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.
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.
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:
PF
PI + 14.7
A
= --------------------- – 14.7
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.
PO ³(2PF – PI)+ 50
Where:
PO = Pressure in the oxygen cylinder, psig
50 = Required minimum over pressure, psi
³ 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).
7. Open the oxygen cylinders valve. Bleed oxygen into the mix gas cylinders at a
maximum rate of 70 psi minute until the desired PF 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 PF to be
taken at the same temperature as PI. 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.
figure16-1 Mixing by Cascading.
|
figure16-2 Mixing with Gas Transfer System.
|
After filling a mixed-gas cylinder, it may be
necessary to increase or decrease the percentage of oxygen in the cylinder.
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:
F
P 1.00 Oo
´( – )
(1.00 – Of)
= ------------------------------------
]
Where:
F = Final cylinder pressure
P = Original Cylinder pressure
Oo = 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.
F
1, 000 ´(1.00 – 0.16)
1.00 – 0.20
= ----------------------------------------------------
1, 000 ´ 84
0.80
= ---------------------------
840
0.80
=
= 1, 050 psi
Add 50 psi of oxygen to obtain a cylinder pressure of 1,050 psi.
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:
F
P´Oo
Of
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.
F
1, 000 ´ 0.20
0.16
= -------------------------------
200
0.16
= ----------
= 1, 250 psi
Add 250 psi of helium to obtain a cylinder pressure of 1,250 psi.
These mixing procedures also apply to mixing by means of an oxygen-transfer
pump. Instead of being bled directly from an oxygen cylinder into a helium
cylinder, oxygen may be drawn from a cylinder at low pressure by the oxygentransfer
pump until the proper cylinder pressure is reached. This allows most of
the oxygen in the cylinder to be used, and it also conserves gas.
Continuous-flow mixing is a precalibrated mixing
system that proportions the amounts of each gas in a mixture by controlling the
flow of each gas as it is delivered to a common mixing chamber. Continuous-flow
gas mixing systems perform a series of functions that ensure extremely accurate
mixtures. Constituent gases are regulated to the same pressure and temperature
before they are metered through precision micro-metering valves. The valve
settings are precalibrated and displayed on curves that are provided with every
system and relate final mixture percentages with valve settings. After mixing, the
mixture is analyzed on-line to provide a continuous history of the oxygen
percentage. Many systems have feedback controls that automatically adjust the
valve settings when the oxygen percentage of the mixture varies from preset tolerance
limits. The final mixture may be supplied directly to a diver or a chamber or
be compressed into storage tanks for later use.
Mixing by volume is a technique where known volumes of
each gas are delivered to a constant-pressure gas holder at near-atmospheric pressure.
The final mixture is subsequently compressed into high-pressure cylinders.
Mixing by volume requires accurate gas meters for measuring the volume of each
gas added to the mixture. When preparing mixtures with this technique, the gases
being mixed shall be at the same temperature unless the gas meters are temperature
compensated.
The volumes of each of the constituent gases are calculated based on their desired
percentages in the final mixture. For example, if 1,000 scf of a 90 percent
helium/10 percent oxygen mixture is needed, 900 scf of helium will be added to
100 scf of oxygen. Normally, an inflatable bag large enough to contain the
required volume of gas at near-atmospheric pressure is used as the mixing
chamber. The pure gases, which are initially contained in high-pressure cylinders,
are regulated at atmospheric pressure, metered, and then piped into the mixing
chamber. Finally, the mixture is compressed and stored in high-pressure flasks or
cylinders.
Provided that the temperatures of the constituent gases are essentially the same,
extremely accurate mixtures are possible by using the volume technique of
mixing. Additionally, care must be taken to ensure that the mixing chamber is
either completely empty or has been filled with a known mixture of uncontaminated
gas before mixing.
Mixing by weight is most often employed where small,
portable cylinders are used. This proportions the gases in the final mixture by the
weight that each gas adds to the initial weight of the container. When mixing by
weight, the empty weight of the container must be known as well as the weight of
any gases already inside the container. The weight of each gas to be added to the
container must be calculated using the procedures described in the U.S. Navy
Diving-Gas Manual. Although the accuracy of the mixture when using this technique
is not affected by variations in gas temperature, it is directly dependent on
the accuracy of the scale being used to weigh the gases. This accuracy shall be
known and the operator must be aware of its effect on the accuracy of the composition
of the final mixture. As a safeguard, the final mixture must be analyzed for
composition using an accurate method of analysis.
