AME 9 End of Unit

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.

Correct Answer:

A. Tachometer

Explanation:

• A. Correct: A tachometer is specifically designed to measure rotational speed, making it suitable for measuring the speed of a ship’s main propeller shaft in RPM.
• B. Incorrect: Pressure transducers measure pressure, not rotational speed.
• C. Incorrect: Strain gauges measure strain or deformation, not rotational speed.
• D. Incorrect: Temperature sensors measure temperature, not rotational speed.
• E. Incorrect: Flowmeters measure the flow rate of fluids, not rotational speed.

Correct Answer:

B. A floating buoy that rises and falls with the liquid level, activating a switch to send an electrical signal to a control system.

Explanation:

• A. Incorrect: A weighted ball typically does not produce an electrical or digital output signal; it usually provides a direct mechanical action.
• B. Correct: A floating buoy or float switch moves with the liquid level and triggers an electrical switch when it reaches a certain point. This switch sends a signal to a remote control system, which can then adjust the liquid level by controlling pumps or valves.
• C. Incorrect: A submerged probe might measure pressure changes, but it is not a flotation device and does not directly control the liquid level through flotation.
• D. Incorrect: A rotating vane typically measures flow rate rather than liquid level and does not operate based on flotation principles.

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:

C. A strain gauge measures strain by detecting changes in electrical resistance as the material to which it is attached is stretched or compressed.

Explanation:

• A. Incorrect: A strain gauge does not operate based on changes in capacitance; it operates based on changes in resistance.
• B. Incorrect: While changes in electrical resistance can be used to measure temperature (as in thermistors), a strain gauge specifically measures mechanical strain, not temperature.
• C. Correct: The primary operation of a strain gauge involves measuring strain through changes in electrical resistance as the material is deformed.
• D. Incorrect: Strain gauges do not measure pressure through magnetic flux; they measure strain through resistance changes.

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. Ceramic materials and metal oxides.

Explanation:

• C. Correct: Thermistors are commonly constructed from ceramic materials and metal oxides. These materials have properties that change significantly with temperature, allowing thermistors to accurately measure temperature variations.
• A. Incorrect: Silicon and gallium arsenide are used in semiconductor devices, not typically in thermistors.
• B. Incorrect: Iron and cobalt are not standard materials for thermistor construction; they are more commonly used in magnetic and structural applications.
• D. Incorrect: Aluminum and copper are metals used in various electrical applications but are not typical materials for thermistor construction.

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.

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. Ultrasound waves are transmitted towards the fluid surface, and the time it takes for the waves to reflect back to the sensor is used to calculate the fluid level.

Explanation:

• A. Incorrect: Ultrasound waves are used to measure the time it takes for the waves to reflect back from the fluid surface, not to reach the bottom of the tank.
• B. Correct: This method accurately describes how ultrasound level sensors work. The sensor emits ultrasound waves towards the fluid surface, and the time delay of the reflected signal is used to determine the fluid level.
• C. Incorrect: Ultrasound level measurement does not rely on energy absorption by the fluid.
• D. Incorrect: Ultrasound waves are directed towards the fluid surface rather than along the tank walls to measure fluid level.

Correct Answer:

A. The bi-metallic strip bends in response to temperature changes, and this bending movement operates a mechanical pointer or switch to indicate the temperature.

Explanation:

• A. Correct: Bi-metallic strips consist of two different metals bonded together. When the temperature changes, the metals expand or contract at different rates, causing the strip to bend. This bending can move a mechanical pointer or operate a switch, providing a direct temperature indication.
• B. Incorrect: Bi-metallic strips do not generate an electrical signal. They rely on mechanical bending to measure temperature, rather than producing an electrical output.
• C. Incorrect: Bi-metallic strips do not change color with temperature. They function based on physical bending rather than color change.
• D. Incorrect: Bi-metallic strips do not measure temperature by changing thermal conductivity. They use mechanical deformation due to different thermal expansion rates of the metals involved.