AME 9 End of Unit

Correct Answer:

A. Strain gauge

Explanation:

• A. Correct: Strain gauges are used to measure the deformation or strain in materials, including the twist or torsion of a propeller shaft.
• B. Incorrect: Thermocouples measure temperature, not mechanical deformation.
• C. Incorrect: Ultrasonic thickness gauges measure material thickness, not twist or deformation.
• D. Incorrect: Pressure transducers measure pressure, not mechanical deformation.
• E. Incorrect: Flowmeters measure the flow rate of fluids, not mechanical deformation or twist.

Correct Answers:

A. Limited effectiveness in tanks with complex internal structures. E. Difficulty in measuring fluid levels in tanks with foam or vapor presence.

Explanation:

• A. Correct: Ultrasound can be less effective in tanks with complex internal structures, such as baffles, which can cause signal reflections and errors.
• B. Incorrect: Ultrasound can measure the level of opaque fluids, so this is not a limitation.
• C. Incorrect: While temperature and pressure can affect accuracy, they are not the primary limitations in most applications.
• D. Incorrect: While ultrasound can be accurate, its performance can vary depending on the fluid density, making this statement incorrect as a limitation.
• E. Correct: The presence of foam or vapor can interfere with the ultrasound signal, making it difficult to obtain accurate measurements.

Correct Answer:

A. Flowmeter

Explanation:

• A. Correct: A flowmeter is specifically designed to measure the flow rate of fluids, making it suitable for measuring the flow rate of a water maker.
• B. Incorrect: A pressure transducer measures pressure, not flow rate.
• C. Incorrect: A thermocouple measures temperature, not flow rate.
• D. Incorrect: A strain gauge measures strain, not flow rate.
• E. Incorrect: A tachometer measures rotational speed, not flow rate.

Correct Answer:

D. They may give inaccurate readings if the liquid has a variable dielectric constant.

Explanation:

• A. Incorrect: While temperature can affect many sensors, capacitance probes are primarily affected by changes in the dielectric constant of the liquid they measure, rather than temperature alone.
• B. Incorrect: Capacitance probes are generally compact and do not require a large amount of space.
• C. Incorrect: Although corrosion is a concern in marine environments, proper material selection and protection usually mitigate this issue for capacitance probes.
• D. Correct: Capacitance probes rely on the dielectric constant of the liquid to measure level accurately. If the dielectric constant varies, it can lead to inaccurate readings.

Correct Answer:

A. Platinum Wire Wrapped Around a Ceramic Core: The sensing element consists of a thin platinum wire wound around a ceramic core. The resistance of the platinum wire changes with temperature, and this change is used to measure temperature.

Explanation:

• A. Correct: The PT100 probe uses a sensing element made of a thin platinum wire wound around a ceramic core. Platinum is chosen for its stable and predictable change in resistance with temperature, providing accurate temperature measurements.
• B. Incorrect: Copper wire is not used in PT100 probes. While copper wire does change resistance with temperature, it does not have the stability and precision required for PT100 sensors.
• C. Incorrect: This describes a thermocouple, not a PT100 probe. Thermocouples use two different metals to create a voltage based on temperature differences.
• D. Incorrect: While resistance film on a substrate can be used in some temperature sensors, it is not the typical construction for PT100 probes. PT100 sensors rely on platinum wire for its accuracy and stability.

Correct Answer:

A. Radar Level Sensors, Ultrasonic Sensors, Capacitance Sensors, Float Switches, Laser Level Sensors

Explanation:

• A. Correct:
  • Radar Level Sensors: Use radio waves to detect the level of the fuel in the tank.
  • Ultrasonic Sensors: Use ultrasonic waves to measure the distance to the surface of the fuel.
  • Capacitance Sensors: Measure changes in capacitance caused by the fuel level.
  • Float Switches: Use a float that moves with the fuel level and activates switches to indicate levels.
  • Laser Level Sensors: Use laser beams to measure the distance to the fuel surface.
• B. Incorrect:
  • Thermocouples, Strain Gauges, Pressure Transducers, Vibration Sensors, Fiber Optic Sensors: While some of these sensors are used in various industrial applications, they are not typically used for remote level monitoring of fuel oil tanks.
• C. Incorrect:
  • Thermometers, pH Meters, Magnetic Field Sensors, Humidity Sensors, Sound Level Meters: These instruments are used for different types of measurements and are not suitable for level monitoring of fuel tanks.
• D. Incorrect:
  • Speed Sensors, Proximity Sensors, Displacement Sensors, Optical Sensors, Gas Detectors: These sensors are used in various applications but do not generally serve the purpose of remotely monitoring fuel oil levels.

