A Venturi meter measures flow rate indirectly by measuring a pressure difference, and the instrument used to sense that difference is a manometer (typically a U-tube differential manometer) connected between the inlet section and the throat.

The working principle is the Bernoulli energy balance combined with continuity. As the fluid accelerates through the converging cone into the narrow throat, its velocity rises (continuity: A₁V₁ = A₂V₂) and, by Bernoulli's equation, its static pressure falls. The pressure drop ΔP = P₁ − P₂ between inlet and throat is directly related to the velocity and hence the discharge:

Q = C_d · A₂ · √(2gΔh / (1 − (A₂/A₁)²))

where Δh is the manometric head corresponding to ΔP and C_d is the coefficient of discharge (about 0.95–0.98 for a well-made Venturi). Because the measurement depends entirely on comparing two pressures across the device, a differential-pressure device — the manometer — is exactly what is required.

The other instruments do not fit:

• A Bourdon gauge reads a single (gauge) pressure at one point, not the difference between two points, so it cannot give ΔP directly.
• A rotameter is itself an independent variable-area flowmeter (a float in a tapered tube); it is an alternative to the Venturi, not a device that reads a Venturi's pressure difference.
• A Pitot tube senses stagnation (velocity) pressure at a point to estimate local velocity; it is not connected across the Venturi's inlet and throat and does not measure this differential pressure.

Hence the correct instrument is the manometer.