Ponies are a vestigial left-over from the days of single outlet double manifolds, in case you hose the single reg, completely un-necessary in modern days when we have excellent isolation manifolds. It is an irrevocable separation of your gas supply, mounted on your back where it represents a huge entanglement risk. You can't reach it to ditch it, untangle it or open/close the supply valve, another huge risk. It is completely the wrong place for a back-up regulator, and since you cant reach it, you have to turn it on and leave it on during the dive where it can happily leak away unnoticed, again, a huge risk.
Pony bottles, and the invisible demons they are supposed to address, are used by the typical dive store interested only in generating sales and profits, and actually introduce bigger risks than they are alleged to address! The safety demons used
to sell these things are easily discounted and addressed by a bit of simple, logical thought.
1) If you are diving a single tank, you are, by definition, doing a no-stop dive with no overhead, which means at any
point during your dive you are free to make a direct ascent to the surface and probably not get bent. Where is the need for the additional expense and risk imposed by a pony? If you are using a pony bottle to change any part of that equation, you are a statistic looking for a place to happen. A buddy that is worth a darn removes even more of the "justification" for
a pony.
2) You have to buy a pony mount, a bottle (of limited size and actual utility, see below), additional 1st and 2nd stage and an additional SPG. In order to dive doubles you have to buy an additional bottle (of useful size and utility), a manifold,
an additional 1st and 2nd stage regulator and a set of bands. A iso manifolded set of dubs gives you *and your buddy* infinitely more rescue options. An AL40 contains a useful amount of gas, hung stage style it can be ditched or handed off,
is left turned off during the dive, thus assuring there is actually gas there "in the unlikely event" of an OOA, and can be used in infinitely more useful ways.
As the diver progresses and wished to get into staged decompression diving, the pony will end up on the floor in the garage or get Ebayed off to the next sucker anyway. A stage bottle mount consists of some cord, rubber hose and a couple hose clamps, and can be made for <$20. No-brainer.
Also, ponies seem to be the vehicle by which all kinds of baloney practices and mindsets get their foot in the door.
Most divers will begin to use their pony on every dive, and justify this use with any of a number of rediculous reasons.
This is typical of "the King and his new clothes". People who bought and use pony's defend their purchase and use with
great vigor, even in the face of overwhelming information to the contrary, another indicator to apply Rule #1 immediately.
I don't know how many times I have heard people tell the "I beat death again!" type stories because they went for their pony reg and it was either turned off or empty (used it on the last dive).
"When you least expect it, you're selected!"
Back mounted pony's should be going the way of the Dodo, as should those who advocate them. Instructors who use, sell and recommend them should be avoided like the clap. There is a much better, safer way. Back mounted pony bottles are a bad answer to a stupid question. If one must insist on carrying a pony it should, at very least, be carried stage style.
Use a set of iso manifolded doubles and/or a front mounted stage bottle. Anything else is simply mental masturbation.
A back mounted pony indicates, in no uncertain terms, possession of an entirely wrong mindset, and gives every reason to thumb the dive in the parking lot. When you hear some yak quoting the TDI manual where they advise filling the pony with 40% OEA "in case an un-planned decompression profile develops" run like a scalded dog. There are more smoking holes in that single statement than an Afghani outhouse.
More than you ever wanted to know about gas consumption:
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From the US Navy Diving Manual, Revision 4, Volume One, Chapter Three:
3-4.5.2 Respiratory Rate. The number of complete respiratory cycles that take place in 1
minute is the respiratory rate. An adult at rest normally has a respiratory rate of
approximately 12 to 16 breaths per minute.
3-4.5.3 Total Lung Capacity. The total lung capacity (TLC) is the total volume of air that
the lungs can hold when filled to capacity. TLC is normally between five and six liters.
3-4.5.4 Vital Capacity. Vital capacity is the volume of air that can be expelled from the
lungs after a full inspiration. The average vital capacity is between four and five
liters.
3-4.5.5 Tidal Volume. Tidal volume is the volume of air moved in or out of the lungs
during a single normal respiratory cycle. The tidal volume generally averages
about one-half liter for an adult at rest. Tidal volume increases considerably during
physical exertion, and cannot exceed the vital capacity.
3-4.5.6 Respiratory Minute Volume. The respiratory minute volume (RMV) is the total
amount of air moved in or out of the lungs in a minute. The respiratory minute
volume is calculated by multiplying the tidal volume by the rate. RMV varies
greatly with the body's activity. It is about 6 to 10 liters per minute at
complete rest and may be over 100 liters per minute during severe work.
