Gases are subject to three closely interrelated factors—temperature, pressure, and volume. As the kinetic theory of gases points out, a change in one of these factors must result in some measurable change in the other factors. Further, the theory indicates that the kinetic behavior of any one gas is the same for all gases or mixtures of gases. Consequently, basic laws have been established to help predict the changes that will be reflected in one factor as the conditions of one or both of the other factors change. A diver needs to know how changing pressure will effect the air in his suit and lungs as he moves up and down in the water. He must be able to determine whether an air compressor can deliver an adequate supply of air to a proposed operating depth. He also needs to be able to interpret the reading on the pressure gauge of his tanks under varying conditions of temperature and pressure. The answers to such questions are calculated using a set of rules called the gas laws. This section explains the gas laws of direct concern to divers.
Boyle's Law. Boyle's law states that at constant temperature, the absolute pressure and the volume of gas are inversely proportional. As pressure increases the gas volume is reduced; as the pressure is reduced the gas volume increases. Boyle's law is important to divers because it relates to change in the volume of a gas caused by the change in pressure, due to depth, which defines the relationship of pressure and volume in breathing gas supplies.
C = a constant P = absolute pressure
V = volume
Boyle's law can also be expressed as: P1V1 = P2V2 Where:
P1 = initial pressure
VI = initial volume P2 = final pressure V2 = final volume
When working with Boyle's law, pressure may be measured in atmospheres absolute. To calculate pressure using atmospheres absolute:
p = Depth fsw -i- 33 fsw p = psig + 14.7psi Pata = ------------------3---3-----f--s---w-------------------- or Pata = ----------1---4---.--7---p---s---i----------
Sample Problem 1. An open diving bell with a volume of 24 cubic feet is to be lowered into the sea from a support craft. No air is supplied to or lost from the bell. Calculate the volume of the air in the bell at 99 fsw.
1. Rearrange the formula for Boyle's law to find the final volume (V2):
2. Calculate the final pressure (P2) at 99 fsw:
3. Substitute known values to find the final volume:
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