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Things you need to know about pitot tube

 

Thinking of how speed is measured in applications like aircraft is measured, well pitot tube is the instrument designed for this task. this instrument which is known as a pitot probe consists of a tube with a short right-angled bend. The device is used to calculate velocity depending on the fluid flow. Pitot tubes are used in anemometers to measure airspeed in wind tunnels and aboard aircraft in flight. They are used to measure the flow of liquid, air, and gas.

Today you’ll get to understand the definition, function, applications, components, diagram, equation, types, and working of a pitot tube. You’ll also get to know the following:

  • Static pressure
  • Dynamic pressure
  • Stagnation pressure
  • Flow velocity
  • Single-port pitot tubes
  • Averaging pitot tubes
  • How to measure airflow with a pitot tube, finally
  • You get to know the advantages and disadvantages of a pitot tube.

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What is a pitot tube?

A pitot tube is a flow measurement instrument for measuring the velocity (speed) of a flowing fluid. It was invented by Henri Pitot who was a French engineer in the 18th century. The device was modified to its modern form in the mid-19th century by a French scientist Henry Darcy.  A pitot tube is a slender tube, having two holes on it. The front hole is placed in the airstream, helping to measure what is known as stagnation pressure. the side hole measures what’s called static pressure. Measurement between these two pressures is what gives us dynamic pressure, which can be used to calculate airspeed. Well, this will be further explained.

Pitot tubes which are flow sensor instruments can be an inexpensive alternative to an orifice plate. Its accuracy ranges from 0.5% to 5% FS, which is similar to that of an orifice. Also, its flow rangeability of 3:1 (although some work at 4:1) is more like the capacity of the orifice plate. The difference between these two devices is that orifice measures the full flow stream, while the pitot tube can detect the flow velocity at only one point of the flow stream.

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Applications of a pitot tube

Pitot tubes can now be found in many applications not only in aviation. It is popular that the instrument is used in aviation, they are also common in industrial machinery, boats, and even sports cars. In fact, you can even employ a pitot tube to your project that requires measurement of the velocity of a flow. However, planes often use pitot-static tubes that contain two openings instead of a separate tube and static ports.

A single pitot tube has the ability to measure static pressure and stagnation pressure. The static ports at the side of the pitot tube help to achieve this all-in-one capability. This eliminates the need for separate static ports. I will elaborate more on this below this article.

Just as earlier mentioned, pitot tune is widely used to determine the airspeed of an aircraft, and the water speed of a boat. It is used to measure liquid, air, and gas flow velocities in certain industrial applications. basically, the instrument is used in wind tunnel experiments and on airplanes to measure flow speed. It is used in a wide range of flow measurement applications such as airspeed in racing cars and air force fighter jets. In industrial applications, pitot tubes are used to calculate airflow in pipes, ducts, and stacks. Also, liquid flow in pipes, weirs, and open channels.

Finally, in applications pitot tubes are used to measure fluid flow velocity by measuring static and dynamic pressure difference. This can be achieved by converting the kinetic energy in a fluid flow to potential energy.

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Component of a pitot tube

Diagram of a pitot tube:

Pitot tube equation

The principle is based on the Bernoulli Equation where each term of the equation can be interpreted as pressure

p + 1/2 ρ v2 + ρ g h

      = p + 1/2 ρ v2 + γ h

      = constant along a streamline … (1)

Where:

p = static pressure (relative to the moving fluid) (Pa)

ρ = density of fluid (kg/m3)

v = flow velocity (m/s)

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γ = ρ g = specific weight (N/m3)

g = acceleration of gravity (m/s2)

h = elevation height (m)

Each term of the equation has the dimension force per unit area N/m(Pa) – or in imperial units lb/ft2 (psi).

Static Pressure – The term P indicates static pressure. it is static relative to the moving fluid and can be measured through a flat opening in parallel to the flow

Dynamic Pressure – The second term – 1/2 ρ v2 – is called dynamic pressure.

