AME 9 End of Unit

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:

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:

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. Temperature gauges and thermostats

Explanation:

• A. Correct: Bi-metallic strips are commonly used in temperature gauges and thermostats. They are designed to bend with temperature changes, making them suitable for mechanical temperature measurement and control.
• B. Incorrect: Bi-metallic strips are not typically used in pressure sensors. Pressure sensors generally use different technologies like strain gauges or piezoelectric materials.
• C. Incorrect: Bi-metallic strips are not used in flow meters. Flow meters typically use methods such as electromagnetic, ultrasonic, or turbine principles.
• D. Incorrect: Bi-metallic strips are not used as vibration dampers. Vibration damping is usually handled by materials or devices specifically designed for that purpose, such as rubber mounts or damping pads.

Correct Answer:

A. To Compensate for Lead Wire Resistance: The additional wires are used to compensate for the resistance of the lead wires, ensuring accurate temperature measurements by measuring only the resistance of the sensing element itself.

Explanation:

• A. Correct: PT100 probes use three or four wires to separate the current-carrying wires from the measurement wires. This configuration eliminates errors caused by the resistance of the lead wires, ensuring that the temperature measurement reflects only the resistance of the sensing element.
• B. Incorrect: The extra wires do not enable measurement across multiple temperature ranges. The primary function is to compensate for lead wire resistance.
• C. Incorrect: The additional wires are not for improving mechanical strength but for ensuring measurement accuracy by accounting for lead wire resistance.
• D. Incorrect: The extra wires do not enable connecting multiple probes; they are used for accurate measurement of a single probe by compensating for lead wire resistance.

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.

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. Non-contact measurement, reducing the risk of contamination. C. Ability to measure fluid levels in tanks with corrosive or hazardous materials.

Explanation:

• A. Correct: One of the main advantages of ultrasound level measurement is that it is a non-contact method, which means there is no direct contact with the fluid, reducing the risk of contamination.
• B. Incorrect: Ultrasound measurement is generally not highly sensitive to temperature changes, which is not typically considered an advantage.
• C. Correct: Ultrasound is effective in measuring fluid levels in tanks containing corrosive or hazardous materials, as it does not require direct contact with the fluid.
• D. Incorrect: While environmental factors can affect measurements, regular calibration due to such sensitivity is not an advantage but rather a limitation.
• E. Incorrect: Ultrasound can measure fluid levels in a variety of fluids, not just transparent ones.

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:

A. Thermocouple

Explanation:

• A. Correct: Thermocouples are widely used for measuring high temperatures, including diesel engine exhaust temperatures, due to their wide temperature range and accuracy.
• B. Incorrect: Pressure gauges measure pressure, not temperature.
• C. Incorrect: Ultrasonic level sensors measure liquid levels, not temperature.
• D. Incorrect: Flowmeters measure the flow rate of liquids or gases, not temperature.
• E. Incorrect: Strain gauges measure deformation or strain, not temperature.

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. 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.