3-4.5.7 Maximal Breathing Capacity and Maximum Ventilatory Volume. The maximal
breathing capacity (MBC) and maximum ventilatory volume (MVV) are the greatest
respiratory minute volumes that a person can produce during a short
period of extremely forceful breathing. In a healthy young man, they may
average as much as 180 liters per minute (the range is 140 to 240 liters per
minute).
3-4.5.8 Maximum Inspiratory Flow Rate and Maximum Expiratory Flow Rate. The
maximum inspiratory flow rate (MIFR) and maximum expiratory flow rate (MEFR)
are the fastest rates at which the body can move gases in and out of the
lungs. These rates are important in designing breathing equipment and computing gas
use under various workloads. Flow rates are usually expressed in liters per
second.
3-4.5.9 Respiratory Quotient. Respiratory quotient (RQ) is the ratio of the
amount of carbon dioxide produced to the amount of oxygen consumed during cellular
processes per unit time. This value ranges from 0.7 to 1.0 depending on diet
and physical exertion and is usually assumed to be 0.9 for calculations. This
ratio is significant when calculating the amount of carbon dioxide produced as oxygen
is used at various workloads while using a closed-circuit breathing apparatus.
The duration of the carbon dioxide absorbent canister can then be compared to
the duration of the oxygen supply.
3-4.5.10 Respiratory Dead Space. Respiratory dead space refers to the part
of the respiratory system that has no alveoli, and in which little or no exchange of gas
between air and blood takes place. It normally amounts to less than 0.2 liter. Air
occupying the dead space at the end of expiration is rebreathed in the following
inspiration. Parts of a diver's breathing apparatus can add to the volume of the dead
space and thus reduce the proportion of the tidal volume that serves the purpose of
respiration. To compensate, the diver must increase his tidal volume. The problem can
best be visualized by using a breathing tube as an example. If the tube
contains one liter of air, a normal exhalation of about one liter will leave the tube
filled with used air from the lungs. At inhalation, the used air will be drawn right
back into the lungs. The tidal volume must be increased by more than a liter to draw
in the needed fresh supply, because any fresh air is diluted by the air in the dead
space. Thus, the air that is taken into the lungs (inspired air) is a mixture of
fresh and dead space gases.
3-4.6 Alveolar/Capillary Gas Exchange. Within the alveolar air spaces, the
composition of the air (alveolar air) is changed by the elimination of carbon dioxide
from the blood, the absorption of oxygen by the blood, and the addition of water
vapor. The air that is exhaled is a mixture of alveolar air and the inspired air that
remained in the dead space. The blood in the capillary bed of the lungs is exposed to
the gas pressures of alveolar air through the thin membranes of the air sacs and the capillary
walls. With this exposure taking place over a vast surface area, the gas pressure of the
blood leaving the lungs is approximately equal to that present in alveolar air.
When arterial blood passes through the capillary network surrounding the
cells in the body tissues it is exposed to and equalizes with the gas pressure of the
tissues. Some of the blood's oxygen is absorbed by the cells and carbon dioxide is
picked up from these cells. When the blood returns to the pulmonary capillaries and
is exposed to the alveolar air, the partial pressures of gases between the
blood and the alveolar air is again equalized. Carbon dioxide diffuses from the blood
into the alveolar air, lowering its pressure, and oxygen is absorbed by the blood
from the alveolar air, increasing its pressure. With each complete round of
circulation, the blood is the medium through which this process of gas exchange occurs. Each
cycle normally requires approximately 20 seconds.
3-4.7 Breathing Control. The amount of oxygen consumed and carbon dioxide
produced increases markedly when a diver is working. The amount of blood
pumped through the tissues and the lungs per minute increases in proportion
to the rate at which these gases must be transported. As a result, more oxygen is
taken up from the alveolar air and more carbon dioxide is delivered to the lungs for
disposal. To maintain proper blood levels, the respiratory minute volume
must also change in proportion to oxygen consumption and carbon dioxide output.
Changes in the partial pressure (concentration) of oxygen and carbon dioxide
(ppO 2 and ppCO 2 ) in the arterial circulation activate central and
peripheral chemoreceptors. These chemoreceptors are attached to important arteries. The
most important are the carotid bodies in the neck and aortic bodies near the
heart. The chemoreceptor in the carotid artery is activated by the ppCO 2 in the
blood and signals the respiratory center in the brain stem to increase or decrease
respiration. The chemoreceptor in the aorta causes the aortic body reflex. This is a
normal chemical reflex initiated by decreased oxygen concentration and increased
carbon dioxide concentration in the blood. These changes result in nerve impulses
that increase respiratory activity. Low oxygen tension alone does not increase
breathing markedly until dangerous levels are reached. The part played by
chemoreceptors is evident in normal processes such as breathholding.
