Which rotameter statement is true?

Which rotameter statement is true?

• A. Rotation of the bobbin within the Thorpe tube is important for accurate function
• B. The Thorpe tube increases in diameter from bottom to top
• C. Its accuracy is affected by changes in temperature and atmospheric pressure
• D. The rotameter for N2O and CO2 are interchangeable

Answer: (A)

The key idea is how the float (bobbin) behaves inside the Thorpe tube when gas flows. The flow rate is read from the height at which the float reaches equilibrium, a balance of forces: gravity pulling the bobbin downward, buoyancy and drag acting upward. The bobbin is designed to rotate freely as it rises, and that rotation is important because it helps keep the gap between the bobbin and the tube wall uniform. A consistent, rotating float reduces variations in drag caused by orientation or minor imperfections in the tube, so the float settles at a height that accurately reflects the chosen flow. If the bobbin didn’t rotate reliably, the drag could vary with orientation or surface irregularities, leading to measurement drift or inaccuracy. While other features of the thorpe tube—like its tapered shape and the need for gas-specific calibration—also play roles in how readings are obtained, the rotation of the bobbin is the factor most directly tied to maintaining accurate flow measurement in routine use.

The key idea is how the float (bobbin) behaves inside the Thorpe tube when gas flows. The flow rate is read from the height at which the float reaches equilibrium, a balance of forces: gravity pulling the bobbin downward, buoyancy and drag acting upward. The bobbin is designed to rotate freely as it rises, and that rotation is important because it helps keep the gap between the bobbin and the tube wall uniform. A consistent, rotating float reduces variations in drag caused by orientation or minor imperfections in the tube, so the float settles at a height that accurately reflects the chosen flow. If the bobbin didn’t rotate reliably, the drag could vary with orientation or surface irregularities, leading to measurement drift or inaccuracy. While other features of the thorpe tube—like its tapered shape and the need for gas-specific calibration—also play roles in how readings are obtained, the rotation of the bobbin is the factor most directly tied to maintaining accurate flow measurement in routine use.