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Pitot Tubes...

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Pitot Tubes

When a fluid stream of uniform velocity flows onto a blunt body, the stream lines looks as follows:

Some of the fluid moves above the body and some of the fluid moves below the body. But the fluid in the center hits the object and stops. The velocity of the fluid at point 2 is zero, and this point is referred to as the stagnation point. 1

A Pitot tube is an instrument for measuring the velocity of a fluid. It was invented by Henri Pitot (1695–1771), and it consists of a tube with a short, right-angled bend, which is placed vertically in a moving fluid with the mouth of the bent part directed upstream. The pressure is measured with a pressure-measuring device. Pitot tubes are used to measure airspeed aboard aircraft in flight and they are also used to measure the flow of liquids.

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A Pitot tube measures a fluid’s

velocity by converting the kinetic energy of the flow into potential energy. This conversion occurs at the stagnation point.

What follows is a schematic of a simple Pitot tube:

Schematic of a simple Pitot tube The device can be used to determine the velocity of a liquid at point 1, upstream from the Pitot tube. The impact of liquid against the opening of the tube at point 2 forces liquid to rise in the vertical leg of the tube to a height Z. Determine the velocity of the fluid at point 1? Point 2 is the stagnation point, where the velocity of flow is zero. This creates an impact pressure termed the dynamic pressure, which forces the fluid into the vertical leg. Writing the energy equation between points 1 and 2, neglecting losses which are very small leads to the following: V12/2g + p1/ + 0 = 0 + p2/ + 0 p1/ is given by the height of the fluid above point 1 and equals K ft of flowing fluid. p2/ is given by the manometer as K + Z, neglecting capillary rise. After substituting values into the equation V12/2g = z and V1 = (2gz)1/2

If this Pitot tube were being used to measure the velocity of flow in a closed conduit, and it indicated a 50 mm rise of water in the tube what is the velocity of flow? V1 = (2gz) 1/2 V1 = (2*g* 9.8 m/s * 0.050m) 1/2 V1 = 0.99 m/s What would be the velocity if the height in the tube were 25 mm? V1 = (2*g* 9.8 m/s * 0.025m) 1/2 V1 = 0.70 m/s What would be the velocity if the height in the tube were 100 mm? V1 = (2*g* 9.8 m/s * 0.10m)1/2 V1 = 1.40 m/s 3

The following picture is of a Pitot tube attached to the underside of an airplane’s wing. The Pitot tube is the L-shaped object. Air flows into this tube during flight and creates a pressure that moves the needle on the air-speed indicator in the cockpit.4

Pitot tube attached to airplane wing

What follows is a schematic drawing of a Pitot tube. The device consists of two concentric tubes. The inner tube is open at the front. This opening is referred to as the stagnation port and it measures the total head. The outer tube is the static tube, which has a few openings on the side of the tube to measure the static head. Each tube has a connection at the top. A measuring device connected to each tube will measure the pressure difference.5

Schematic Diagram of a Pitot Tube (Not to Scale)

A common way to measure the pressure difference is to connect a differential manometer to the two openings. What follows is a cross-section view of a typical Pitot static tube that is attached to a differential manometer.

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Cross-section of a Typical Pitot Static Tube

In addition to being used on airplanes, Pitot tubes are commonly used to determine the volumetric flow of air through a smoke stack. The volumetric flow through a facility’s stack (i.e. power plant or auto manufacturing facility) is used to determine the facility’s compliance with air-use regulations. That is, how much of a given pollutant they are emitting into the air. The volume of the flow determines the concentration of the pollutant.

What follows are images of Pitot tube that could be used for stack testing.

Drawings of Typical Pitot Static Tubes

Photograph of Typical Pitot Static Tube Tip

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Photograph of Typical Pitot Static Tube Tips

Photograph of Typical Pitot Static Tube Tips Holes are drilled into the sides of the stack at particular intervals and the Pitot tube is placed into the stack with the angled bend against the flow of air. The flow is measured many times across the entire diameter of the stack and the values are averaged. If the Pitot tube measured flow only at the center of the stack it would overestimate the volumetric flow since flow is greatest at the center. Conversely, if the flow were measured only near the walls of the stack, it would underestimate the volumetric airflow since flow is less near the walls due to friction. Tall stacks, such as those associated with power plants and large industrial facilities have platforms that are used as staging areas for stack tests. These platforms can be several hundred feet above ground level. If on a cold windy day you see several people huddled on such a platform, they are probably conducting a stack test. References 1. Diagram of Flow Across Blunt Object University of Leeds Engineering Fluid Mechanics (EFM) group web site. efm.leeds.ac.uk/CIVE/CIVE1400/Section3/Bernoulli-apps.htm

2. "Pitot Tube." Britannica Concise Encyclopedia. 2003.” web site britannica.com/ebc/article 3. Streeter, Victor L. et al Fluid Mechanics, 9 th ed, pp 131-132, McGraw Hill, 1998, New York, New York 4. Picture of Pitot tube on the airplane from How Stuff Works website travel.howstuffworks.com/airplane7.htm 5. Schematic of Pitot Tube from the University of Texas Civil Engineering web site ce.utexas.edu/prof/kinnas/319LAB/Lab/lab%2046. Diagram of Cross Section of Typical Pitot Static Tube from efunda (engineering fundamentals)website efunda.com/designstandards/sensors/pitot_tubes 7. Vendor Images of Pitot Tubes from Clean Air Express web site cleanair.com...