Hydrostatic Pressure – The third term – γ h – is called hydrostatic pressure. It represents the pressure due to an elevation change.

Stagnation Pressure – The Bernoulli Equation states that the energy along a streamline is constant – and can be modified to

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p1 + 1/2 ρ v12 + γ h1

    = p2 + 1/2 ρ v22 + γ h2

    = constant along the streamline … (2)

Where:

suffix1 is a point in the free flow upstream

suffix2 is the stagnation point where the velocity in the flow is zero

Flow Velocity – In a measuring point we regard the hydrostatic pressure as a constant where h1 = h2 – and this part can be eliminated. Since v2 is zero, (2) can be modified to:

p1 + 1/2 ρ v12 = p2 … (3)

or

v1 = [2 (p2 – p1) / ρ] 1/2           

    = [2 Δp / ρ] ½ … (4)

Where:

Δp = p2 – p1  (differential pressure)

With (4) it’s possible to calculate the flow velocity in point 1 – the free flow upstream – if we know the differential pressure difference Δp = p2 – p1 and the density of the fluid.

It is common to use ahead instead of pressure. (4) can be modified by dividing with specific weight γ to

v1 = c [2 g Δh]1/2 … (5)

where:

c = coefficient – depending on reference liquid and units used or calculated 

g = acceleration of gravity

Δh = h2 – h1 = height difference (fluid column)

Note! – in the basic equation the head unit is regarding the density of the flowing fluid. For other units and reference liquids – like mm Water Column – check Velocity Pressure Head.

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Working of a pitot tube

The working of a pitot tube is less complex and can easily understand. A pitot tube measures two pressures; static and the total impact pressure. the static pressure is the operating pressure in the pipe, duct, or the environment, upstream to the pitot tube. It is measured at right angles to the flow direction, often in a low turbulence location.

The sum of the static and kinetic pressures is the total impact pressure (PT). It is detected as the flowing stream impacts the pitot opening. To measure this impact pressure, the pitot tube used is often small, and sometimes L-shaped tube. The opening must directly face the oncoming flow stream. The point velocity of approach (VP) is be calculated by taking the square root of the difference between the total pressure (PT) and the static pressure (P). It will then be multiplied by the C/D ratio, where C is a dimensional constant and D is density. It can be mathematically expressed as;

Vp = C(PT – P)½/D

Since the flow rate is obtained by multiplying the point velocity (VP) by the cross-sectional area of the pipe or duct, the velocity measurement must be made at an insertion depth that corresponds to the average velocity. The velocity profile in the pipe changes from elongated (laminar) to flatter (turbulent). This happens because the flow velocity rises. It changes the point of average velocity and requires an adjustment of the insertion depth.

Pitot tube instruments are used only for highly turbulent flows (Reynolds Numbers > 20,000). Therefore, the velocity profile needs to be flat enough so that the insertion depth is not critical.

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Watch the video below to learn more on the working of a pitot tube:

A single-port pitot tube

A single-port pitot tube measures the flow velocity only at a single point in the cross-section of a flowing stream. The probe is to be inserted to a point in a flowing stream where the flow velocity is the average of the velocities across the cross-section. The impact port must face directly into the fluid flow.

Single-port pitot tubes can be made less sensitive to flow direction if the impact port has n internal bevels of about 15 degrees, extending to about 1.5 diameters into the tube. If the pressure differential created by the venturi is too low for accurate detection, the conventional pitot tube can be replaced by a pitot venturi. A double venturi sensor can also be used to produce a higher-pressure differential.

A clean calibrated, clean, and properly inserted single-port pitot tube will offer ±1% of full-scale flow accuracy over a flow range of 3:1. With some loss of accuracy, the instrument can even measure over a range of 4:1. The benefits of a single-port pitot tube include low cost, simplicity in design, no moving parts, and it causes very little pressure loss in the flowing stream. Although it has some limitations which include the errors resulting from velocity profile changes or from plugging of the pressure ports.

