages:
* Most operating fluids are relatively expensive and degrade over a period of time.
* Leaks cannot be tolerated and could pose an environmental threat.
* Oil under pressure can pose a fire and health risk.

Can only be used for discrete or analogue transmission.

53
Q

List the pros and cons of Electrical Transmission and the measuring systems used to transmit a signal?

A

Advantages:
* Easy to install, wiring is simpler to work with than pneumatic or hydraulic piping.
* Very cost effective to run.
* Highly flexible.
* Signal transmission is virtually instantaneous.
* Can multiplex i.e. transmit many different sets of digital data simultaneously.

Disadvantages:
* Requires a high degree of specialist knowledge to maintain.
* Heat sensitive (particularly high temperatures.)
* Easily damaged (particularly by water.)
* High voltages pose a health and fire risk.
* Vulnerable to external electrical and magnetic interference.

Can be used for discrete, analogue and digital transmission

54
Q

List the pros and cons of Fibre Optic Transmission and the measuring systems used to transmit a signal?

A

Advantages:
* Easy to install, simpler to work with than piping.
* Cost effective to run.
* Highly flexible.
* Signal transmission is virtually instantaneous.
* Can multiplex i.e. transmit many different sets of digital data simultaneously.
* Safer than electrical transmission and free from interference.

Disadvantages:
* Requires an additional electrical/light interface or transducer at each end of the transmission line.
* Signal requires boosting at set intervals along transmission path.

Can be used for discrete, analogue and digital transmission

55
Q

Draw and explain a Practical Motion Balance Transmitter?

A

Same as a Motion Balance Transmitter but rearranged so the nozzle is on the outside and a gain adjuster in in the middle.

56
Q

With aid of a drawing explain an Electro-Pneumatic transmission?

A

The electrical signal is first changed into a mechanical movement by an electromagnetic solenoid. The solenoid core, connected to a diaphragm, acts on a nozzle in the same way as a flapper. The nozzle is connected directly on to a device known as a volume booster that acts similarly to a pressure relay but increases the volume of air without increasing the pressure.

As the pressure increases in the nozzle it acts on the top diaphragm, sealing off the vent and opening the needle valve against the spring force. A large volume of air then passes through the needle valve seat, increasing the pressure in the outlet chamber. This change in pressure acts on the lower diaphragm, which moves upwards partially opening the vent and equalising the pressure between the nozzle and the outlet chamber.

p.167

57
Q

What are is the industry standard transmission/ conversion?

A

4 - 20 mA electrical current converting to 0.2 - 1 bar air pressure

58
Q

Sketch and label a P/I Converter?

A

A strain resistance pressure sensor. P - Pressure. I - Current.

Can transduce an analogue change in pressure into a varying electrical signal

59
Q

With aid of a sketch explain a Moving Coil Meter?

A

A moving coil meter consists of a stationary soft iron cylinder with a coil suspended around it. The coil is pivoted between two magnetic pole pieces and is connected to a pointer mechanism. The coil can move freely in the air gap between the pole pieces and the soft iron core.

When the incoming signal current instrument flows through the coil it produces a magnetic flux that reacts with the magnetic flux passing between the magnetic poles and causes the coil to rotate. It is weighted by a return spring.

Cheap, reliable, accurate, can accommodate a wide variety of ranges and accept a directly transmitted analogue signal

p.169

60
Q

Explain with an aid of a sketch of a pneumatic actuator?

A

A pneumatic actuator is made up of two parts, one being diaphragm motor, which converts a change in pneumatic pressure into a linear movement and the seconding being a valve body that varies the area through which the fluid flow must pass.

The diaphragm movement is opposed by a spring, which supplies a force that acts against the air signal pressure. Full valve travel typically occurs with a change of pressure on the diaphragm from 0.2 - 1.0 bar, but can be adjusted by varying the spring tension using the tension adjusting screw.

The size of the diaphragm depends on;
* The valve stroke.
* The operating force required

The valve body is a screw lift valve. this can be a single or double valve seat. The advantage of a single seat port is it closing off is more tight but the force need is much higher, requiring a larger diaphragm.

P.176

61
Q

With aid of a sketch show the 4 different types of Valve plugs?

A
  • Percentage ‘V’ Port
  • Percentage Parabolic
  • Linear
  • Quick Opening

P.178

62
Q

Sketch a graph of the 4 different types of Valve Plugs?

A

P. 179

63
Q

List the advantages and disadvantages of Pneumatic Diaphragm Valves?

