AME 9 End of Unit

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.

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. Using an electromagnetic flow meter with a digital output to a control panel.

Explanation:

• A. Incorrect: An orifice plate with a local differential pressure gauge can measure flow rate, but it does not provide a remote measurement.
• B. Incorrect: A rotameter typically provides a local indication of flow rate and does not transmit data remotely.
• C. Correct: An electromagnetic flow meter can accurately measure flow rate and transmit this data digitally to a remote control panel, making it suitable for remote monitoring.
• D. Incorrect: A Venturi meter can measure flow rate, but when paired with a mechanical dial, it is more suited for local measurement rather than remote measurement.

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:

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. 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:

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. A potentiometer varies resistance within a circuit by adjusting a sliding contact along a resistive track, allowing it to control voltage and current flow.

Explanation:

• B. Correct: A potentiometer operates by varying the resistance in a circuit through a sliding or rotating contact along a resistive element. This adjustment changes the voltage and current flow in the circuit, making it useful for controlling and calibrating electrical signals.
• A. Incorrect: This describes a voltmeter or digital voltmeter rather than a potentiometer. A potentiometer does not provide a direct digital readout of voltage.
• C. Incorrect: This describes a principle more similar to an analog meter or electrical transducer, not a potentiometer. A potentiometer’s primary function is to adjust resistance, not to convert mechanical movement into an electrical signal.
• D. Incorrect: This describes the operation of a temperature sensor or thermistor, not a potentiometer. Potentiometers are not used to measure temperature directly; they adjust resistance to control electrical parameters.

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. 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 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.