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Averaging pitot tubes

Averaging pitot tubes were invented to eliminate the problem of finding the average velocity point. This tube is provided with multiple impacts and static pressure ports. It is designed to extend across the entire diameter of the pipe. The pressures detected by all the impact pressure ports will be joined and the square root of their difference is measured as an indication of the average flow in the pipe.

There is a port closer to the outlet of the combined signal, it has a slightly greater influence than the port that is farthest away. Although for secondary applications where pitot tubes are commonly used may see the error as nothing. A pitot tube can be modifying to meet the needs of a particular application such as the number of impact ports, the distance between ports, and the diameter of the averaging pitot tube.

The sensing ports of the averaging pitot tubes are often too large so that the tube can work as a true averaging chamber. The port openings are optimized to prevent plugging rather than averaging. Although, purging with inert gas is used to keep the ports clean, allowing the sensor to use smaller ports.

Averaging types of pitot tubes offer the same benefits and limitations as single-port tubes. Only that, they are slightly more expensive and a little more accurate, especially if the flow is not formed fully. Furthermore, some averaging pitot sensors can be inserted through the same opening (or hot tap) that accommodates a single-port tube.

pitot tube

How to measure airflow with a pitot tube

Since pitot tubes are well suited for mid to high-velocity airflow measurement, it is important to know how to measure airflow with the instrument. Taking accurate measurements requires density correction and a careful traverse. This is because the accuracy is dictated by the pressure measurement device which is mounted to the pitot tube. Although more economical method (hot wire and vane) can be used to measure airflow in low flow applications. However, for high flow or high-temperature applications, the pitot tube is ideal.

Just as stated above, a pitot tube measures total pressure and static pressure to determine the velocity pressure. This process also derived the air velocity. The tube is inserted into the duct with the tip pointed toward the airflow. The positive port of the manometer is connected to the total pressure port (Pt) and the negative to the static pressure port (Ps). This manometer will then display the velocity pressure that can be converted to velocity.

measurement of airflow with pitot tube

Modern pitot tubes are designed with proper nose or tip and the distance between the nose, static pressure taps, and the stem is sufficient. This will minimize turbulence and interference, making it be used without correction or calibration factors.

Ensuring accurate velocity pressure reading, the pitot tube tip must be pointed directly into (parallel with) the air stream. With the correct alignment of the pitot tube, the velocity pressure indication will be at its maximum.

Note that accurate readings cannot be taken in a turbulent air stream. Pitot tubes shout at least be inserted to 8-1/2 duct diameters downstream from elbows, bends, or other obstructions that can cause turbulence. For a precise measurement, the straightening vanes should be located 5 duct diameters upstream from the pitot tube if available.

Air velocity is not equal across a sectional area of a duct or laminar. Therefore, a traverse of the duct needs to be performed to determine an average velocity. The friction closer to the walls of the duct will slow down the airflow as they scrub the duct walls. A defined pattern will be followed to ensure accurate measurement.

pitot tube

Advantages and disadvantages of a pitot tube

Advantages:

  • The instrument is portable
  • It contains no moving parts.
  • Despite its huge work, it’s inexpensive
  • Have a simple design and it is reliable.
  • Low permanent pressure loss.
  • Ease of installation into an existing system.
  • Suited for different environmental conditions, including high temperatures and a wide range of pressures.

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Disadvantages:

Despite the good advantages of the pitot tube, some limitation still occurs. Below are the disadvantages of pitot tubes in their various applications.

  • Low rangeability
  • Low accuracy.
  • Reading can be easily obstructed, depending on the fluid.
  • Vibration can give a wrong reading and can even damage the instrument.

Conclusion

A pitot tube is a great component for taking flowing fluid measurements. In this post, we’ve examined the definition, function, applications, equation, types, and working of a pitot tube. We also saw the advantages and disadvantages of the pitot tube instruments.

I hope you get a lot from this article, if so, kindly share it with other students. Thanks for reading. See you next time!

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