An important early step in any scuba dive is computing the air supply requirement.
The air supply requirement is a function of the expected duration of the dive at a
specific working depth. The duration of the air supply in the scuba cylinders
depends on the depth at which the air is delivered. Air consumption rate increases
with depth.
The duration of the air supply of any given cylinder or
combination of cylinders depends upon:
-
The diver’s consumption rate, which varies with the diver’s work rate,
-
The depth of the dive, and
-
The capacity and recommended minimum pressure of the cylinder(s).
Temperature is usually not significant in computing the duration of the air supply,
unless the temperature conditions are extreme. When diving in extreme temperature
conditions, Charles’/Gay-Lusac’s law must be applied.
There are three steps in calculating how long a diver’s air supply will last:
1. Calculate the diver’s consumption rate by using this formula:
C
D+33
33
= ---------------- ´ RMV
Where:
C = Diver’s consumption rate, standard cubic feet per minute (scfm)
D = Depth, fsw
RMV = Diver’s Respiratory Minute Volume, actual cubic feet per minute
(acfm) (from Figure 7-6)
2. Calculate the available air capacity provided by the cylinders. The air capacity
must be expressed as the capacity that will actually be available to the diver,
rather than as a total capacity of the cylinder. The formula for calculating the
available air capacity is:
Va
Pc – Prm
14.7
= -------------------- ´(FV ´ N)
Where:
Pc = Measured cylinder pressure, psig
Prm = Recommended minimum pressure of cylinder, psig
FV = Internal volume (scf)
N = Number of cylinders
Va = Capacity available (scf)
3. Calculate the duration of the available capacity (in minutes) by using this
formula:
Duration= Va
C
Where:
Va = Capacity available, scf
C = Consumption rate, scfm
Sample Problem. Determine the duration of the air supply of a diver doing
moderate work at 70 fsw using twin 72-cubic-foot steel cylinders charged to 2,250
psig.
1. Calculate the diver’s consumption rate in scfm. According to Figure 7-6, the
diver’s consumption rate at depth is 1.4 acfm.
C
D+33
33
= ---------------- ´ RMV
70 + 33
33
= ------------------ ´ 1.4
= 4.37 scfm
2. Calculate the available air capacity provided by the cylinders. Table 7-1
contains the cylinder data used in this calculation:
-
Floodable Volume = 0.420 scf
-
Rated working pressure = 2250 psig
-
Reserve pressure for twin 72-cubic-foot cylinders = 250 ps
V
Pc – Prm
14.7
= -------------------- ´(FV ´ N)
2250 – 250
14.7
= -------------------------- ´(0.420 ´ 2)
= 114 scf
3. Calculate the duration of the available capacity.
Duration
Va
C
= ------
= -------------------------
= 26 minutes
The total time for the dive, from initial descent to surfacing at the end of the
dive, is limited to 26 minutes.
Compressed air meeting the established
standards can usually be obtained from Navy sources. In the absence of
appropriate Navy sources, air may be procured from commercial sources. Usually,
any civilian agency or firm which handles compressed oxygen can provide pure compressed air. Air procured from commercial sources must meet the requirements
of Grade A Source I or Source II air as specified by FED SPEC BB-A-
1034B. Refer to Table 4-2 in Chapter 4 for the air purity requirements.
NOTE. | | Paragraph 7-4.5 addresses safety precautions for charging and handling
cylinders. |
Scuba cylinders shall be charged only with air that meets diving air purity standards.
A diving unit can charge its own cylinders by one of two accepted methods:
(1) by cascading or transferring air from banks of large cylinders into the scuba
tanks; or (2) by using a high-pressure air compressor. Cascading is the fastest and
most efficient method for charging scuba tanks. The NAVSEA/00C ANU list lists
approved high-pressure compressors and equipment authorized for scuba air
sources.
The normal cascade system consists of supply flasks connected together by a
manifold and feeding into a scuba high-pressure whip. This whip consists of a
scuba yoke fitting, a pressure gauge, and a bleed valve for relieving the pressure in
the lines after charging a cylinder. A cascade system, with attached whip, is shown
in Figure 7-7.
Scuba charging lines shall be fabricated using SAE 100R7 hose for 3,000 psi
service and SAE 100R8 hose for 5,000 psi service. The service pressure of the
scuba charging lines shall be no greater than the working pressure of the hose
used.
The working pressure of a hose is determined as one-fourth of its burst pressure.
While this criteria for working pressure was developed based on the characteristics
of rubber hose, it has also been determined to be appropriate for use with the
plastic hoses cited above.
Fleet units using charging lines shall not exceed the rated working pressure of the
hose. If the charging line working pressure rating does not meet service requirements,
restrict the service pressure of the hose to its working pressure and initiate
replacement action immediately.
The use of strain reliefs made from cable, chain, 21-thread, or 3/8-inch nylon,
married at a minimum of every 18 inches and at the end of the hose, is a required
safety procedure to prevent whipping in the event of hose failure under pressure.
Marrying cord shall be 1/8-inch nylon or material of equivalent strength. Tie
wraps, tape, and marlin are not authorized for this purpose.