As a result of the regulatory process and the adjustments they cause, the
blood leaving the lungs usually has about the same oxygen and carbon dioxide
levels during work that it did at rest. The maximum pumping capacity of the heart
(blood circulation) and respiratory system (ventilation) largely determines the
amount of work a person can do.
3-4.8 Oxygen Consumption. A diver's oxygen consumption is an important
factor when determining how long breathing gas will last, the ventilation rates
required to maintain proper helmet oxygen level, and the length of time a canister
will absorb carbon dioxide. Oxygen consumption is a measure of energy expenditure
and is closely linked to the respiratory processes of ventilation and carbon
dioxide production. Oxygen consumption is measured in liters per minute (l/min) at
Standard Temperature (0°C, 32°F) and Pressure (14.7 psia, 1 ata), Dry Gas (STPD). These rates of
oxygen consumption are not depth dependent. This means that a fully charged MK16 oxygen bottle
containing 360 standard liters (3.96 scf) of usable gas will last 225 minutes at an oxygen
consumption rate of 1.6 liters per minute at any depth, provided no gas leaks from the rig.
Minute ventilation, or respiratory minute volume (RMV), is measured at BTPS
(body temperature 37°C/98.6°F, ambient barometric pressure, saturated with
water vapor at body temperature) and varies depending on a person's activity
level, as shown in Figure 3-6. Surface RMV can be approximated by multiplying the
oxygen consumption rate by 25. Although this 25:1 ratio decreases with
increasing gas density and high inhaled oxygen concentrations, it is a good
rule-of-thumb approximation for computing how long the breathing gas will last.
Unlike oxygen consumption, the amount of gas exhaled by the lungs is depth
dependent. At the surface, a diver swimming at 0.5 knot exhales 20 l/min of
gas. A scuba cylinder containing 71.2 standard cubic feet (scf) of air
(approximately 2,000 standard liters) lasts approximately 100 minutes. At 33 fsw, the diver
still exhales 20 l/min at BTPS, but the gas is twice as dense; thus, the
exhalation would be approximately 40 standard l/min and the cylinder would last only half as
long, or 50 minutes. At three atmospheres, the same cylinder would last only
one-third as long as at the surface.
Carbon dioxide production depends only on the level of exertion and can be
assumed to be independent of depth. Carbon dioxide production and RQ are
used to compute ventilation rates for chambers and free-flow diving helmets.
These factors may also be used to determine whether the oxygen supply or the
duration of the CO 2 absorbent will limit a diver's time in a closed or semi-closed
system.
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Boiled down to gravy for SCUBA divers:
1 Liter = 0.03531467 cuft
1 cuft = 28.31685 liter
Go to 33 feet, mark your time and pressure on a slate. Stay still for ten minutes at the same depth.
Record how much pressure you used.
Now, mark the time and the pressure again, and swim normally for ten minutes at 33 feet.
Record how much pressure you used.
Let's say:
95 cu ft bottle @ 2400 PSI
95/2400 = .0395 cuft per PSI
Let's again assume you dropped 200 psi in ten minutes at rest .0395 X 200 =
7.9 cuft used at 2 atm.
7.9/2 = 3.95 cuft used at 1 atm
3.95/10 = .395 cuft per minute SAC at rest, at 1 atm.
Say you used 400 psi during the swim.
400 X .0395 = 15.8/2 = 7.9/10 = .79 cuft per minute working SAC at 1 atm. Now, remember this number can change drastically
in the "bad" direction as temperature goes down and work load/panic/stress/hangover factor goes up.
Take your working SAC, multiply by the depth in ATM, and you get the amount of gas you *probably* need for that portion of the dive. Double or triple it for safety and for your buddy in case of a complete gas supply loss.
You would use the at rest SAC to determine deco gas requirements.
And, of course, all this changes from day to day, dive to dive.
The Navy notes 2.5 cuft per minute, swimming against a 1.2 knot current, and this was calculated using a very fit young diver:
99 fsw = 4 ATM
2.5 x 4 = 10 cuft per minute, which makes a 13 or 19 cuft pony a joke that's not funny if things go seriously wrong.
I have seen a new diver (father and daughter, dad was freaking) go through an AL80 in 9 minutes, and our deepest depth
was 60 feet.
Best is to do this several times, over several dives, and for different (longer) lengths of time. After a while, you will
get a realistic window of your average SAC rate.
Scott
