A pitot tube (| p | I | t | oʊ) (also pitot tube or Pitotsche tube ; German technical term: dynamic pressure probe ), named after Henri de Pitot , is a straight or L-shaped, semi-open tube for measuring the total pressure of fluids or gases. It is used, among other things, to measure the speed of airplanes, helicopters and racing vehicles.
In order to be able to measure speeds, pitot tubes are usually also equipped with a static pressure probe. Such measuring units are called Pitot tubes . Its best-known representative is the Prandtl pitot tube , which is used in aviation under the name Pitot tube for speed measurement in the pitot static system .
Theoretical foundations of the Prandtl pitot tube
A pitot tube works according to the fundamentals of fluid dynamics and is a classic example of the practical application of Bernoulli's equations . It consists of a tube that is aligned parallel to the flow in such a way that the flow hits a tube opening frontally. The rear part of the tube is firmly connected to a pressure measuring device.
The flow rate of a liquid or a gas is measured through the pitot tube as a function of the back pressure . This is based on the following considerations (shown here on a U-tube manometer ): Types of pressure
- The total pressure is the pressure that is exerted on a moving body by the medium, liquid or gas surrounding it, in the direction of movement of the medium. This total pressure is measured by the pitot tube when
- a medium flowing around is brought to a standstill (example: flow measurement in liquids) or
- a stationary medium is brought exactly to the speed of the object by a moving object. (Example: the pitot tube on aircraft).
- It does not matter whether the pitot tube or the medium is moved during the measurement. Both result in a representation of the relative flow around the pitot tube, and only the relative speed is important. The total pressure measured by the pitot tube can be further subdivided into dynamic pressure and static pressure in a flowing medium.
- The back pressure (also dynamic pressure ) is the pressure that the flowing medium exerts by its velocity and its mass (density). It characterizes the proportion of kinetic energy in the flowing medium. The faster the flow and the greater the mass (density) of the flow, the greater the dynamic pressure.
- The static pressure represents the share of the potential energy in the total energy of the medium. It corresponds to the air pressure of the still ambient air in which the pitot tube is located. In the stationary medium, the static pressure is equal to the total pressure, since the dynamic pressure becomes 0.
The sum of dynamic pressure and static pressure is always recorded in the pitot tube . The total pressure is measured with the pitot tube. In connection with a measurement of the static pressure and a differential pressure sensor, the flow velocity of a medium can be calculated according to Bernoulli's law if its density is known.
Layout and function
All pitot tubes have the same operating principle with a similar structure. A spindle-shaped metal body is attached to a measuring support by means of a holder . Inside the metal body there are channels that are connected to the environment via bores . The ducts are connected to suitable measuring devices via the pipe support . In aircraft flying under instrument flight conditions, heating by means of heating coils is provided, since the position of the pitot tube favors icing and thus the risk of an instrument failure.
At the tip of the open tube, the stagnation point , there is a flow velocity of 0. A device that has only one opening at the stagnation point, is in the German language as the actual pitot tube (engl. Pitot tube ), respectively.
If the measuring tube is used to measure the static pressure alone, then it is a static probe that can be identified through openings on the side of the measuring tube and a closure at the stagnation point.
The combination of pitot tube and static probe is usually called Prandtl tube in German-speaking countries . This term is unknown in the English-speaking world, where one always speaks of a pitot tube .
Pitot tubes are used where a simple and exact flow measurement is to be carried out.
Airplanes move in an air pressure environment that varies as they climb, descend, accelerate, or slow down. This means that a variable total air pressure, consisting of dynamic pressure and static air pressure, occurs on the pitot tube. The speed of an aircraft can be shown in the airspeed indicator relative to the dynamic pressure. The total pressure is recorded by the pitot tube and passed on to the airspeed indicator via the total pressure line. The higher the back pressure, the higher the speed.
