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CHAPTER 14 — Breathing Gas Mixing Procedures 14-1 CHAPTER 14 Breathing Gas Mixing Procedures 14-1 INTRODUCTION 14-1.1 Purpose. The purpose of this chapter is to familiarize divers with the techniques used to mix divers’ breathing gas. 14-1.2 Scope. This chapter outlines the procedures used in mixing divers’ breathing and treat ment gas. 14-2 MIXING PROCEDURES Two or more pure gases, or gas mixtures, may be combined by a variety of tech­ niques to form a final mixture of predetermined composition. This section discusses the techniques for mixing gases. Aboard ships, where space is limited and motion can affect the accuracy of precision scales, gases are normally mixed by partial pressure or by continuous-flow mixing systems. The methods of mixing by volume or weight are most suitable for use in shore ­ based facilities because the procedure requires large, gas ­ tight holding tanks and precision scales. 14-2.1 Mixing by Partial Pressure. Mixing gases in proportion to their partial pressures in the final mixture is the method commonly used at most Navy facilities. The basic principle behind this method is Dalton’s Law of Partial Pressures, which states that the total pressure of a mixture is equal to the sum of the partial pressures of all the gases in the mixture. The partial pressure of a gas in a mixture can be calculated using the ideal ­ gas (perfect ­ gas) method or the real ­ gas method. The ideal ­ gas method assumes that pressure is directly proportional to the temperature and density of a gas. The real ­ gas method additionally accounts for the fact that some gases will compress more or less than other gases. Compressibility is a physical property of every gas. Helium does not compress as much as oxygen. If two cylinders with the same internal volume are filled to the same pressure, one with oxygen and the other with helium, the oxygen cylinder will hold more cubic feet of gas than the helium cylinder. As pressure is increased, and/or as tempera ture is decreased in both cylinders, the relative difference in the amount of gas in each cylinder increases accordingly. The same phenomenon results when two gases are mixed in one cylinder. If an empty cylinder is filled to 1,000 psia with oxygen and topped off to 2,000 psia with helium, the resulting mixture contains more oxygen than helium. Being aware of the differences in the compressibility of various gases is usually sufficient to avoid the problems that are often encountered when mixing gases.

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