Physics of Diving. Our Normal Environment - Air
Physics of Diving. Our Normal Environment - Air
The diver is affected by increasing water pressure as he descends and this manifests itself in several ways. Some will be noticed quickly: others will take longer to become apparent. Both the diver’s body and his equipment will be affected. Divers should have a clear understanding of how the laws of physics apply to them and to their equipment. Without this knowledge they put themselves at risk.
Before considering the diving environment, it is necessary to look at the atmosphere in which we normally live and the gases which make up the air we breathe.
ATMOSPHERIC PRESSURE. The earth is surrounded by an envelope of air which we call the atmosphere. Air is a mixture of gases, and like all matter, it has mass. A mass exerts a force on those things which lie beneath it, and at sea level the atmosphere presses down with a force of approximately 1 kilogram for every square centimetre of the earth’s surface. Gas pressure is commonly measured in units of bar and our own atmosphere exerts a pressure at sea level of approximately 1 bar.
Atmospheric Pressure = 1.02 bar (1 bar approx.)
Atmospheric pressure varies slightly with changes in weather and diminishes with altitude until it reaches zero at the extreme limit of the atmosphere. At about 5000 m above sea level, for example, the atmospheric pressure is about 0.5 bar.
Our bodies do not suffer in any way from this pressure which is applied to every square centimetre of their surface-we are born to it!
GAUGE PRESSURE. When a pressure is to be measured, it is normal practice to relate it to ambient pressure. Thus a simple gauge would read zero at an atmospheric pressure of 1 bar. An aqualung contents gauge would perhaps read 200 bar, but this really means 200 bar above the normal atmospheric pressure of 1 bar. Such a recording would be known as a Gauge Pressure.
ABSOLUTE PRESSURE. If the above gauge were related to true zero as found in a vacuum it would read 201 bar - the extra 1 bar being atmospheric pressure.
Such a gauge reading would be termed an Absolute Pressure.
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
In diving physics, it is normal to work in absolute terms, and the reasons for doing so will be soon apparent.
COMPOSITION OF AIR. The air we breathe is a mixture of gases comprising:
Nitrogen (N2) approx. 79% (say, 4/5)
Oxygen (02) approx. 21% (say, 1/5)
There are traces of Carbon Dioxide (C02) and other rare or inert gases, but in such small quantities that they can be ignored. All gases are compressible, having neither shape nor volume.
On the other hand, liquids have a definite volume and mass and may be considered to be incompressible at the pressures we are to consider.