Correct Answer:

A. A Bourdon tube can be used to measure the temperature of the engine room exhaust gases, typically located near the exhaust manifold.

Explanation:

• A. Correct: Although Bourdon tubes are primarily used for pressure measurement, in some specific applications, they might be used in conjunction with other systems to measure temperature, such as monitoring exhaust gases where accurate pressure and temperature data are crucial.
• B. Incorrect: The measurement of fuel oil temperature would typically be done with a thermometer designed for liquids, not a Bourdon tube.
• C. Incorrect: Temperature measurements of seawater intake are generally performed with temperature sensors or probes designed for such conditions, not with a Bourdon tube.
• D. Incorrect: Measuring the temperature of boiler water typically requires temperature sensors designed for high-temperature environments, rather than a Bourdon tube.

Correct Answer:

B. The rotation of the impeller generates pulses, which are counted by a microcontroller and then converted into a digital readout of flow rate in litres per minute.

Explanation:

• A. Incorrect: While some flowmeters might use magnetic sensors, the generation of a magnetic field by the impeller itself does not directly convert into a digital readout without electronic processing.
• B. Correct: In an impeller-type flowmeter, the rotation of the impeller creates pulses as it interrupts or triggers a sensor (often magnetic or optical). These pulses are counted by a microcontroller, which then calculates the flow rate and displays it as a digital value in litres per minute.
• C. Incorrect: An impeller-type flowmeter does not produce a varying resistance; instead, it typically generates pulses related to the flow rate.
• D. Incorrect: While the impeller's rotation is related to the flow rate, the output is not directly digital; it requires processing (such as counting pulses) to convert the rotational speed into a digital flow rate display.

Explanation:

• A. Correct: This is the correct formula. The strain ($\epsilon$) is calculated by the change in resistance ($\Delta R$) divided by the original resistance ($R$), multiplied by the gauge factor (GF).
• B. Incorrect: This formula is for calculating percentage elongation or strain in terms of length change, not specifically related to strain gauges.
• C. Incorrect: This formula is for calculating stress ($\sigma$), not strain.
• D. Incorrect: This is the formula for calculating the equivalent resistance of two resistors in parallel, not related to strain gauges.

Correct Answer:

C. Thermistors operate by changing their electrical resistance in response to temperature changes.

Explanation:

• C. Correct: The principle of operation of a thermistor is based on its electrical resistance changing with temperature. The resistance of a thermistor decreases or increases depending on whether it is a Negative Temperature Coefficient (NTC) or Positive Temperature Coefficient (PTC) thermistor.
• A. Incorrect: Changes in capacitance are not the principle of operation for thermistors. This principle applies to other types of temperature sensors.
• B. Incorrect: Magnetic field strength is not used in the operation of thermistors. This principle might apply to other types of sensors, like magnetic field sensors.
• D. Incorrect: Changes in light absorption are not related to thermistors. This principle is more related to optical temperature sensors or photodetectors.

Correct Answers:

A. Monitoring the strain on the ship’s hull to detect structural weaknesses.

C. Monitoring the load on a ship's cargo crane to prevent overloading.

Explanation:

• A. Correct: Strain gauges are used to monitor the strain on a ship's hull, which can help in detecting structural weaknesses or potential failures.
• B. Incorrect: Strain gauges are not used for measuring temperature; thermocouples or resistance temperature detectors (RTDs) would be used for that purpose.
• C. Correct: Strain gauges are commonly used to monitor the load on cargo cranes to ensure that they are not overloaded, which is crucial for safety.
• D. Incorrect: Strain gauges do not measure speed; devices like tachometers are used for this.
• E. Incorrect: Strain gauges do not measure pH levels; specialized sensors are required for pH measurement.