CHAPTER 16 — Breathing Gas Mixing Procedures 16-1
CHAPTER 16
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16-1
INTRODUCTION
16-1.1
Purpose.
The purpose of this chapter is to familiarize divers with the techniques
used to mix divers’ breathing gas.
16-1.2
Scope.
This chapter outlines the procedures used in mixing divers’ breathing and
treatment gas.
16-2
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.
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 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.
16-2 U.S. Navy Diving Manual—Volume 3
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 deter-
mine 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) P
I
.
2.
Calculate the pressure in the mixed-gas cylinder(s) after mixing, using the fol-
lowing equation:
Where:
P
F
= Final mix cylinder pressure, psig*
P
I
= 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), P
o
.
4.
Determine if there is sufficient pressure in the oxygen cylinder(s) to accom-
plish mixing with or without an oxygen transfer pump.
Where:
P
O
= 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).
P
F
P
I
14.7+
A
----------------------
14.7=
P
O
2P
F
P
I
()
50+
CHAPTER 16 — Breathing Gas Mixing Procedures 16-3
Figure 16-1.
Mixing by Cascading.
16-4 U.S. Navy Diving Manual—Volume 3
Figure 16-2.
Mixing with Gas Transfer System.
CHAPTER 16 — Breathing Gas Mixing Procedures 16-5
7.
Open the oxygen cylinders valve. Bleed oxygen into the mix gas cylinders at a
maximum rate of 70 psi minute until the desired P
F
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 P
F
to be
taken at the same temperature as P
I
. However, because of the compressibility
effects, more oxygen will normally have to be bled into the mixed-gas cylin-
ders 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.
After filling a mixed-gas cylinder, it may be
necessary to increase or decrease the percentage of oxygen in the cylinder.
16-2.3.1
Increasing the Oxygen Percentage.
To increase the oxygen percentage:
1.
Subtract the known percentage of oxygen from 100 to obtain the existing per-
centage 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 percent-
age 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
P1.00O
o
()×
1.00 O
f
()
-------------------------------------
=
16-6 U.S. Navy Diving Manual—Volume 3
Where:
F = Final cylinder pressure
P = Original Cylinder pressure
O
o
= Original oxygen % (decimal form)
O
f
= 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.
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
O
o
= Original oxygen % (decimal form)
O
f
= Final oxygen % (decimal form)
F
1 000
,
1.00 0.16
()×
1.00 0.20
-----------------------------------------------------
=
1 000
,
84
×
0.80
---------------------------
=
840
0.80
----------
=
1 050 psi
,
=
F
PO
o
×
O
f
----------------
=
CHAPTER 16 — Breathing Gas Mixing Procedures 16-7
Sample Problem.
For a cylinder containing 1,000 psi of a 20 percent oxygen
mixture and a 16 percent oxygen mixture is desired.
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 oxygen-
transfer 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.
16-2.4
Continuous-Flow Mixing.
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 toler-
ance limits. The final mixture may be supplied directly to a diver or a chamber or
be compressed into storage tanks for later use.
16-2.5
Mixing by Volume.
Mixing by volume is a technique where known volumes of
each gas are delivered to a constant-pressure gas holder at near-atmospheric pres-
sure. 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 tempera-
ture 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.
F
1 000
,
0.20
×
0.16
-------------------------------
=
200
0.16
----------
=
1 250 psi
,
=
16-8 U.S. Navy Diving Manual—Volume 3
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 uncontami-
nated gas before mixing.
16-2.6
Mixing by Weight.
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 tech-
nique 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 compo-
sition of the final mixture. As a safeguard, the final mixture must be analyzed for
composition using an accurate method of analysis.
16-3
GAS ANALYSIS
The precise determination of the type and concentration of the constituents of
breathing gas is of vital importance in many diving operations. Adverse physio-
logical reactions can occur when exposure time and concentrations of various
components in the breathing atmosphere vary from prescribed limits. Analysis of
oxygen content of helium-oxygen mixtures shall be accurate to within +
0.5
percent.
The quality of the breathing gas is important in both air and mixed-gas diving. In
air diving, the basic gas composition is fixed, and the primary consideration is
directed toward determining if gaseous impurities are present in the air supply (i.e.
carbon monoxide, hydrocarbons) and the effects of inadequate ventilation (carbon
dioxide). Using analytical equipment in air diving is not routine practice. Analyt-
ical equipment is generally employed only when it is suspected that the air supply
is not functioning properly or when evaluating new equipment.
Gas analysis is essential in mixed-gas diving. Because of the potential hazards
presented by anoxia and by CNS and pulmonary oxygen toxicity, it is mandatory
that the oxygen content of the gas supply be determined before a dive. Oxygen
analysis is the most common, but not the only type of analytical measurement that
is performed in mixed-gas diving. In deep diving systems, scrubbing equipment
performance must be monitored by carbon dioxide analysis of the atmosphere.
Long-term maintenance of personnel under hyperbaric conditions often necessi-
tates the use of a range of analytical procedures. Analyses are required to
determine the presence and concentration of minor quantities of potentially toxic
impurities resulting from the off-gassing of materials, metabolic processes, and
other sources.
CHAPTER 16 — Breathing Gas Mixing Procedures 16-9
16-3.1
Instrument Selection.
Selecting an instrument for analyzing hyperbaric atmo-
spheric constituents shall be determined on an individual command basis. Two
important characteristics are accuracy and response time. Accuracy within the
range of expected concentration must be adequate to determine the true value of
the constituent being studied. This characteristic is of particular importance when
a sample must be taken at elevated pressure and expanded to permit analysis. The
instrument's response time to changes in concentration is important when
measuring constituents that may rapidly change and result in quick development
of toxic conditions.
Response times of up to 10 seconds are adequate for monitoring gas concentra-
tions such as oxygen and carbon dioxide in a diving apparatus. When monitoring
hyperbaric chamber atmospheres, response times of up to 30 seconds are accept-
able. The instruments used should accurately measure concentrations to within
1/10 of the maximum allowable concentration. Thus, to analyze for carbon
dioxide with a maximum permissible concentration of 5,000 ppm (SEV), an
instrument with an accuracy of at least 500 ppm (SEV) must be used.
In addition to accuracy and response time, portability is a factor in choosing the
correct instrument. While large, permanently-mounted instruments are acceptable
for installation on fixed-chamber facilities, small hand-carried instruments are
better suited for emergency use inside a chamber or at remote dive sites.
16-3.2
Techniques for Analyzing Constituents of a Gas.
The constituents of a gas may
be analyzed both qualitatively (type determination) and quantitatively (type and
amount) using many different techniques and instruments. Guidance regarding
instrument selection can be obtained from NAVSEA, NEDU, or from instrument
manufacturer technical representatives. Although each technique is not discussed,
the major types are listed below as a reference for those who desire to study them
in detail.
Mass spectrometry
Colorimetric detection
Ultraviolet spectrophotometry
Infrared spectrophotometry
Gas chromatography
Electrolysis
Paramagnetism
16-10 U.S. Navy Diving Manual—Volume 3
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