What measurement device is a variable-area flowmeter used in closed tubes?

A variable-area flowmeter measures the flow rate of liquids or gases by allowing the area through which the fluid flows to change as the flow rate varies. The rotameter, which is a specific type of variable-area flowmeter, is designed with a tapered tube and a float that rises or falls based on the flow rate. As the flow increases, the float moves upward, expanding the area for flow, and vice versa. This direct indication of flow rate makes rotameters an effective tool for monitoring flow in closed systems.

The other devices mentioned serve different functions. A pressure gauge measures the pressure within a system rather than flow; a telemetry system is used for remote data transmission and does not measure flow directly; and a weir gauge is used to measure flow over weirs, primarily in open channels, rather than in closed tubes. This distinction underscores why the rotameter is the correct choice for this particular question about measuring flow in closed conduits.