Pitot tube

A pitot tube (also pitot tube, english pitot tube ), named after Henri Pitot, is a straight or L-shaped, open on one side tube for measuring the total pressure of liquids or gases.

Pitot tubes are often also equipped with a static pressure probe to measure so that speeds can. Such measuring units are called pitot tube. Her most famous representative is the Prandtl Pitot tube, which is used in aviation under the name pitot tube for measuring speed in the pitot -static system.

Theoretical foundations of the Prandtl pitot tube

A pitot tube is working according to the principles of fluid dynamics and is a classic example of the practical application of Bernoulli's equations. It consists of a pipe which is aligned parallel to the flow in such a way that the flow impinges upon a front pipe opening. The rear part of the tube is fixedly connected to a pressure measuring device.

The flow rate of a liquid or a gas is measured by the Pitot tube ( pressure tube ) as a function of the dynamic pressure. The following considerations are ( here on a U- tube manometer shown): print types

In the pitot tube, the sum of the dynamic pressure and static pressure is always recorded. Thus, with the pitot tube, the total pressure is measured. In conjunction with a measurement of the static pressure and a differential pressure sensor can be calculated according to the Bernoulli law, the flow rate of a medium, when the density is known.

Design and function

All pitot tubes, including those that are used in aviation, have the same mode of action, with a similar structure. A spindle-shaped metal body is secured by means of a bracket on a measurement support. Inside the metal body are channels that are connected via holes with the environment. About the carrier pipe, the channels are connected to suitable measuring instruments. For an aircraft flying under instrument flight conditions, heating is provided by means of heating coils, as the position of the pitot tube icing, and thus the risk of instrument failure favors.

At the top of the front open pipe, the stagnation point is a flow rate of 0. A device that only has an opening at the stagnation point is referred to in the German language as the actual pitot tube (English pitot tube ).

If the measuring tube used alone to measure the static pressure, so it is a static probe, which can be seen through lateral openings on the measuring tube and fastening at the stagnation point.

In German-speaking countries, the combination of pitot and static probe is usually called Prandtl tube. This designation is in the English language is unknown, there are always speaks of a pitot tube.

Applications

Pitot tubes are used in those applications where a simple and accurate flow measurement is to be performed.

Aviation

Aircraft move in a pressure environment that varies when they rise, fall, speed up or slow down the speed. This means that the Pitot tube, a variable total air pressure comprising the dynamic pressure and the static pressure occurs. The speed of an aircraft can be represented relative to the dynamic pressure in the airspeed indicator. The total pressure is absorbed by the pitot tube and forwarded by the total pressure line to the airspeed indicator. The higher the back pressure, the higher the speed.

Since the pitot tube but in addition to the desired back pressure still provides the static pressure of the environment, this must be removed by pressure equalization again in order to display only the dynamic pressure can.

A pitot tube is mounted in a prominent location on the aircraft outside where the slightest disturbance of the air flow can be expected. This can for example be below a wing with the opening outside the surface air flow or jets at the tip of the nose. The mounting position should aim in particular to keep the static error as small as possible. Often wind tunnel tests must be performed to determine the optimum position. For larger aircraft and other military aircraft security reasons several pitot tubes at various points.

The air flow passes in a straight line and unaffected in the tube, which is surrounded at Prandtl tubes having lateral orifices for measuring static pressure. The dynamic pressure is measured, for example, with an attached pressure cell or a transducer and according to the Bernoulli equation in the airspeed indicator as appropriate airspeed (IAS, Eng. Indicated Air Speed ​​) appears.

In the late 1930s, aircraft reached speeds in the transonic range, it was found that those connected to the pitot tube instruments indicated a higher by about 10% speed than was actually present. This was due to compressibility effects, since the Bernoulli's equation only applies for incompressible media. Uncompensated measurement systems provide useful measurements only in the lower speed range. With increasing flight speed - from about Mach 0.3 - performs the compressibility of the air, that is, the density change of the air compression, in a measurement error, which indicates a higher rate than is actually available.

By means of tables, or analog or computer calculation must then be made a velocity correction to gain from the IAS is an equivalence speed (EAS, Eng. Equivalent air speed ). This is an equivalent to the air flowing around the wing dar. aircraft at speeds well above Mach 0.3 usually have speed indicators that displays this corrected for the compression speed as a multiple of the Mach number.

Pitot types of aircraft

Mechanical variant

In the mechanical shape runs, as described above, a hose or tube from the pitot tube ( Pitot tube ) to a pressure cell in the airspeed indicator and also a tube from the static port directly to all three barometric instruments. Here there is no electrical energy is required, which is, for example, in sailplanes beneficial. Pitotrohrform this is independent of the size is still common in aircraft, which are not digitized data, the display in the cockpit. The distance from the pitot tube to the meter should this be low in order to keep the volume of gas in the system low and so as to ensure a fast response of the instrument.

