Pressure measurement – purpose of a differential manometer A differential manometer is specifically used to measure the following quantity in fluid systems and pipelines:

Introduction:Differential manometers are classic hydrostatic devices used to compare pressures at two locations. Unlike simple piezometers that read gauge pressure at a single tap, a differential manometer directly provides the pressure difference, which is essential for flow measurement, loss calculations, and instrument calibration.

Given Data / Assumptions:

• Two pressure taps connect to limbs of a U-tube filled with a manometric fluid (often mercury or another immiscible, heavier liquid).
• Fluids are static inside the manometer; temperature effects are negligible.
• Manometric fluid does not mix with the line fluid.

Concept / Approach:Hydrostatic balance relates level differences to pressure differences. For a U-tube with different fluids in each limb, the vertical height difference h of the manometer columns converts to Δp via density contrasts. The instrument inherently reads Δp, not absolute p.

Step-by-Step Solution:1) Connect taps from two points A and B to the manometer limbs.2) Allow levels to stabilize; measure height difference h between reference interfaces.3) Write Δp = p_A − p_B = (rho_m − rho_f) * g * h for a typical arrangement (signs depend on configuration).4) Use Δp to compute head loss, flow rate (with devices like venturimeters), or system gradients.

Verification / Alternative check:Swapping the connections should change the sign of Δp but preserve magnitude, confirming that the device measures difference, not absolute pressure.

Why Other Options Are Wrong:

• Atmospheric pressure: measured by a barometer, not a differential manometer.
• Single-point pipe/channel pressure: a piezometer or pressure gauge is used.
• Venturimeter throat only: differential manometers compare two taps (upstream vs throat).
• Absolute pressure: requires a vacuum reference (barometer or absolute transducer).

Common Pitfalls:Using an inappropriate manometric fluid (mixing or wetting issues) or misreading levels due to capillarity and parallax.

Final Answer:difference of pressures between two points in a pipe