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Air Pressure Sensor

Technical Information

Catalogue No: C1122
Category: Air Data
Object Type: Sensor/Transducer
Object Name: Air Pressure Sensor
Part No: 7882AB
Serial No: 1036
Manufacturer: Weston Aerospace
Division: Unknown
Platform(s):
Year of Manufacture: 2000
Dimensions:
Width (mm):
79 
Height (mm):
43 
Depth (mm):
76 
Weight (g):
231 
Location: Main Object Store
Inscription(s):

weston aerospace
Tel: +44(0)1252 544433
Type 7882AB
SN: 1036
DoM: 04/08/00 13:03
A Roxboro Group Company
Marconi Avionics Ltd
P/N 60401-003-10
────────────────
[etched on the unshielded sensor]
262kPa
028336
7881 2ZF
S/N 925361
────────────────
[etched on the shielded sensor]
130kPa
126971
7881 12D

Notes:

The Weston pressure sensor is based around a vibrating cylinder, where the natural
frequency of cylinder depends upon the applied pressure. This is classed as a
‘vibrating element’ sensor and exhibits exceptional measurement performance by
virtue of its operating mechanism.
The advantage of the vibrating cylinder technique lies in the mechanism that converts
from pressure to frequency change. When pressure is applied, the stress generated
in the wall of the cylinder affects the frequency directly and, although strain is
generated in the material, this has only a small effect on the frequency. The
consequence of this is that hysteresis and creep of the material have very little effect
on the pressure measurement. In addition, no other mechanism is required to read
out the measurement (like a strain gauge). The cylinder acts as the complete
measurement system and it is only necessary to maintain it in resonance so that the
frequency can be measured.
The consequence of the directness of this measurement technique is that high
measurement accuracy <0.01% of full scale pressure (FSP) is achieved with very low
drift rates <0.01%FSP/year.

Air data systems provide accurate information on quantities such as pressure altitude, vertical speed, calibrated airspeed, true airspeed, Mach number, static air temperature and air density ratio. This information is essential for the pilot to fly the aircraft safely and is also required by a number of key avionic subsystems which enable the pilot to carry out the mission. It is thus one of the key avionic systems in its own right and forms part of the essential core of avionic sub systems required in all modern aircraft, civil or military.

The air data quantities; pressure, altitude, vertical speed, calibrated airspeed, true airspeed, Mach number etc. are derived from three basic measurements by sensors connected to probes which measure:

Total (or Pitot) pressure
Static pressure
Total (or indicated) air temperature

The total pressure, PT, is measured by means of an absolute pressure sensor (or transducer) connected to a Pitot tube facing the moving airstream. The Pitot pressure is a measure of ram air pressure (the air pressure created by vehicle motion or the air ramming into the tube). When airspeed increases, the ram air pressure is increased, which can be translated by the airspeed indicator.

The static pressure of the free airstream, PS, is measured by an absolute pressure transducer connected to a suitable orifice located where the surface pressure is nearly the same as the pressure of the surrounding atmosphere. The static pressure is obtained through a static port which most often is a flush-mounted hole on the fuselage of an aircraft located where it can access the air flow in a relatively undisturbed area. Some aircraft may have a single static port, while others may have more than one. When the aircraft climbs, static pressure will decrease.

High performance military aircraft generally have a combined Pitot/static probe which extends out in front of the aircraft so as to be as far away as practicable from aerodynamic interference effects and shock waves generated by the aircraft structure. A Pitot-static tube effectively integrates the static ports into the Pitot probe. It incorporates a second coaxial tube (or tubes) with pressure sampling holes on the sides of the probe, outside the direct airflow, to measure the static pressure. Some civil transport aircraft have Pitot probes with separate static pressure orifices located in the fuselage generally somewhere between the nose and the wing.

From the measurements of static pressure PT and total pressure PS it is possible to derive the Pressure Altitude, Vertical Speed, Calibrated Airspeed and Mach number. Measurement of the air temperature is made by means of a temperature sensor installed in a probe in the airstream and from this a function called Total Air Temperature can be calculated.

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