Flow Measurement

# Pitot Tube Working Principle

## What is a Pitot tube?

Pitot tubes are a form of insertion flow meter that is widely used in all process industries. Pitot tubes are employed in anemometers to determine airspeed in wind tunnels and in airplanes.

## Pitot tube Working Principle

• Generally, we can say that this Pitot tube we are using to measure  the velocity of working fluid  such as liquid or gas works on the basis of the law of Bernoulli’s principle,
• We already know that Bernoulli’s law states that the pressure gets varied with the rise in the speed or velocity of the working fluid.
• The fluid alone exerts pressure on its environment, which is alluded to as static pressure. The static pressure is maximum when the fluid’s velocity is zero.

## Construction of Pitot tube

• Generally, we know that a Pitot tube is an instrument used to measure the fluid velocity of liquids and gases positioned towards the flow’s path.
• A Pitot tube is a cylindrical tube having one side open, and the other side closed.
• The construction part of the Pitot tube includes two hollow tubes to detect the pressure at different regions or locations.
• These hollow tubes can be mounted individually in a pipe or installed together in a single enclosure considered as a single device.
• The fluid enters through the input opening, and it’s possible that there are two extra pores in the Pitot tube that operate as static pressure sources.
• One tube detects the gauge pressure or impact pressure, and another tube detects only static pressure basically at the wall of the pipe.
• The orientation of the Pitot tube used to measure the static pressure must be installed perpendicular to the border, leading to a clear barrier.

## Working of Pitot tube

• When the working fluid flows through the pipeline and remains in this location.
• Here the fluid is allowed to flow in one chamber, and another chamber of the tube is filled with the fluid having constant pressure separated by a diaphragm.
• The change in level between both the liquid in the tube, as well as the free surface, is employed to determine dynamic pressure.
• The flow velocity and hence the flow rate is determined by measuring the impact and static pressures while coupled to the appropriate differential pressure meter.
• The flow rate is determined by taking the square root of the pressure.
• The method for determining flow rate from pressure is affected by some factors like
1. Location of the static tap
2. Tube design.

To eliminate this characteristic, the Pitot-static probe contains static holes in the tube system. The velocity of the flowing fluid within the tube can be easily measured by using the relation shown below

U= √ (2δP/ρ)

Where

• U is fluid flow rate
• ΔP is Diff_pressure
• ρ is fluid density used

## What are the advantages of the Pitot tube?

1. Pitot tube is portable
2. It contains no moving parts.
3. It is economical.
4. Low-pressure loss.
5. Installation is easy for an existing system.
6. They constantly deliver precision under various environmental circumstances.
7. They outlast several other fluid measuring tools in terms of durability.
8. Compared to orifice plates, they are more affordable.

## What are the disadvantages of the Pitot tube?

1. Accuracy is low
2. Lower range
3. Strange particles present in a fluid can easily jam the Pitot tube and interrupt regular reading.
4. Less accurate
5. Low range

## What are the applications of Pitot Tube?

The Pitot tube is used in

1. Utility streams.
2. Air duct
3. Pipe system.
4. Aircraft to determine the velocity of airflow.
5. To map flow profiles in a channel or duct.

### How does the Pitot tube work?

The front hole evaluates system pressure, whereas the peripheral apertures determine stagnation pressure. The distinction between these two measurements is termed dynamic pressure, and it is used to calculate airspeed.

### Where is the Pitot tube used?

Pitot tubes are used to measure

1. Airspeed aboard aircraft in flight
2. In the wind tunnel
3. The flow of liquids.

### When was the Pitot tube invented?

The Pitot tube was discovered by French engineer Henry Pitot in the 18th century. But, later modified by Henry Darcy into a modern shape during the 19th century.

### Can the Pitot tube measure static pressure?

A Pitot tube determines the velocity of environmental air using both gauge pressure & static pressure, by inserting it into a duct with its end facing toward airflow.

### How to measure pressure using a Pitot tube?

The differential pressure determined between both pressures gives the dynamic pressure. The difference in level between the liquid in the tube and the free surface is used to calculate dynamic pressure.