Short Answer:

A Pitot tube is a simple device used to measure the velocity of fluid flow at a particular point in a pipeline or open channel. It works on the principle of Bernoulli’s theorem, which relates pressure and velocity in a moving fluid. The Pitot tube measures the difference between stagnation pressure and static pressure, which helps in calculating the flow velocity.

In simple words, a Pitot tube converts the kinetic energy of a moving fluid into pressure energy. This pressure difference is then measured using a manometer, allowing engineers to determine the speed of the flowing fluid accurately.

Detailed Explanation:

Pitot Tube

A Pitot tube is one of the simplest and most commonly used instruments for measuring the velocity of a fluid (either liquid or gas). It was invented by Henri Pitot, a French engineer, in the 18th century and later modified by Henry Darcy to improve its accuracy.

The Pitot tube is based on Bernoulli’s principle, which states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or potential energy. It is widely used in fluid mechanics, hydraulics, and aerodynamics to determine flow velocity in pipes, ducts, and even around aircraft.

1. Principle of Pitot Tube

The working of a Pitot tube depends on Bernoulli’s theorem, which can be expressed as:

In the case of horizontal flow, the elevation head   remains constant, so:

According to this principle, when a moving fluid is brought to rest (stagnation) at a point, its velocity head is converted into pressure head. The Pitot tube measures this difference between the stagnation pressure (total pressure) and the static pressure, which is used to find the velocity of flow.

The velocity of the fluid can be calculated by the formula:

where,

•  = velocity of flow (m/s)
•  = coefficient of Pitot tube (usually close to 1)
•  = difference in head between stagnation and static pressure (m)
•  = acceleration due to gravity (9.81 m/s²)

2. Construction of Pitot Tube

A Pitot tube consists of two main tubes that measure two different types of pressure:

1. Stagnation Tube (Impact Tube):
   • This tube faces directly into the flow of the fluid.
   • The fluid entering this tube comes to rest at its opening, creating a stagnation pressure (the sum of static and dynamic pressures).
2. Static Tube:
   • This tube has small holes on its sides, positioned parallel to the fluid flow.
   • It measures the static pressure, which acts perpendicular to the flow direction.

Both tubes are connected to a differential manometer, which measures the difference between stagnation pressure and static pressure. This difference is proportional to the fluid’s velocity head.

3. Working of Pitot Tube

The working process of the Pitot tube can be explained step-by-step:

1. The impact opening of the Pitot tube faces directly into the fluid stream, and the flowing fluid enters the tube.
2. The velocity of the fluid at the tube entrance becomes zero (stagnation point), resulting in an increase in pressure — known as stagnation pressure.
3. The static tube, positioned parallel to the flow, measures the static pressure of the fluid.
4. The difference between stagnation pressure and static pressure represents the dynamic pressure, which is related to the velocity of the flow.
5. The pressure difference is measured using a manometer, usually containing mercury or water, connected between the two tubes.
6. Using the measured head difference ( ), the velocity of the flow can be calculated from the formula:

or, if calibration is needed,

where   is the coefficient of the Pitot tube (usually between 0.98 and 1.0).

4. Derivation for Velocity Formula

Let,

•  = static pressure
•  = stagnation pressure

From Bernoulli’s theorem:

Rearranging,

If the pressure difference   is measured using a manometer,

where   is the density of the manometric fluid and   is the difference in manometer reading.

Substituting into the Bernoulli equation gives:

This formula allows calculation of the flow velocity using the measured head difference and the densities of both the manometer fluid and the flowing fluid.

5. Advantages of Pitot Tube

1. Simple Construction: It is small, lightweight, and easy to handle.
2. Low Cost: Very economical compared to other flow measuring devices.
3. No Moving Parts: Reduces maintenance and operational issues.
4. Accurate for High Velocity: Suitable for measuring high-speed fluid flows, such as in air ducts and aircraft.
5. Easy Installation: Can be easily inserted into the flow without disturbing it significantly.

6. Disadvantages of Pitot Tube

1. Not Suitable for Low Velocities: Accuracy decreases at very low flow speeds.
2. Affected by Flow Disturbances: Any turbulence or unsteady flow can cause measurement errors.
3. Not Suitable for Dirty or Viscous Fluids: The openings can clog easily.
4. Requires Calibration: The coefficient of discharge must be determined experimentally for accurate results.
5. Limited Range: It measures velocity only at a single point, not across the entire flow section.

7. Applications of Pitot Tube

1. Aircraft Speed Measurement: Used to measure the airspeed of aircraft by comparing stagnation and static pressures.
2. Industrial Piping Systems: Measures flow velocity of gases or liquids in closed pipelines.
3. Ventilation Systems: Used to determine air velocity in ducts and exhaust systems.
4. Hydraulic Laboratories: Commonly used for experimental studies of fluid flow.
5. Open Channel Flow: Used for measuring local velocities in rivers, canals, and flumes.

8. Comparison with Venturi and Orifice Meters

While all three instruments are based on Bernoulli’s principle:

• The Venturi meter measures total discharge through a pipe.
• The Orifice meter also measures discharge but with higher energy loss.
• The Pitot tube, in contrast, measures velocity at a specific point, not total discharge, and causes negligible head loss.

Conclusion

In conclusion, a Pitot tube is a simple yet highly effective instrument for measuring fluid velocity at a specific point. Based on Bernoulli’s principle, it works by comparing the stagnation and static pressures of the flowing fluid. The device is easy to construct, inexpensive, and ideal for measuring high-speed flows in pipes, ducts, and even in aircraft. Despite its limitations at low velocities and with dirty fluids, the Pitot tube remains a widely used and valuable tool in fluid mechanics and engineering applications.