GAS MIXING

In making gas storage volume calculations it must be recognized that the

storage volume will not flow gas to the chamber when at equilibrium pressure

with the chamber unless provisions are included to transfer the gas. In Part

2 of the U.S. Navy Diving-Gas Manual (Reference (1)), tables of gas

properties are presented for convenient use in the design of diving systems.

These tables show how gasses such as helium, oxygen, and air deviate from the

ideal gas laws. The pressure range covered is 0 to 1500 feet of seawater for

chamber calculations, and 500 to 5000 psi for gas storage calculations. The

temperature range covered is 30 deg. F to 130 deg. F. Properties of 5 pure

gasses and of 25 mixtures of heliox are tabulated.

Required storage volume will dictate the size and number of storage flasks

required. Flask shape and volume for the various sizes used by the U.S. Navy

is included in military specification MIL-F-22606.

4. GAS MIXING. When gasses, other than air, are used for breathing at depth,

a method of accurately mixing the various gasses must be devised.

Commercial mix rig systems are available which utilize a continuous flow

mixing process. This method has proved most accurate, repeatable and a

producer of a large quantity of gas in a short period. Figure 6-15 shows how

the pure gasses flow at normal storage pressures (1100 psi or greater) to the

input regulator (1), the gas is then heated to a specified and controlled

temperature in heaters (2), so that the flow metering valves (3) always

measure gas at the same pressure and temperature. This eliminates the need

for computing temperature and pressure corrections. After passing through

the metering valves, the three gas streams are mixed in a turbulent mixing

chamber (4), then passed through a regulator (5) that controls the

flowmeter-outlet pressure at 950 psi. A sample of the mixed gas is withdrawn

through line (6) and passed through a gas analyzer. The main gas stream

passes through regulator (7), where its pressure is regulated at the value

needed to supply the diver. An accumulator (9) provides for mixed-gas

storage. With the above pressure levels, the rate of flow of the mixture is

160 scfm, equivalent to 6 cfm at depth for a depth of 850 feet. The mixer

output may be used to supply the PV directly or may be routed through a

compressor to storage.