Some of the major physical hazards are related to the effects of pressure. Pressure is defined as force per unit area. i.e.
PRESSURE = FORCE
If a force is spread over twice the area, the pressure is halved. This explains why, for example, wide tires are preferable for driving on beaches. The weight of the vehicle (force) when spread over a large area causes less pressure on the sand. This vehicle is less likely to sink into the sand than one with narrow tires.
Gases exert pressure because they are made up of lots of fast moving molecules. The greater the number and the faster they move, the greater the pressure. Pressure on a Submerged Diver The pressure acting on a submerged diver has two components:
- The atmosphere above the water, termed atmospheric pressure,
- The weight of the water above the diver, termed hydrostatic pressure.
Divers’ depth gauges are calibrated only to read the hydrostatic pressure (the depth of water) and so they read zero at sea level. They do not read the 1 atmosphere (1 ATA) above them. Thus the “gauge pressure” is always 1 atmosphere less than the true or “absolute” pressure. We will now elaborate.
The atmosphere above the earth is some 150 km high. Although air is very light, this amount of air has significant weight and exerts substantial pressure on the earth’s surface.
Atmospheric pressure at sea level is referred to as “one atmosphere” or “one bar”. It is the same as 101.3 kPa, 1 kg/cm2, 760mm Hg and 14.7 psi. At higher altitudes, atmospheric pressure is reduced, a factor which has a significant effect on diving in mountain lakes (see Chapter 6).
Fig. 2.2 Atmospheric and Hydrostatic Pressures (depth)
added and thus converted to Absolute Pressure
Water is much denser than air and 10 meters (or 33 ft) of sea water exerts the same pressure (weight) as the whole 150 km of atmospheric air i.e. 1 ATA. For every additional 10 meres the diver descends, the water will exert a further pressure, equivalent to another atmosphere (1 ATA).
Common units of pressure (approximately):
As described above, hydrostatic pressure in diving is generally measured by a pressure or depth gauge. Such a gauge is normally set to register a pressure of zero at sea level and so it ignores the pressure due to the atmosphere (1ATA). The pressure registered by a gauge at 10 meters sea water depth would thus be one atmosphere gauge (1ATG) or equivalent units. Gauge pressure is converted to absolute pressure by adding 1 atmosphere pressure.
With a mixture of gases, the proportion of the total pressure contributed by each of the gases is termed its partial pressure (its part of the pressure). The partial pressure contributed by each gas is proportional to its percentage of the mixture. Each gas contributes the same proportion to the total pressure of the mixture, as is its proportion in the composition of the mixture. e.g. air at 1 ATA contains 21% oxygen, hence the partial pressure of oxygen is 0.21 ATA and
air at 1 ATA contains 78% nitrogen, hence the partial pressure of nitrogen is 0.78 ATA.
The total pressure exerted on a diver at depth will be the pressure due to the atmosphere acting on the surface of the water (atmospheric pressure) plus the pressure due to the depth of the water itself (hydrostatic pressure). The total pressure acting on the diver is termed the “absolute pressure”. It is often expressed in terms of atmospheres and is called “atmospheres absolute” or “ATA”. To calculate the absolute pressure acting on a diver at a given depth in terms of atmospheres, divide the depth in meters by 10 (since every 10 m. sea water exerts 1 atmosphere pressure) and add 1 (the pressure of the atmosphere above the water). e.g. the absolute pressure at 40 meters is [40 ÷ 10] + 1 = 5 ATA (The depth in feet, divided by 33 + 1 also calculates absolute pressure, for those in the USA, e.g. the absolute pressure at 99 ft is 99/33 +1 = 4 ATA).
The majority of this text was contributed via the Free and Open Works of Dr. Edmond's Diving Medicine- http://www.divingmedicine.info please visit their site for a FREE Downloadable PDF of their entire works.