Pitot Tube [PDF]

Zitiervorschau

I.

Introduction What is a pitot tube?

Basically, a pitot tube is a pressure measurement instrument used to measure flow speed or the fluid flow velocity. It is widely used to determine the airspeed of an aircraft, water speed of a boat, and to measure liquid, air and gas flow velocities in industrial applications. The pitot tube is used to measure the local flow velocity at a given point in the flow stream and not the average flow velocity in the pipe or conduit. It’s a slender tube that has two holes in it. The front hole is placed in the airstream to measure what’s called the stagnation pressure. The side hole measures the static pressure. By measuring the difference between these pressures, you get the dynamic pressure, which can be used to calculate airspeed. II.

History and Trivias Who invented the pitot tube?

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-19 th century by the French scientist Henry Darcy. Why is it most often located on the left wing of an aircraft? To give accurate airspeed readings, the pitot tube of an aircraft is placed at a location where there is relatively undisturbed airflow. Behind the propeller would clearly be a bad location! A location under the wing is pretty good spot. It is located on the left wing since it is a shorter distance to the instrument panel than the right side. But some aircrafts have pitot tubes on both sides. Can’t pilots just use Global Positioning System (GPS) to measure their airspeed? Airspeed readings from a GPS can be off by less than 25% than the readings from a pitot tube and this amount of error can create a dangerous situation. III.

Equipment, Parts and Characteristics

Pitot tubes on aircraft commonly have heating elements called pitot heat to prevent the tube from becoming clogged with ice.

IV.

Types

A variety of special forms of the pitot tube have been evolved. Included are the compact tube for boundary layer measurements and shielded totalpressure tubes. The latter are insensitive to angle of attack up to 40 degrees. A reversed pitot tube, also known as pitometer, has one pressure opening facing upstream and the other facing downstream. The pitot-venturi flow element is capable of developing a pressure differential 5 to 10 times that of a standard tube. V.

Typical Standard Design

See Table 10-4 (Comparison of Flowmeter Technologies), pp. 10-15 of Perry’s Chemical Engineer’s Handbook. VI.

Calculations Velocity (V0) at the tip of a pitot static tube is given by:

V 0=C √2 gh=C √2 gc ( PT −P S )/ ρ0 With gases at velocities above 60 m/s, compressibility becomes important and the equation should be:

pi p0 ¿ ¿ 2 gc k p0 ( )¿ k−1 ρ0 V 0=C √ ¿ Where C = coefficient, dimensionless gc = dimensionless constant h = dynamic pressure g = local acceleration due to gravity pi = impact pressure p0 = local static pressure P0 = fluid density

V M= √ kRT

For isentropic flow:

The relationships between total and static temperature and pressure are given by the following relationship:

TT k −1 2 =1+ M TS 2 PT k −1 2 (k−1 )/ k =(1+ M ) PS 2

VII.

Equipment for Measuring Gas Velocity

The hot-wire anemometer consists essentially of an electrically heated fine wire (generally platinum) exposed to the gas stream whose velocity is being measured. The hot-film anemometer has been developed for applications in which use of the hot-wire anemometer presents problems. It consists of a platinum-film sensing element deposited on a glass substrate. Various geometries can be used. The heated-thermocouple anemometer measures gas velocity from the cooling effect of the gas stream flowing across the hot junctions of a thermopile supplied with constant electrical power input. A glass coated bead thermistor anemometer can be used for measurement of low fluid velocities. The laser-Doppler anemometer measures local fluid velocity from the change in frequency of radiation, between a stationary source and a receiver, due to scattering by particles along the wave path.