to oxygen and preventing the normal transport of oxygen to our tissues. Carbon monoxide is produced by the incomplete oxidation of carbon or carbon-containing fuels, especially in internal-combustion engines and furnaces. Its concentration in unpolluted air is roughly 100-200 parts per billion (the higher concentrations are in the more industrialized Northern Hemisphere). Water Vapor The amount of water vapor in the atmosphere is referred to as humidity. This is the most variable characteristic of air. The amount of water vapor that the air can retain is governed almost entirely by temperature. The higher the temperature, the more water the atmosphere can hold. When the air holds the maximum amount of water vapor possible, it is said to be saturated. Humidity is usually expressed as relative humidity, the percent to which the air is saturated with water vapor at its current temperature. As air is cooled, its relative humidity will rise because the total amount of water vapor the air is able to contain is decreasing. If cooling continues below the saturation temperature (dew point), the moisture will condense as clouds, fog, or precipitation. A fogged diving mask is an example of condensation of the moisture in your expired breath on the cooler faceplate of the mask. Water vapor is an important consideration in scuba breathing air. As air is compressed by the compressor, its temperature increases. The increased concentration of water in the smaller volume of air is able to remain as vapor because of the higher temperature. But as the compressed air cools, the water vapor will condense into liquid water. To prevent this condensation from occurring in your equipment, as well as to remove possible oil vapor, the air is passed through intercoolers after each compressor stage and through a desiccant in the end filtration system. The air in your cylinder is therefore very dry. When you breathe scuba air, your body must remoisturize the inspired air and raise its relative humidity to 100%. The moisture provided to the air by the respiratory organs is a major cause of dehydration in divers. You should drink plenty of water before and after diving to compensate for your moisture losses. Chapter 3- Diving Physics MEASURING THE PHYSICAL WORLD Much of our ability to understand and compare things and happenings in the world relies on our ability to measure them. Measurements can be more or less exact depending on our need for accuracy. In everyday usage, we often give approximations that may be more meaningful for our purposes than an exact measurement. “A 20-minute walk” is probably more useful information about the distance into town than saying it is 541.5 meters or 1,776 feet 6 inches. Science, on the other hand, requires exact measurements to compare quantities or phenomena. Much of the history of measurement systems is the quest for uniform, easily related, standard units of measurement. Measurement systems relate to several basic concepts: 1. Length – the distance between two points. Area and volume are determined in units of length. 2. Mass – the property of a body that is a measure of its inertia (resistance to a change in motion). Mass is commonly equated with weight in a given gravitational field. 3. Time – duration. 4. Energy – the capacity to do work. Temperature, a measure of hotness, is produced by the average kinetic energy of the molecules of a body. 5. Force – any action that tends to cause change in motion of a body or to distort it. Two systems of measurement are in use in most of the world today. The most widely used is the International System of Units (SI, for Le Systeme internationale). The SI came into being by international agreement in October 1960. It grew naturally out of the metric system as a need for increased accuracy, and standardization became necessary. SI units are nearly equivalent to their predecessor metric units, and SI is still commonly called the metric system. The metric system had its origins during the late 18th century among scientists who sought to replace the many regional systems of measurement with a rational, scientifically based one. The SI/metric system has two distinct advantages. All of its units are related, so that it is not necessary to use complex calculations when making conversions between them. Also, the system is a decimal system, and all units Diving Physics 71
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