Since the pitot tube supplies the static pressure of the environment in addition to the desired dynamic pressure, this must be removed again by pressure compensation in order to be able to represent the dynamic pressure alone.
A pitot tube is attached in an exposed position on the outside of the aircraft where the slightest disturbance to the air flow is to be expected. This can be, for example, below a wing with the opening outside of the surface air flow or, in the case of jets, at the tip of the nose. The mounting position should aim to keep the static error as low as possible. Often wind tunnel tests have to be carried out to determine the optimal position. In larger aircraft and also in military aircraft, there are several pitot tubes in different places for safety reasons.
The air flow penetrates in a straight line and unaffected into the pipe, which in Prandtl pipes is surrounded by openings on the side for measuring the static pressure. The dynamic pressure is measured, for example, with a connected pressure cell or a transducer and displayed in the airspeed indicator as the corresponding air speed (IAS, indicated air speed ) in accordance with the Bernoulli equation .
When airplanes reached speeds in the transonic range at the end of the 1930s , it was found that the instruments connected to the pitot tube displayed a speed about 10% higher than was actually available. The reason was compressibility effects, since the Bernoulli equation only applies to incompressible media. Uncompensated measuring systems only deliver usable measured values in the lower speed range. With increasing airspeed - from about Mach 0.3 - the compressibility of the air, i.e. the change in density of the air due to compression, leads to a measurement error that indicates a higher speed than is actually present.
A speed correction must then be carried out by means of tables, or analog or computer calculation, in order to obtain an equivalent air speed (EAS ) from the IAS . This represents an equivalent to the air flowing around the wing. Airplanes with speeds well above Mach 0.3 usually have speed displays that show this speed, corrected by the compression, as a multiple of the Mach number .
Pitot tube types for aircraft
In the mechanical form, as described above, a hose or tube runs from the pitot tube to a pressure cell in the airspeed indicator and also a tube from the static port directly to all three barometric instruments . No electrical energy is required here, which is an advantage in gliders, for example. This pitot tube shape is still common today in aircraft, regardless of size, which do not display the data in digital form in the cockpit. The distance from the pitot tube to the display instrument should be small in order to keep the gas volume in the system low and to ensure that the instrument responds quickly.
In the second case, electrical energy is required. The opening for the dynamic pressure is connected to one half of the transducer via a channel. This is where the total pressure comes in. There are further small bores on the side of the pitot tube, which finally run together to the other half of the transducer via separate channels. The static pressure is applied here.
The transducer ( differential pressure sensor) measures the pressure difference using a pressure transducer. The pressure difference is relatively small, so that only very sensitive transducers can be used. Suitable: piezoresistive or capacitive transducers. With the first, the resistance of a ceramic measuring plate is measured, with the second, the capacitance of a capacitor that changes due to pressure fluctuations.
This information is then standardized by amplifiers, converted into an analog electrical signal and passed on to the airspeed indicator, which uses it to generate a speed indication in accordance with Bernoulli's law. In modern digitalized cockpits, the electronics form what is known as a bus participant , which makes the measurement data available directly to the computers without analog conversion.
In military aviation, extreme flight situations are common, for example with combat aircraft. A normal pitot tube is no longer exposed to the frontal flow, so that under these conditions the stagnation point moves to the side and makes a reliable measurement impossible. For this purpose, special forms of pitot tubes have been developed in which there are further openings around the central stagnation point opening, which enable an exact measurement even if the stagnation point is shifted. Alternatively, so-called keel probes are used, in which the actual Pitot tube is embedded in a Venturi nozzle in order to improve the flow at the measuring point.
There are also movable arrangements that automatically adjust to the direction of flow. For use in the wind tunnel, miniaturized forms are used, which, however, generally do not have a static probe.
