In a Thorpe tube rotameter, how does the bore diameter change from bottom to top?

In a Thorpe tube rotameter, how does the bore diameter change from bottom to top?

• A. It increases from bottom to top
• B. It decreases from bottom to top
• C. It remains constant
• D. It varies with the gas

Answer: (A)

The bore of a Thorpe tube becomes larger as you move upward. In this flowmeter, a float rises in a vertically oriented, tapered tube so that at higher flow rates the float sits higher in sections with a larger cross-sectional area. When the gas pushes the float up, entering progressively wider portions reduces the flow velocity and drag around the float, allowing a new equilibrium at a higher position. This increasing diameter from bottom to top enables the float to rise with increasing flow, producing a usable scale. The exact gas density can shift buoyancy and calibration, but the bore geometry itself is fixed and widens upward.

The bore of a Thorpe tube becomes larger as you move upward. In this flowmeter, a float rises in a vertically oriented, tapered tube so that at higher flow rates the float sits higher in sections with a larger cross-sectional area. When the gas pushes the float up, entering progressively wider portions reduces the flow velocity and drag around the float, allowing a new equilibrium at a higher position. This increasing diameter from bottom to top enables the float to rise with increasing flow, producing a usable scale. The exact gas density can shift buoyancy and calibration, but the bore geometry itself is fixed and widens upward.