Question

Which of the following measuring device is not suitable for the measurement of the rate of flow of liquids

• A. Nutating-Disc Meter
• B. Hot-Wire Anemometer
• C. Laser-Doppler Anemometer
• D. Venturi Meter

Hint:

Hot-wire anemometers are designed for gas flow measurement, as their heat transfer sensitivity is less effective in liquids where conduction dominates.

Answer 1

The Correct Option is B

Step 1: Understand flow measurement devices.
Flow measurement devices are designed for specific fluids (liquids or gases) based on their operating principles. We need to identify which device is not suitable for measuring the flow rate of liquids. Step 2: Analyze each device.
Nutating-Disc Meter: A positive displacement flow meter commonly used for liquids (e.g., water meters). It measures flow by the movement of a disc that “nutates” as liquid passes through, displacing a fixed volume per cycle. It is suitable for liquids.
Hot-Wire Anemometer: Measures flow velocity by the cooling effect of a fluid on a heated wire. The wire’s resistance changes with temperature, which is correlated to flow velocity. This device is primarily used for gases (e.g., air) because the heat transfer characteristics in gases are more predictable and sensitive to velocity changes. In liquids, heat transfer is dominated by conduction and convection, making the hot-wire anemometer less accurate and unsuitable for liquid flow measurement.
Laser-Doppler Anemometer: A non-intrusive device that measures flow velocity using the Doppler shift of laser light scattered by particles in the fluid. It can be used for both liquids and gases, as long as the fluid contains scattering particles (e.g., in water with suspended particles).
Venturi Meter: Measures flow rate based on the pressure drop across a constricted section of pipe, using Bernoulli’s principle. It is widely used for liquids (e.g., water, oil) and gases, making it suitable for liquid flow measurement.
Step 3: Evaluate the options.
(1) Nutating-Disc Meter: Incorrect, as it is designed for measuring liquid flow rates. Incorrect.
(2) Hot-Wire Anemometer: Correct, as it is primarily used for gases and is not suitable for liquid flow measurement due to poor sensitivity and accuracy in liquids. Correct.
(3) Laser-Doppler Anemometer: Incorrect, as it can measure flow rates of both liquids and gases. Incorrect.
(4) Venturi Meter: Incorrect, as it is suitable for measuring liquid flow rates. Incorrect. Step 4: Select the correct answer.
The hot-wire anemometer is not suitable for the measurement of the rate of flow of liquids, matching option (2).