Pressure, Atmosphere and Instrumentation

Pressure instruments are widely used in aviation. This article introduces the basic components of pressure as relevant to gliding. We shall then discuss how the pressure varies in the atmosphere, and the implications of this on the functioning of pressure instruments.

The author finds it very hard to discuss such topics without the aid of mathematical formulae. Every effort shall be made to explain the physical background of all the definitions and theorems used. If there is something you cannot understand, please consult an instructor who may determine what level of mathematical understanding (if any) is necessary. You do not need to be a physicist or engineer to fly gliders.

Fundamental Thermodynamics

Thermodynamics is a basic science that deals with energy. Its most notable application being the generation of power through heat engines. However, the only bit of thermodynamics we need to understand pressure is the Ideal Gas Law.

States and thermodynamic properties

Air can be liquified, but for all aviation purposes air is a gas. We shall not define a gas but the reader should have a good understanding of the concept through life experience.

Any substance of engineering interest can have states. For example, water can be ice, liquid water, or water vapour. If you are asked the question "What is the state of water at room temperature?" you may intuitively answer "liquid water". However, believe it or not, by reaching this conclusion you are making the assumption that the pressure that this water is at is at everyday atmospheric level. If the same water is put into vacuum while maintained at the same temperature, it will boil.

Therefore, to fully define the state that a substance is at, two thermodynamic properties must be specified. The most common thermodynamic properties can be:

• Temperature \(T\)
• Pressure \(p\)
• Density \( \rho \)

and a bunch of others. At least two must be known to fully define the state of a substance.

It is very important to understand that, by calling a thermodynamic property a property, it is implied that it belongs to the substance. It does not depend on the way you look at it, or, in physical language, it is independent of the frame of reference. For example, when you stand still you think the air is still at a temperature of 25°C, but if you drive past in a car you will think the air is moving. This does not, however, affect the fact that the air is still at 25°C. We shall see later that we can define some other things that depends on motion, known as the stagnated quantities, sometimes incorrectly referred to as "stagnation properties".

Ideal gas

Air can be modelled as an ideal gas. An ideal gas is a gas for which the following relationship holds:

\[ p = \rho R T \]

where:

• \(p\) is the pressure
• \(\rho\) is the density
• \(R\) is a constant that is a property of the gas itself, and
• \(T\) is the temperature.

Therefore, if we want to find the density of some air, both the pressure and the temperature needs to be specified (it is possible to substitute one or both with something else, but we shall not investigate this complexity). For example, atmospheric air and the air in a tyre are at the same temperature, but since the air in the tyre has a higher pressure (if you remember pumping it in), it has a higher density than atmospheric air.