Pitot tube, what is it and how to choose the best one?

November 2, 2021

The pitot tube , also known as a pitot probe, is a flow measuring device used to measure fluid flow velocity. The Pitot tube was invented by the French engineer Henri Pitot in the early 18th century and was modified to its modern form in the mid-19th century by the French scientist Henry Darcy. It is widely used to determine the speed of an airplane, the speed of the water of a boat, and to measure the flow rates of liquids, air and gas in certain industrial applications.

Although the Pitot tube is one of the simplest flow sensors, it is used in a wide range of flow measurement applications, such as airspeed in Air Force race cars and fighter jets. In industrial applications, Pitot tubes are used to measure air flow in pipes, ducts, and stacks, and liquid flow in pipes, weirs, and open channels.

While accuracy and range capability are relatively low, Pitot tubes are simple, reliable, inexpensive, and suitable for a variety of environmental conditions, including extremely high temperatures and a wide range of pressures.

The Pitot tube is an inexpensive alternative to an orifice plate . Accuracy ranges from 0.5% to 5% FS, which is comparable to that of a hole. Their 3: 1 flow capacity, and some operate at 4: 1, is also similar to the orifice plate capacity. The main difference is that while an orifice measures total flow, the Pitot tube detects the flow velocity at a single point in the flow. An advantage of the thin pitot probe is that it can be inserted into existing, pressurized pipes, called hot-tapping, without requiring a shutdown.

Next we will talk about the pitot tube, its uses and other details that make it an indispensable instrument in the laboratory world, such as the Capillary Tube . Join us!

What is a Pitot Tube?

The Pitot tube is a mechanical device used to measure flow velocity at any point in a pipe or channel. It was invented by the French engineer Henri Pitot and modified by the French scientist Henry Darcy. The Pitot tube works on a basic principle of fluid mechanics and that is, if a fluid stops flowing, all its kinetic energy is converted into pressure energy. It helps us measure that pressure energy of the stopped fluid.

What is a Pitot Tube used for?

Basically, it is used in wind tunnel experiments and in airplanes to measure flow velocity. It is a thin tube that has two holes. The front hole is placed in the air stream to measure what is called stagnation pressure. The side hole measures static pressure. By measuring the difference between these pressures, you get the dynamic pressure, which can be used to calculate air velocity. In an aircraft, the Pitot tube can be mounted in a number of ways, including protruding from the edge of the wing or protruding from the fuselage.

The actual tube on the aircraft is about 10 inches long with a 1/2-inch diameter. Several small holes are drilled around the outside of the tube and a central hole is drilled in the axis of the tube. The outer holes are connected to one side of a device called a pressure transducer. The center hole of the tube is kept separate from the outer holes and is connected to the other side of the transducer.

The transducer measures the pressure difference in the two groups of tubes by measuring the tension in a thin element using an electronic tension meter. The Pitot tube is mounted on the aircraft so that the center tube always points in the direction of travel and the outer holes are perpendicular to the center tube.

How does the Pitot Tube work?

Pitot tubes were invented to measure liquid flow or air velocity . Basically, a differential pressure (d / p) flow meter, a Pitot tube measures two pressures: static pressure and total impact pressure. Static pressure is the operating pressure in the pipe, duct, or environment, upstream of the Pitot tube.

It is measured at right angles to the flow direction, preferably at a low turbulence location. Total impact pressure (PT) is the sum of the static and kinetic pressures and is sensed as the flowing current impacts the pitot opening. To measure impact pressure, most Pitot tubes use a small tube, sometimes L-shaped, with the opening directly facing the approaching flow stream.

Approach velocity (VP) can be calculated by taking the square root of the difference between total pressure (PT) and static pressure (P) and multiplying it by the C / D ratio, where C is a dimensional constant and D is density.

When the flow rate is obtained by multiplying the point velocity (VP) by the cross-sectional area of ​​the pipe or conduit, it is critical that the velocity measurement is made at an insertion depth that corresponds to the average velocity. As flow velocity increases, the velocity profile in the pipe changes from elongated to flatter. This changes the average velocity point and requires an insertion depth adjustment.

Pitot Tube Types

  • Simple pitot tube
  • Static source
  • Tubo Pitot-static

Pitot Tube Applications

It is used to measure the speed of an airplane , the speed of a ship, fluid flows in different industries. It is also used where very high precision is not required and to measure the flow profile in a duct or channel.

Pitot Tube Advantages

  • It is easy and inexpensive to install and remove
  • It has no moving parts to help minimize friction losses.
  • It is small in size
  • It is low cost
  • Causes very little pressure loss.

Pitot Tube Limitations

  • Any foreign material in flow can change the accurate reading.
  • Its accuracy is not that good
  • Has low range capacity
  • Has low sensitivity
  • It works best in high speed flow.
  • A change in the speed profile can cause significant changes in the readings
  • Cannot be used with dirty fluids.
  • Its sensitivity is disturbed by the direction of flow.

Theory of operation

The basic Pitot tube consists of a tube that points directly to the flow of fluid . Since this tube contains liquid, the pressure can be measured; the moving fluid stops as there is no outlet to allow the flow to continue. This pressure is the stagnation pressure of the fluid, also known as the total pressure or (particularly in aviation) the Pitot pressure. The measured stagnation pressure cannot be used to determine the fluid flow rate.

Pitot tube and planes

A static pitot system is a pressure-sensitive instrument system most often used in aviation to determine an aircraft’s speed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a static port, and the pitot-static instruments.

Errors in pitot static system readings can be extremely dangerous as information obtained from the pitot static system, such as airspeed, is potentially critical to safety.

Industrial uses of Pitot tubes

In industry, the flow rates that are measured are often those that flow in conduits and pipes where measurements with an anemometer would be difficult to obtain. In these types of measurements, the most practical instrument to use is the Pitot tube.

The Pitot tube can be inserted through a small hole in the duct with the Pitot connected to a tube water meter or some other differential pressure meter to determine the flow rate within the channeled wind tunnel.

One use of this technique is to determine the volume of air that is delivered to a conditioned space.

Why is a pitot tube important for measuring air?

Airplanes have a certain operational envelope, and it is dangerous to fly too low speed because it can stop, and it is also dangerous to fly too high speed due to structural reasons.

Pitot Tube Installation

Pitot tubes can be used as permanently installed flow sensors or as portable monitoring devices that provide periodic data. Permanently installed carbon steel or stainless steel units can operate at pressures up to 1400 PSIG. And they are inserted into the pipeline through flanged or bolted connections. Their installation generally occurs prior to plant start-up, but they can be connected to an operational process.

Dr. Loony Davis5
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Born and raised in Brussels in an English family, I have always lived in a multicultural environment. After several work experiences in marketing and communication, I came to Smart Water Magazine, which I describe as the most exciting challenge of my career.
I am a person with great restlessness and curiosity to learn, discover what I do not know, as well as reinvent myself daily, someone who is curious about life and wants to know. I enjoy sharing knowledge.
This is my personal project but I also collaborate in other blogs, it is the case, the most important web on water currently exists in the US, if you are interested you can read my articles here.

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