Figure 7.7. Cascading System for Charging Scuba Cylinders.
|
Normally, scuba tanks are
charged using the following operating procedures (OPs), which may be tailored to
each unit:
1. Determine that the cylinder is within the hydrostatic test date.
2. Check the existing pressure in the scuba cylinder with an accurate pressure
gauge.
3. Attach the cylinder to the yoke fitting on the charging whip, and attach the
safety strain relief.
4. For safety and to dissipate heat generated in the charging process, when
facilities are available, immerse the scuba cylinder in a tank of water while it
is being filled. A 55-gallon drum is a suitable container for this purpose.
5. Tighten all fittings in the system.
6. Close the bleed valve.
7. Place reserve mechanism lever in the open (lever down) position.
8. Open the cylinder (on/off) valve. This valve is fully opened with about two
turns on the handle, counter-clockwise. However, the valve must not be used
in a fully open position as it may stick or be stripped if force is used to open a
valve that is incorrectly believed to be closed. The proper procedure is to open
the valve fully and then close or back off one-quarter to one-half turn. This
will not impede the flow of air.
9. Open the supply flask valve.
10. Slowly open the charging valve. The sound of the air flowing into the scuba
cylinder is noticeable. The operator will control the flow so that the pressure in
the cylinder increases at a rate not to exceed 400 psig per minute. If unable to
submerge scuba cylinders during charging, the charging rate must not exceed
200 psig per minute. The rate of filling must be controlled to prevent
overheating; the cylinder must not be allowed to become too hot to touch.
11. Monitor the pressure gauge carefully. When the reading reaches the rated
pressure for the scuba cylinder, close the valve on the first cylinder and take a
reading.
12. Close the charging valve.
13. Close the on/off valve on the scuba cylinder.
14. Ensure that all valves in the system are firmly closed.
15. Let the scuba cylinder cool to room temperature. Once the cylinder is cool, the
pressure will have dropped and you may need to top off the scuba cylinder.
Follow this procedure to top off a scuba
cylinder:
1. Open the on/off valve on the scuba cylinder.
2. Select a supply flask with higher pressure than the scuba rated limit.
3. Open the supply valve on the flask.
4. Throttle the charging valve to bring the scuba cylinder up to the rated limit.
5. Close all valves.
6. Open the bleed valve and depressurize the lines.e.
7. When air has stopped flowing through the bleed valve, disconnect the scuba
cylinder from the yoke fitting.
8. Reset the reserve mechanism (lever in up position).
In the absence of high-pressure air systems, large-volume air compressors can be
used to charge scuba cylinders directly. However, few compressors can deliver air
in sufficient quantity at the needed pressure for efficient operation. Small
compressors should be used only if no other suitable source is available.
If a suitable compressor is available, the basic charging procedure will be the same
as that outlined for cascading except that the compressor will replace the bank of
cylinders. Special considerations that apply when using air compressors are:
-
The compressor must be listed in the NAVSEA/00C ANU list if it is not part
of a certified system.
-
The compressor must deliver air that meets the established purity standards.
-
The compressor shall be equipped with ANU particulate filters. Chemically
active filters are not authorized.
-
An engine-driven compressor must always be mounted so there is no danger
of taking in exhaust fumes from the engine, stack gas, or other contaminated
air from local sources.
-
Only approved diving compressor lubricants are to be used in accordance with
PMS procedures or manufacturer’s recommendations.
Additional information on using air compressors is found in paragraph 8-6.2.2.
The following safety
rules apply to charging and handling scuba cylinders:
-
Carry cylinders by holding the valve and body of the cylinder. Avoid carrying
a cylinder by the backpack or harness straps as the quick-release buckle can be
accidentally tripped or the straps may fail.
-
Do not attempt to fill any cylinder if the hydrostatic test date has expired or if
the cylinder appears to be substandard. Dents, severe rusting, bent valves,
frozen reserve mechanisms, or evidence of internal contamination (e.g., water
scales or rust) are all signs of unsuitability. See CGA Pamphlet C-6, Standards
for Visual Inspection of Compressed Gas Cylinders.
-
Always use gauges to measure cylinder pressure. Never point the dial of a
gauge to which pressure is being applied toward the operators face.
-
Never work on a cylinder valve while the cylinder is charged.
-
Make sure that the air reserve mechanism is open (lever down) before
charging.
-
Use only compressed air for filling conventional scuba cylinders. Never fill
scuba cylinders with oxygen. Air is color-coded black, while oxygen is colorcoded
green.
-
Tighten all fittings before pressurizing lines.
-
When fully charged, close the air reserve (lever up). Mark the filled tank to
indicate the pressure to which it was charged.
-
Handle charged cylinders with care. If a charged cylinder is damaged or if the
valve is accidentally knocked loose, the cylinder tank can become explosive projectile. A cylinder charged to 2,000 psi has enough potential
energy to propel itself for some distance, tearing through any obstructions in
its way.
-
Store filled cylinders in a cool, shaded area. Never leave filled cylinders in
direct sunlight.
-
Cylinders should always be properly secured aboard ship or in a diving boat.