A

Advantages:
* Robust.
* Can be used in hazardous areas.
* Will fail safe on loss of air signal.
* Fast acting.
* Relatively cheap to repair.

Disadvantages:-
* Require positioners and boosters to function accurately.
* Require complicated locking devices if the valve needs to be
held in position (fail set) in the event of an air failure.
* Relatively large.
* Require long runs of expensive air piping (with leakage and
loss of signal possible).
* Require a clean/dry air supply.

64
Q

Valve Hysteresis is caused from the following?

A

Resistance
- Comes from the friction of the valve stem guides and seals,
together with pressure forces from the controlled medium.

Inertia
- Comes from the mass of the moving components in the valve.

Capacitance
- Comes from the delay in build-up of pressure in the diaphragm
chamber from the relatively low volume air signal transmissions passing from the controller to the valve.

65
Q

How can we remove Hysteresis Losses and what functions do they provide?

A

By installing Air Pressure Boosters and Valve Positioners

  • By boosting the air pressure signal from the controller to ensure that the valve position signal from the process controller is executed quickly and accurately (reduce capacitance & resistance losses).
  • Adjust the amount that the valve spindle will travel for a given change in the air pressure signal i.e. effectively adjusting the gain or amplification of the valve (reduce resistance losses).
  • Position and hold the valve spindle in some position relative to the control air pressure signal supplied to it (reduce inertia losses).
66
Q

List the different types of actuators?

A
  • Pneumatic Diaphragm Valve Actuator
  • High Power Valve Actuator (Piston Actuator)
  • Electrical Driven Actuator
67
Q

Explain with an aid of a sketch how a High Power Valve Actuator works?

A

As air pressure increases in the pilot valve, Hydraulic fluid is able to flow and move the piston. As the piston moves, the feedback linkage produces a force on the spring assembly which will then return the piston to a neutral position.

P.185

68
Q

Explain with an aid of a sketch, how an electrical driven valve actuator works?

A

Like a pneumatic actuator, but the valve spindle is replaced with a threaded spindle that is liked to an internal geared wheel. This is driven by a combination of reduction gears and AC motors depending on the size and power required.

A potentiometer is fitted to give an accurate feed-back of the position of the spindle and allows precise control over the valve.

P.186

69
Q

With aid of a sketch, Explain Two Step Control

A

Referred to as an on/off control. It is the simplest type of process control action and consists of a controller (human or otherwise) switching energy into or out of a process and allowing the energy transfer to continue until it is no longer needed.

Examples are; A light switch; A simple water heater; A compressor and air bottle.

P.188

70
Q

List the problems with two step control?

A
  • It is difficult to establish a steady state.
  • There is no control over what happens within the process between the time that the energy is switched into or out of the process - the ‘dead band.’
  • If the system being controlled has a low energy demand capacity and a high energy supply capacity i.e. if the compressor is too big for the air demand, then the switching cycle can be very fast and lead to instability.
  • If the on/off limits are set too close to the desired value, then the
    correcting element will rapidly switch on or off as the actual value
    moves about the desired value – more instability
71
Q

What is the difference between a narrow and wide Dead band?

A

*A narrow ‘dead band’ = increased accuracy with less stability

  • A wide ‘dead band’ = reduced accuracy with increased stability.
72
Q

Explain Proportional Control?

A

A controllers ability to change the gain between the input and output signals. This allows the output from the controller to be proportional to the input signal.

73
Q

Explain the term Proportional Band?

A

The term proportional band is used to describe the amplification or gain that is applied to a control system.

74
Q

Explain the term Integral?

A

Integral action steadily corrects the difference between the desired value and the actual value as long as an error exists.

75
Q

Explain the term Derivative Control?

A

Derivative action is used to speed up the reaction of the controller to fast acting changes in the process. It produces a control output signal that is proportional to the speed of change of the error signal between the desired and actual values of the process. It adjusts the controller output to the rate of change of the input signal.

This is the difference between the set value and the actual valve. Also known as the error.

76
Q

Describe a Three Term Control and the benefit of it?

A

Three term control combines proportional, integral and derivative (P + I + D)

  • A rapid controller response to the rate of change between the desired and actual values of the process.
  • Adjustment of the gain of the control system.
  • Elimination of the resultant offset between the desired and actual
    values of the process.
77
Q

What are the advantages of using Electronic control over Pneumatic control?

A
  • Compact, robust control units.
  • Fast, direct signal transmission from sensors and to control
    actuators.
  • Simplicity of maintenance.
  • Cheaper to install.

Applied Marine Engineering (9 decks)

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