What is Pitot Tube | Insertion flow Meter­­; Principle, Construction, and Working

The pitot tube is commonly used to measure the rate of liquid flow (Speed, Velocity)/ velocity head. it is also called a differential pressure measuring device.

Principle of Pitot Tube

It measures the velocity at one point only. it is based on the principle of Bernoulli’s theorem.

When it is inserted into the center of the pipe, the velocity of flow is increased at that point, and change in the pressure head. which was further calculated through a manometer.

Then By putting the value in the equation and finding out the value of the velocity head.

Construction of Pitot Tube

• It is also called the Insertion flow meter
• It is small compared to the pipe.
• One tube is perpendicular to the flow direction and the other tube is connected parallel to the direction of the flow.
• The manometer is connected with this to measure the pressure head difference.

Working:

Firstly two tubes are inserted into the pipe as given above image. The pitot tube is used to measure the velocity head of the flow. The fluid velocity is increased at the narrow constriction, resulting in decreased pressure. Tube at right angles to the flow only helps in measuring the pressure head. The tube that points upstream measures pressure heads and velocity heads. the difference in the above reading indicates the velocity head. therefore, the reading (R) of the manometer measured the velocity head.

In the tube, the velocity of the fluid is increased due to a constricted (narrow) tube, resulting in a decrease in pressure head (ΔH) difference.

ΔH measured in a manometer and then put this value in the given formula and calculate the velocity head or rate of flow of liquid (fluid).

Where,

Vact. = Velocity of the fluid (Velocity Head)

Cv= Coefficient of the pitot tube

ΔH= Pressure head difference

Although, the pitot tube measures the velocity at one particular point, eg. at the point of the insertion. the average velocity (across the cross-section of the pipe) may be obtained either by following ways.

1. A pitot tube may be inserted at the center of the pipe and it measures the maximum velocity.
2. An adjustable pitot tube may be used to take readings from different points in the cross-section.

Related Post: Venturi Meter

Application

• Pitot tubes are widely used in Aircraft to measure the airspeed of the aircraft.
• Fluid flow measurement in industrial applications.
• Used for measuring the boat speed inside the water.

Advantages:

• The pitot tubes measured the velocity at one point only.
• It is a low-cost device
• No maintenance cost

Disadvantages:

• Directly it does not give the fluid velocity.
• The pitot tubes themselves cause more disturbance.
• For, gases, the reading is extremely small. for gases working on low pressure, some form of multiplying gauge must be used.