Because of its exposed position in the air flow, the pitot tube of an aircraft is susceptible to dirt, insects, water and icing. Pitot tubes from parked aircraft are therefore provided with a protective coating that must be removed before take-off. For aircraft that can fly in instrument flight conditions , a heated pitot tube is mandatory due to the risk of icing. A defective pitot tube is the likely cause of the crash of flights Birgenair flight 301 in February 1996 and Air France flight 447 on June 1, 2009. Adhesive strips on the pitot tubes overlooked by maintenance personnel are considered to be the cause of the crash of Aeroperú flight 603 on June 2. October 1996.
Pitot tubes are also used in high-speed motor vehicles when a speed measurement is required that is independent of the tire speed.
Formula 1 is a typical application. Here, wind direction and wind strength play a role in the setup of the vehicle. Pitot tubes are also used in long-distance tests of prototypes. In principle, similar devices are used in motor vehicles as in aviation, but these are always electronic.
Similar to the measuring devices for airplanes, the pitot tube can also be used to measure wind speeds. Since the pitot tube inherently at very low wind speeds barely a measurement result delivers, but practically has no upper limit and quickly responds to wind speed changes, the meter is particularly suitable for strong winds and gusts measurement and is used in conjunction with a differential pressure gauge as anemometer for use .
The measured value display can be evaluated both electronically and via a differential pressure measuring cell. Pitot tubes for meteorology are practically always designed to be heated.
The pitot tube is suitable as a simple measuring device for measuring speed in many media. Pitot tubes are widespread in industry; such probes are usually used in a closed tube system. Probes are available that are suitable for ambient pressures of up to 800 bar and temperatures from −250 ° C to 1300 ° C. It can be designed as a measuring beam, with a number of measuring openings in the stagnation point in order to be able to determine the flow path over a cross-section with a probe. The material of the probes is selected according to the application. The more common name for such sensors is pitot tube.
Pitot measuring tubes are also used to determine the amount of extinguishing water available from hydrants. This makes use of the fact that with a constant cross-section of the outlet opening of a hydrant, the speed of the exiting liquid (here: extinguishing water) is directly proportional to the exiting volume flow. The pressure of the escaping extinguishing water can be converted into the volume flow using the following formula:
- : Volume flow
- k: constant (1733 for DN 50 or 2816 for DN 65 outlet openings)
- p: pressure of the pitot measuring tube
The use of a water meter is prohibited in this application, as the water meter usually has a different cross-section than the hydrant connection opening and has its own flow resistance (in the event of a fire, the fire department does not use a water meter).
Measurement of the flow velocity in air lines (ducts and pipes). Usually a curve is created for each measuring device, which takes into account the internal resistances in the measuring device and thus provides very reliable and precise data.
In 1732 Henri de Pitot published a draft for a " machine for measuring the speed of flowing water and the wake of ships ". This principle has remained in use to this day.
However, Pitot's development still had weaknesses. It consisted of two tubes lying next to each other, one of which was bent by 90 ° at the lower end in order to be directed into the water flow, while the second, straight tube took up the static pressure. With this arrangement, however, the pipe for static pressure measurement was within the turbulence caused by the bent pipe in front of it. In addition, there were theoretical deficiencies regarding the conversion of the pressure difference into the flow rate. Due to constant fluctuations, only very imprecise measurements could be made.
In 1775 James Lind measured wind speeds with the help of a Pitot tube anemometer . A U-shaped tube was bent at the front end again by 90 ° and filled with a liquid, as shown in the figure above. The penetrating air pushed the water on the rear pipe of the Us upwards on a scale. In order to increase the sensitivity, William Snow-Harris enlarged the air inlet significantly in 1858 .
From 1856 onwards, the instrument developed by Pitot was decisively further developed by Henry Darcy by attaching valves, applying a vacuum over the pipes, relocating the inlet of the static pipe to the side - and thus outside the turbulence of the Pitot pipe - and a new calculation formula the flow rate developed. Darcy's further development was also primarily used to measure flowing water.
Ludwig Prandtl developed the version of the pitot tube, which is still in use today. How it works is described above.
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