AME 9 End of Unit

Correct Answer:

B. The strain gauge is connected in a Wheatstone bridge circuit to measure the small changes in resistance accurately, which are then converted into pressure readings.

Explanation:

• A. Incorrect: While voltage changes can be measured, this method is not as precise as using a Wheatstone bridge for small resistance changes.
• B. Correct: The Wheatstone bridge circuit is commonly used with strain gauges because it allows for the precise measurement of very small changes in resistance, which correspond to pressure changes.
• C. Incorrect: A simple voltage divider circuit does not provide the accuracy needed for measuring small changes in resistance due to pressure.
• D. Incorrect: While amplifiers are sometimes used, the key component for accurate measurement is the Wheatstone bridge, which this option does not mention.

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:

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.

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. 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.

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:

A. Two Dissimilar Metals Produce a Voltage: A thermocouple operates by generating a voltage when two dissimilar metals are joined together and exposed to different temperatures, with the voltage being proportional to the temperature difference.

Explanation:

• A. Correct: A thermocouple consists of two different metals joined at one end. When this junction is exposed to different temperatures, it produces a voltage that can be measured. The voltage generated is directly related to the temperature difference between the junctions, allowing for temperature measurement.
• B. Incorrect: A thermocouple does not operate based on the expansion of a single metal but on the voltage produced by the junction of two dissimilar metals.
• C. Incorrect: Changes in electrical resistance are not the basis of thermocouple operation; that principle is more relevant to resistance temperature detectors (RTDs).
• D. Incorrect: Thermocouples do not operate based on magnetic fields; they generate voltage from the temperature difference at the junction of two dissimilar metals.

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 Answers:

A. Monitoring fuel tank levels.
C. Detecting the presence of water in the bilge.

Explanation:

• A. Correct: Capacitance probes are commonly used to monitor liquid levels, such as in fuel tanks.
• B. Incorrect: Flow rate measurement typically involves devices like impeller flowmeters or ultrasonic flow sensors, not capacitance probes.
• C. Correct: Capacitance probes can detect the presence of liquids, making them suitable for bilge water detection.
• D. Incorrect: Temperature measurement is typically done using thermocouples or resistance thermometers, not capacitance probes.
• E. Incorrect: Pressure measurement is typically performed with pressure sensors like Bourdon tubes, not capacitance probes.

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.

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.