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 Answer:

B. To detect and measure temperature changes in various systems.

Explanation:

• B. Correct: Thermistors are widely used on vessels to measure and monitor temperature changes in various systems, such as engine cooling systems, fuel tanks, and environmental monitoring.
• A. Incorrect: Thermistors do not measure the concentration of dissolved gases. This task is typically performed by gas sensors.
• C. Incorrect: Thermistors do not measure pH levels. pH sensors or probes are used for this purpose.
• D. Incorrect: Thermistors are not used to measure the speed of the vessel. Speed measurement is typically done using speed logs or other specialized instruments.

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. 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:

A. Platinum and 100 Ohms at 0°C: The "PT" stands for platinum, and "100" indicates that the probe has a resistance of 100 ohms at 0°C.

Explanation:

• A. Correct: The PT100 probe is named for its use of platinum ("PT") and its resistance of 100 ohms at 0°C. This naming convention helps identify the specific type of resistance thermometer and its base resistance value.
• B. Incorrect: The "100" does not refer to the measurement range of the probe, but rather to its resistance value at 0°C.
• C. Incorrect: The "100" in PT100 does not refer to the number of wire turns in the sensing element; it refers to the resistance value at 0°C.
• D. Incorrect: The "100" does not indicate the length of the sensing element but rather the resistance at 0°C. The probe length is not specified by this designation.

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:

C. By detecting changes in the capacitance between the probe and the tank wall as the liquid level rises or falls, which is then converted into an electrical signal.

Explanation:

• A. Incorrect: Capacitance probes do not generate or use magnetic fields to measure liquid levels.
• B. Incorrect: Capacitance probes measure changes in capacitance, not resistance, to determine liquid levels.
• C. Correct: A capacitance probe works by detecting changes in the capacitance between the probe and the tank wall. As the liquid level changes, the dielectric constant between the probe and the wall changes, altering the capacitance. This change is then converted into an electrical signal that can be used to measure the liquid level.
• D. Incorrect: This describes the operation of an ultrasonic level sensor, not a capacitance probe.

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. 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.

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.