The Pressure Instruments

What are the pressure instruments ?

The pressure instruments are The pressure instruments are just sensitive barometers.
As such they measure the difference between two air pressures and communicate the difference to the pilot by moving a pointer around a scale.
They are adapted for use in aircraft by the addition of scales which are calibrated to indicate pressure differences in relevant units. The units are chosen dependant upon the use to which the instrument is put (it would be incorrect to measure Altitude in Knots).

How It Works

The Barometer

 
The barometer works by measuring the difference between the air pressure inside a sealed capsule and the air pressure inside the casing which contains the capsule (the casing air pressure is mechanically simulated in the case of the altimeter). The capsule is made from an easily deformed material and acts somewhat like a balloon in that when it is squeezed it changes shape. The capsule is manufactured in such a way as to enable this deformation to take place in a controlled manner it will contract or expand along one axis only. This expansion or contraction is used to drive a pointer around a scale by using a system of gears and springs.

Example Cont

Web References

Nasa Article Describing Pitot and Static Pressure Types of Pitot Tube Barometers Principle Of Operation

Bibliography
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