Instrumentation — differential manometers are primarily used to measure which of the following?

Introduction / Context:
Manometers are simple yet precise pressure-measuring devices based on hydrostatics. Differential manometers use a U-tube (or similar arrangement) filled with manometric fluid to compare pressures at two locations directly.

• Incompressible manometric fluid (e.g., mercury or colored water).
• Two tap points connected to each limb.
• Quasi-static readings; negligible dynamic effects.

Concept / Approach:

The height difference between the columns reflects the pressure difference according to p1 − p2 = rho_m * g * (Δh) corrected for the densities of the working fluids. This makes differential manometers ideal for measuring pressure drops across fittings, Venturi meters, orifices, and filters.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Cross-check with calibrated differential pressure transmitters; for steady laminar pipe flow, compare to Hagen–Poiseuille prediction to validate.

Why Other Options Are Wrong:

Atmospheric pressure is measured by barometers; very low absolute pressures by McLeod or Pirani gauges; “pressure in water channels…” is generic and not specific to differential measurement.

Common Pitfalls (misconceptions, mistakes):

Using an inappropriate manometric fluid causing capillary or vapor issues; ignoring temperature effects on density.

Final Answer:

difference in pressure at two points