Mixing Procedures

Two or more pure gases, or gas mixtures, may be combined by a variety of techniques 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.

16-2.1 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 realgas 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 temperature 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.

When using the ideal-gas procedures, a diver should add less oxygen than is called for, analyze the resulting mixture, and compensate as required. The U.S. Navy Diving-Gas Manual (NAVSEA 0994-LP-003-7010, June 1971) should be consulted for procedures to accurately calculate the partial pressures of each gas in the final mixture. These procedures take into consideration the compressibility of the gases being mixed. Regardless of the basis of the calculations used to determine the final partial pressures of the constituent gases, the mixture shall always be analyzed for oxygen content prior to use.

16-2.2 Ideal-Gas Method Mixing Procedure. Gas mixing may be prepared one cylinder at a time or to and from multiple cylinders. The required equipment is inert gas, oxygen, mix cylinders or flasks, an oxygen analyzer, and a mixing manifold. A gas transfer system may or may not be used. Typical mixing arrangements are shown in Figure 16-1 and Figure 16-2. To mix gas using the idea-gas method:

1. Measure the pressure in the inert-gas cylinder(s) PI.

2. Calculate the pressure in the mixed-gas cylinder(s) after mixing, using the following equation:

Where:

Pf = Final mix cylinder pressure, psig*

Pi = Inert gas cylinder pressure, psig

A = Decimal percent of inert gas in the final mixture

* PF cannot exceed the working pressure of the inert gas cylinder.

3. Measure the pressure in the oxygen cylinder(s), Po.

4. Determine if there is sufficient pressure in the oxygen cylinder(s) to accomplish mixing with or without an oxygen transfer pump.

Where:

PO = Pressure in the oxygen cylinder, psig 50 = Required minimum over pressure, psi > means greater than or equal to

5. Connect the inert-gas and oxygen cylinder(s) using an arrangement shown in Figure 16-1 or Figure 16-2.

6. Open the mix gas cylinders valve(s).

■ 1 I.

Figure 16-1. Mixing by Cascading.

Figure 16-2. Mixing with Gas Transfer System.

7. Open the oxygen cylinders valve. Bleed oxygen into the mix gas cylinders at a maximum rate of 70 psi minute until the desired Pp is reached.

8. Close the oxygen and mixed-gas cylinder valves. The heat of compression will have increased the temperature of the mixed-gas cylinders and will give a false indication of the pressure in the cylinder. The calculation requires the Pp to be taken at the same temperature as Pp However, because of the compressibility effects, more oxygen will normally have to be bled into the mixed-gas cylinders than expected. Therefore, allow the cylinders to stand for at least six hours to permit the gases to mix homogeneously, or if equipment is available, roll the cylinder for at least one hour. Analyze the gas mixture to determine its oxygen percentage. The percentage of oxygen should be near or slightly below the desired percentage.

9. Add oxygen as necessary and reanalyze the mixture. Repeat this step until the desired mixture is attained.

16-2.3 Adjustment of Oxygen Percentage.

necessary to increase or decrease the

16-2.3.1 Increasing the Oxygen Percentage.

After filling a mixed-gas cylinder, it may be percentage of oxygen in the cylinder.

To increase the oxygen percentage:

1. Subtract the known percentage of oxygen from 100 to obtain the existing percentage of helium.

2. Multiply the helium percentage by the cylinder pressure to obtain the pressure of helium in the cylinder.

3. Subtract the desired oxygen percentage from 100 to obtain the desired percentage of helium.

4. Divide the existing helium pressure (Step 2) by the desired helium percentage (Step 3) in decimal form. (This step gives the cylinder pressure that will exist when enough oxygen has been added to yield the desired percentage.)

5. Add oxygen until this pressure is reached.

6. Allow temperature and pressure to stabilize and add more oxygen, if necessary.

The following formula sums up the computation:

Where:

F = Final cylinder pressure P = Original Cylinder pressure 0o = Original oxygen % (decimal form) Of = Final oxygen % (decimal form)

Sample Problem. An oxygen cylinder contains 1,000 psi of a 16 percent oxygen mixture, and a 20 percent oxygen mixture is desired.

--------------1---.-0---0----------0---.--2---0---------------

Add 50 psi of oxygen to obtain a cylinder pressure of 1,050 psi.

16-2.3.2 Reducing the Oxygen Percentage. To reduce the oxygen percentage, use the following procedure:

1. Multiply oxygen percentage (decimal form) by the cylinder pressure to obtain the psi of oxygen pressure.

2. Divide this figure by the desired oxygen percentage (decimal form). This yields the final pressure to be obtained by adding helium.

3. Add helium until this pressure is reached.

4. Allow temperature and pressure to stabilize and add more helium, if necessary.

The following formula sums up the computation:

Where:

F = Final cylinder pressure

P = Original Cylinder pressure

Oo = Original oxygen % (decimal form)

Of = Final oxygen % (decimal form)

Sample Problem. For a cylinder containing 1,000 psi of a 20 percent oxygen mixture and a 16 percent oxygen mixture is desired.

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