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Pressure Modules for Dynamic Pressure

3.5.3 Pressure Modules for Dynamic Pressure

Basic Principles

The air velocity is determined by the dynam-

ic pressure and the static pressure, which

develops when a Prandtl Pitot tube is placed

in an air flow.

The total pressure strikes the opening of the

Pitot tube and is available at the terminal (+)

of the pressure module.

The pure static pressure is measured via

side slots and is available at the terminal (-).

The pressure difference, i.e. the dynamic

pressure is a measure for the velocity in the

flow. The dynamic pressure is calculated

and indicated.

The dynamic pressure relates to the air velocity as follows:

1. Density of the air:

with ρ

= 1.292 kg/m

(density at 0°C)

T = air temperature in °C

2. Air velocity (valid up to approx. 40 m/s):

with p = dynamic pressure in Pa

k = probe-related coefficient

Prandtl tube: k = 1

cylindrical probe: k = 1.7

3. Air velocity with consideration of the compressibility of the air

(also valid at more than 40 m/s):

with c = speed of sound in air

(331 + 0.6 x T m/s)

The formulae show the influence of the air temperature to the density of the air

and, consequently, to the result of the measurement of the dynamic pressure.

Furthermore, the deviation of the atmospheric pressure pa from the normal

pressure 1013mbar also has an effect on the result. The following factor can

be used for correcting the velocity:

K ≈ 1 + (1013 - pa) · 0.0005 (in 1st approximation)

3-5-14 ALMEMO

Sensors

static pressure

total pressure

=

273

273

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/ 2

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1013mbar

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