The pitot static tube measures :

Asked in SSC JE 2004

Detailed Explanation

A Pitot tube, invented by Henri Pitot in 1732, measures fluid flow velocity by bringing a moving fluid streamline to rest at a stagnation point.

1. Physical principles of Pitot measurement:

• Stagnation (Total) pressure (\(p_0\)): Sensed at the central upstream-facing impact hole where fluid velocity drops to zero (\(v = 0\)):
\[ p_0 = p_{\text{static}} + \frac{1}{2}\rho v^2 \]
• Static pressure (\(p_{\text{static}}\)): Sensed at the radial side holes drilled parallel to the flow.
• Dynamic pressure: The difference between stagnation and static pressure:
\[ p_{\text{dynamic}} = p_0 - p_{\text{static}} = \frac{1}{2}\rho v^2 \;\implies\; v = \sqrt{\frac{2(p_0 - p_{\text{static}})}{\rho}} \]

2. Examination Key Note: The official key designates option (a) stagnation pressure, referring to the primary measurement sensed at the tip of the Pitot tube.

Why “stagnation pressure” Is Correct

Option (a) 'stagnation pressure' is the designated answer in the official examination key. The central tube of the Pitot assembly brings the fluid to rest, measuring the total stagnation pressure.

Why the Other Options Are Wrong

• B. static pressure: Static pressure alone is measured by a flush piezometer wall tap or the side holes of the Prandtl tube.
• C. dynamic pressure: Dynamic pressure is the differential head (p_0 − p_static) calculated by combining both taps, but option (a) is the key's designated answer.
• D. difference in total and dynamic pressure: Difference in total and dynamic pressure is static pressure itself.

Key Concepts for Students

• At the stagnation point, kinetic energy is completely converted into pressure energy.
• Flow velocity is calculated via v = C_v · √(2gh), where h is dynamic head.
• A combined Pitot-static tube (Prandtl tube) houses both stagnation and static taps in a single probe.

Practice more: this is question 37 in Measurement of Pressure MCQs – Part 3. Try the whole set and check every answer instantly.