What device is commonly used to measure flow rate in hydraulic systems?

• A. Thermocouple
• B. Manometer
• C. Pressure gauge
• D. Rotameter

Answer: (D)

Measuring flow rate involves determining how much liquid passes per unit time. A rotameter uses the variable-area principle: a tapered tube with a float inside. As the upstream flow increases, the float rises until the drag from the moving liquid balances the float’s weight. The float settles at a height that corresponds to a specific flow rate, which you read on a scale marked on the tube. This creates a direct, visual readout of volumetric flow without needing external power, making it a simple and common choice for hydraulic systems.

The other options don’t measure flow directly: a thermocouple detects temperature, a manometer measures pressure difference, and a pressure gauge measures pressure. None of these by itself provides the amount of liquid passing per unit time.