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 A sample of the mixed gas is withdrawn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