Electronic version

In the second case, electric power is required. Here, the opening for the dynamic pressure over a channel with one half of the measured value transducer is connected. Here the total pressure comes on. At the pitot tube located on the side more small holes that will eventually run on separate channels together the other half of the measured value converter. This is due to the static pressure.

The transducer (differential pressure sensor ) measures the pressure difference by means of a pressure sensor. The pressure difference is relatively low, so that only a very sensitive sensor can be used. In question piezoresistive or capacitive transducers come. The first time the resistance of a ceramic measuring plate is measured, the second, the capacity of a variable by the pressure fluctuation capacitor.

This information is then normalized by the amplifier is converted into an analog electrical signal and sent to the airspeed indicator, which forms thereof according to Bernoulli's law, a speed indication. In modern digital cockpit electronics is one of the so-called bus node that represents the measured data without analog conversion directly to the computers available.

Special shapes

In the military aviation extreme attitudes are, for example, in fighter aircraft frequently. In this case, a normal pitot tube is not in the air stream frontal, so under these conditions, the stagnation point moves to the side and makes a reliable measurement impossible. There are special forms for this was developed by Pitot tubes, which are arranged around the central stagnation point opening more openings that allow for a shift of the stagnation point an exact measurement. Alternatively so-called keel probes are used, in which the actual pitot tube is embedded in a venturi, to improve the flow at the measurement point.

There are also mobile devices, which automatically adjust to the flow direction. For use in the wind tunnel are miniaturized forms used, however, the refrain usually on a static probe.

Possibilities of error

Due to its prominent position in the airflow the pitot tube of an airplane is prone to dirt, insects, water and icing. Pitot tubes of parked aircraft are therefore provided with a protective coating that must be removed before the start. For aircraft that can fly in instrument flight conditions is prescribed because of the risk of icing a heated pitot tube. A defective pitot tube is considered probable cause of the crash of Flight 301 flights Birgenair in February 1996, Air France Flight 447 on June 1, 2009.

Motor vehicles

Pitot tubes are also used in fast-moving motor vehicles for use when a speed measurement is required, which is independent of the tire speed.

A typical case is the formula 1 Here the wind direction and wind speed for the setup of the vehicle play a role. Even for long -track testing of prototypes pitot tubes are used. For motor vehicles, in principle similar devices as in the aerospace industries, but these are always made ​​electronically.

Wind measurement

Analogously to the measuring instruments for aircraft, the pitot tube may also be used to measure wind speeds. Since the pitot tube on principle at very low wind speeds provides hardly a measurement result, but practically has no upper limit and quickly responds to wind speed changes, the meter is especially suited for high-wind and gust measurement and comes in conjunction with a differential pressure gauge as anemometer for use.

The measured value display can be both electronically and analyzed by differential pressure cell. Pitot tubes for meteorology are executed almost always heated.

Process engineering

The pitot tube is a simple instrument for measuring speed in many media. Pitot tubes are widely used in industry, the use of such probes is usually in a closed pipe system. It offered probes for ambient pressures up to 800 bar and temperatures from -250 ° C up to 1300 ° C suitable. An embodiment as the measurement beams is possible, and at the stagnation point, a number of measuring openings are provided in order to determine the flow profile over a cross-section with a probe can. The material of the probe is chosen according to the application. The more common name for such sensors is pitot tube.

Fire protection

Pitot tubes are measured and used to determine the available quantity of fire water hydrant. This method takes advantage of the fact that at a constant cross- section of the outlet opening of a fire hydrant, the speed of the exiting fluid ( here: fire water ) is directly proportional to the exiting flow. The pressure of the exiting extinguishing water can be converted using the following formula in the flow:

The use of a water meter is ruled out in this application, because the water meter is usually a different cross section than the hydrant opening and a separate flow resistance ( in case of fire use the fire no water meter ).

History of development

Henri de Pitot published in 1732 a proposal for a "machine for measuring the velocity of flowing water and of the wake of ships ". This principle remained to this day in use.

Pitots development but had weaknesses. It consisted of two adjacent tubes, one of which was bent at the lower end 90 °, to be directed into the water stream, while the second straight tube recorded static pressure. By this arrangement, however, the tube was for static pressure measurement within the turbulence caused by the bent tube in front of it. These theoretical shortcomings came on the implementation of the pressure difference in the flow rate. Also, due to constant fluctuations only quite inaccurate measurements could be made.

1775 James Lindt measured using a Pitot tube anemometer wind speeds. Here, a U- shaped tube according to the above figure at the front end was once again bent at 90 ° to the front and filled with a liquid. The air entering squeezed the water pipe at the rear of the U on a scale up. To increase the sensitivity, enlarged in 1858 William Snow Harris the air intake significantly.

As of 1856, developed by Pitot instrument by Henry Darcy has been considerably developed by mounting valves, docked a vacuum above the pipes, moved the inlet of the static tube on the side - and thus outside the turmoil of the pitot tube - and a new formula for calculating the flow rate developed. Even Darcy's development has been used primarily to measure the flowing water.

Ludwig Prandtl developed the design of the pitot tube still in use today. The operation described above.