Differential manometers are used for measuring :
Try it first: pick an option, then read the explanation.
Asked in SSC JE 06/11/2024 (09am-11am)
Detailed Explanation
In fluid pipeline systems and flow meters (such as venturimeters and orifice plates), determining the pressure drop across two sections is essential. Differential manometers connect directly across the two points to measure this differential head.
1. Functional purpose of a differential manometer:
- Both limbs are connected to pressurized fluid ports (e.g., points \(A\) and \(B\)).
- Neither limb is open to the atmosphere.
- The manometric fluid displacement reflects the net difference in piezometric head: \((p_A/\gamma + z_A) - (p_B/\gamma + z_B)\).
2. Governing hydrostatic balance:
\[ p_A - p_B = g \cdot \Delta h \cdot (\rho_m - \rho_f) \]
Where \(\Delta h\) is the difference in manometric levels, \(\rho_m\) is the density of the manometric fluid, and \(\rho_f\) is the flowing fluid density.
Why “difference of pressure between two points” Is Correct
By definition and operating principle, a differential manometer connects between two distinct locations in a piping system to directly measure the difference of pressure (\(\Delta p = p_A - p_B\)) between those two points.
Why the Other Options Are Wrong
- A. velocity at a point in a fluid: Velocity at a point is measured using a Pitot tube or hot-wire anemometer.
- B. difference of velocity between two points: Difference of velocity is calculated from continuity equation or measured with laser Doppler velocimetry.
- D. difference in density of fluid between two points.: Fluid density differences are measured using hydrometers, pycnometers, or densitometers.
Key Point
Differential manometers measure the difference in pressure between two points: Δp = p_A − p_B.
Key Concepts for Students
- Differential manometers do not measure absolute or gauge pressure at either point individually.
- Inverted U-tube differential manometers use a light fluid or air for measuring very small pressure differences.
- When used with venturimeters, the manometer deflection h gives differential head directly: h = x(S_m/S_w − 1).
Practice more: this is question 5 in Measurement of Pressure MCQs – Part 1. Try the whole set and check every answer instantly.