Which statement about rotameters is false?

Which statement about rotameters is false?

• 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: (D)

Rotameters measure flow by balancing buoyancy and drag on a float as gas rises through a tapered tube; the float’s vertical position directly indicates the flow rate. The tube’s diameter increases as you move upward, so a given flow will push the float higher to reach equilibrium. Each gas has its own density and viscosity, so the instrument is calibrated for specific gases. Swapping a rotameter between gases like N2O and CO2 would give incorrect readings because their flow characteristics differ, making the statement about interchangeability false. The float (bobbin) is free to rotate inside the tube, which helps minimize friction and allows the float to settle at the correct height for a given flow, supporting accurate measurement. Temperature and atmospheric pressure affect gas density and volume, so readings can deviate from the calibration unless conditions are accounted for.

Rotameters measure flow by balancing buoyancy and drag on a float as gas rises through a tapered tube; the float’s vertical position directly indicates the flow rate. The tube’s diameter increases as you move upward, so a given flow will push the float higher to reach equilibrium. Each gas has its own density and viscosity, so the instrument is calibrated for specific gases. Swapping a rotameter between gases like N2O and CO2 would give incorrect readings because their flow characteristics differ, making the statement about interchangeability false. The float (bobbin) is free to rotate inside the tube, which helps minimize friction and allows the float to settle at the correct height for a given flow, supporting accurate measurement. Temperature and atmospheric pressure affect gas density and volume, so readings can deviate from the calibration unless conditions are accounted for.