In a rotameter, at low gas flow, which physical property primarily governs the accuracy of the flow indication?

In a rotameter, at low gas flow, which physical property primarily governs the accuracy of the flow indication?

• A. Gas density
• B. Gas viscosity
• C. Temperature
• D. Humidity

Answer: (B)

In a rotameter, the float rises until the drag from the flowing gas balances the float’s weight minus buoyancy. At low gas flow, the flow around the float is slow and viscous-dominated (low Reynolds number), so the drag force is governed mainly by the gas’s viscosity. Because drag scales with viscosity for a given flow around the float, changes in viscosity directly alter the float’s position for the same actual flow, making viscosity the key factor that sets the accuracy of the flow indication at low flow. Density (buoyancy) still plays a role, but the direct driver of the low-flow calibration is viscosity. Humidity has negligible effect, and temperature can influence viscosity, but the primary property affecting accuracy in this regime is viscosity.

In a rotameter, the float rises until the drag from the flowing gas balances the float’s weight minus buoyancy. At low gas flow, the flow around the float is slow and viscous-dominated (low Reynolds number), so the drag force is governed mainly by the gas’s viscosity. Because drag scales with viscosity for a given flow around the float, changes in viscosity directly alter the float’s position for the same actual flow, making viscosity the key factor that sets the accuracy of the flow indication at low flow. Density (buoyancy) still plays a role, but the direct driver of the low-flow calibration is viscosity. Humidity has negligible effect, and temperature can influence viscosity, but the primary property affecting accuracy in this regime is viscosity.