AME 9 End of Unit

Correct Answer:

A. A Bourdon tube cannot be used to measure temperature as it is specifically designed for pressure measurement.

Explanation:

• A. Correct: A Bourdon tube is specifically designed for measuring pressure. It operates by detecting changes in pressure that cause mechanical movement of the tube, which is not suitable for measuring temperature.
• B. Incorrect: Filling a Bourdon tube with a temperature-sensitive liquid is not a standard method for measuring temperature. This approach does not apply to the operation of a Bourdon tube.
• C. Incorrect: A Bourdon tube does not connect to a temperature sensor for measuring temperature. It is designed for pressure measurement and does not integrate with temperature sensors in this way.
• D. Incorrect: A Bourdon tube operates based on pressure changes, not temperature. It does not convert temperature changes into mechanical movement directly.

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. Pressure gauge

Explanation:

• A. Correct: A pressure gauge is designed to measure local pressure readings, making it suitable for measuring the pressure in a ship’s fire-main.
• B. Incorrect: A flowmeter measures flow rate, not pressure.
• C. Incorrect: A thermocouple measures temperature, not pressure.
• D. Incorrect: A strain gauge measures strain, not pressure.
• E. Incorrect: A tachometer measures rotational speed, not pressure.

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.

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.

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 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. They have limited accuracy and resolution compared to electronic temperature sensors.

Explanation:

• A. Correct: Bi-metallic strips generally have lower accuracy and resolution compared to electronic sensors like thermocouples or RTDs. They are better suited for applications where high precision is not critical.
• B. Incorrect: Bi-metallic strips are sensitive to temperature changes, but they can have a slower response time compared to some electronic sensors. The issue is more about accuracy rather than sensitivity or response time.
• C. Incorrect: Bi-metallic strips are not typically affected by electromagnetic interference, as they operate on mechanical bending rather than electronic signals.
• D. Incorrect: Bi-metallic strips generally do not require frequent calibration. They are mechanical devices that maintain their calibration unless physically damaged or altered.

Correct Answer:

B. By converting rotational movement into a variable resistance through a rotating wiper that contacts a resistive element, which then provides a corresponding voltage output.

Explanation:

• B. Correct: A potentiometer measures rotary movement by having a rotating wiper (contact) that moves along a circular resistive track. As the wiper rotates, it changes the resistance in the circuit. The corresponding voltage output from the potentiometer varies with the angle of rotation, allowing measurement of rotary movement.
• A. Incorrect: A linear resistive track is used for linear movement measurements, not rotary. Potentiometers for rotary movement have a circular resistive track.
• C. Incorrect: This describes a capacitive position sensor, not a potentiometer. A potentiometer does not measure rotary movement by changes in capacitance.
• D. Incorrect: This describes a type of rotary position sensor or analog meter, not a potentiometer. Potentiometers do not use magnetic fields for measuring rotary movement.

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. Linear variable differential transformer (LVDT)

Explanation:

• A. Correct: An LVDT is commonly used for precise and remote position measurement, making it suitable for measuring the position of a diesel engine governor or fuel rack.
• B. Incorrect: Pressure transducers measure pressure, not position.
• C. Incorrect: Thermocouples measure temperature, not position.
• D. Incorrect: Ultrasonic flowmeters measure flow rate, not position.
• E. Incorrect: Proximity sensors detect the presence or absence of an object but are not typically used for precise position measurements.