move into solution than there are dissolved molecules available to migrate out. As molecules enter the liquid, they increase the gas tension, or partial pressure of the gas in the liquid (figure 3-10). The pressure gradient becomes less as the gas tension increases relative to the partial pressure of the gas. In the random motion of the gas molecules, more molecules begin to diffuse out of solution and the ingassing process slows. Eventually the tension reaches a value equal to the partial pressure. At that point the pressure gradient is zero, equilibrium is reached, and there will be no net change in the amount of gas in solution. The liquid is saturated with the gas. Molecules will continue to randomly enter or exit the liquid, but the balance will continue unless there is a change in pressure or temperature. If the partial pressure of the gas on the solution is lowered, offgassing will occur. You can see that ingassing/offgassing is a curve by examining any set of dive tables. Notice how rapidly offgassing occurs right after you exit a dive (how quickly you pass through the first few letter groups) compared to how slowly you outgas as your body approaches equilibrium with atmospheric pressure (how long it takes you to pass through groups “B” and “A” and off the tables). Whenever you dive, the gases you breathe will begin to pass into solution in the tissues of your body. The quantity of a particular gas dissolved is governed by the pressure and the amount of time you breathe the gas at increased pressure, as well as the solubility of that gas in a given tissue type. If you remain submerged long enough, your body will become saturated for that particular depth with the gas. As you start your ascent, you begin to outgas. If the ascent is slow enough, the dissolved gas will be carried to the lungs by your blood and exhaled as it passes out of solution into your alveoli. However, if your ascent is too fast, your body is not able to eliminate the overburden of gas fast enough to prevent bubble formation in the tissues, and decompression sickness may result. Energy Three concepts we met early in this chapter were energy, force, and work. We have defined energy as the capacity to do work and have spoken of kinetic energy (of a body in motion) and potential energy (possessed by a body at rest because of its position). Force is any action that tends to maintain or alter the position of a body or to distort it. It is a push or a pull. Forces are often illustrated with arrows which show their direction and magnitude (represented by the length of the arrow). Work occurs when a mass is moved over a distance by an external force. In this definition, work is accomplished only when an object is displaced, a gas compressed, molecules in an object made to move more rapidly, etc. No work is done unless the object is moved in some way. When you lift a scuba cylinder, you perform work. But if you are simply holding the cylinder without moving it, no energy is transferred to it and no work is performed. Under ordinary conditions, energy can be neither created nor destroyed. It can be transferred, and it can be changed from one form to another. Because all forms of energy can be converted to work, amounts of energy are expressed in units of work (application of force through distance, in units such as joules, calories, foot-pounds, and kilowatt-hours). There are six basic forms of energy: 1. Mechanical: The kinetic and potential energies resulting from the movement of a body. 2. Heat: The energy of molecular motion. 3. Electromagnetic radiation: The energy of electromagnetic waves, such as light, x-rays, and radio waves. 4. Chemical: The energy released from chemical reactions. 5. Electrical: The energy of moving electrons. 6. Nuclear: The energy of atomic forces. NAUI Master Scuba Diver 96 Diving Physics Ingassing Offgassing Gas Tension Time FIGURE 3-10. INGASSING AND OFFGASSING
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