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*note- A convention of this glossary is to bold any numbers with
decimal points to avoid confusion when the decimal is hard to see.
- Absolute atmospheres (or Atmospheres absolute, or
The ambient pressure including the air column over the water. The air column = 1 atm. at
sea level. In sea water, another atmosphere is added each 33FSW. There is an increase in pressure per foot of sea
water equivalent to 1/33 or .03030303. So ATA may be calculated by
multiplying the depth (FSW) by .0303030 and then adding 1 for the air
above the water. i.e. the ATA at 46 FSW = (46 * .0303030) + 1 = 2.3939 ATA.
to convert ATA to FSW. ATA - 1 * 33 = FSW
Axial flow scrubber-
An axial scrubber is a scrubber design
in which the breathing gases move from top to bottom (or vise-versa) through the scrubber.
The most common example is the current US navy fully-closed rebreather the MK16.
An example of a simple axial flow scrubber would be to start with a cofffe can with
a removable top. Punch holes in the top and bottom of the coffee can and fill the
middle with soda lime then put the top on. Seal the entire unit into the breathing
loop. In this example the gases must travel from one side of the can to the other.
Contrast this with "Radial scrubber"
- Breathing loop -
The breathing loop in a rebreather is composed of all the internal areas within
which the diver's breathing gases flow. This includes, the counter-lung,
scrubber, breathing hoses and the divers lungs.
- Counter-lung (abbr.- CL)-
The counter lung is the sealed flexible bag which inflates as the diver exhales and
deflates as the diver inhales. It acts as a storage area for the diver's breathing gases.
The positioning of this bag within the breathing loop can
greatly affect the breathing effort.
- CNS Toxicity (Central Nervous System Toxicity)
- (see Oxygen Toxicity)
- Diluent -
This is the gas used in a closed circuit rebreather to make up volume in the breathing
loop as the diver proceeds to deeper depths and the gases in the breathing loop are
compressed. Depending on the rebreather, and the type of diving, the gas used for diluent
could be air, nitrox, trimix or even heliox.
- EAD (Equivalent Air Depth) -
The depth relative to the partial pressure of
nitrogen in a normal air mixture (21%O2, 79%N2). When there is a
lower than normal fraction of nitrogen in a gas mix, the partial pressures of nitrogen are
lower at any given depth. This allows the diver to feel less narcotic effect from the
nitrogen than when breathing air at the same depth. EAD = (fN2*(d+33))/.79-33 , where d =
- EAN(x) [Enriched Air Nitrox (percent of Oxygen)] -
This is one of the naming conventions for a non-normal mixture of Oxygen and Nitrogen. Air
has approximately 21% Oxygen and 79% Nitrogen, this is the normal mixture of Oxygen and
Nitrogen (also called normoxic). It you have more (or less ) Oxygen in the mix, it is
considered nitrox. If the mixture had 32% Oxygen with the balance as Nitrogen this would
be labeled EAN32.
- FSW (Feet of Salt Water) -
This is a measurement unit of both depth and pressure, however its most precisely used as
a pressure unit. Because the pressure is the same everywhere in the ocean at any given
depth below the surface, these units may be used interchangeably in the ocean. So that at
33 feet of depth the pressure is 33 FSW. You can have a pressure of 33FSW in fresh water,
but you would not be 33 feet below the surface of the fresh water when your gauge read 33
FSW. Fresh water is not as dense as salt water and so you would be deeper that 33 feet
when the gauge read 33 FSW. All common diving gauges read in FSW.
- Fraction of gas (f[x]) -
The percent of a particular gas in a gas mix. Air contains, 21 percent O2 and
79 percent N. In Air the f02 = .21
(21 percent) and the fN2 = .79 (79 percent). You may also hear
the term "fraction of inspired gas". This means the fraction actually inspired,
or "breathed in".
- Fully-closed circuit rebreather -
This type of rebreather does not release any gases from the unit except under the
conditions of ascending from depth as the counter-lung expands
with the reduction in ambient pressure. The advantage is the greatest possible use of the
onboard Oxygen and the maximum Physiological benefits. The disadvantage is the added
complexity of electronics and mechanics to monitor the ppO2
and to inject the proper amounts of diluent and O2 into
the breathing loop.
- Heliox -
A breathing mixture of gases consisting entirely of Helium and Oxygen. This is
used to eliminate Nitrogen narcosis and to control the affects of Oxygen toxicity, by
eliminating the Nitrogen and reducing the Oxygen in the breathing mix. Another benenfit is
reduced effort of breathing due to the lower density of helium. There are a few
disadvantages, for working divers, helium distorts the voice and helium has less
insulating value than an Oxygen/Nitrogen mix which results in divers becoming cold sooner.
There have been some reports of divers feeling jittery when using heliox, and as dives go
beyond about 300 FSW, classic symptoms of HPNS
can begin to appear.
- Hypercapnia -
Hypercapnia is the physiological condition that results from too much C02
(carbon dioxide). In rebreather diving this is usually the product of a poorly functioning
scrubber. However, the condition is not limited
to rebreather users. Typical symptoms are a shortness of breath and a headache. In extreme
cases the final result is unconsciousness and eventual death from lack of Oxygen. Skip breathing, hard work at depth, dead air spaces in the
breathing loop and other problems can lead to hypercapnia.
- Hyperoxic and Hyperoxia -
In general, these terms relate to a more than a normal amount of Oxygen.
Hyperoxic refers to a mixture of gases with higher than normal Oxygen content (above 21%).
- Hyperoxia is the physiological condition associated with breathing too high of a partial pressure of Oxygen. The human body has a limit on
both the partial pressure of Oxygen it can tolerate and the long term dosage of Oxygen.
The partial pressure upper limit is generally considered
to be approximately 1.6 ppO2 but most
divers leave some margin for error and a more typical upper limit is 1.4
ppO2. When high partial pressures of Oxygen are inspired, convulsions may occur
with little or no warning. The long term dosage of oxygen is measured in
units called OTU's.
- Hypoxic and Hypoxia -
In general, both of these terms relate to a less than normal amount of
Oxygen. Hypoxic refers to a mixture of gases with a lower than normal fraction of oxygen (less than 21%).
- Hypoxia is the physiological condition associated with breathing too low of a partial pressure of Oxygen. When the ppO2
of oxygen falls below about .12, there is often not enough Oxygen to
- HPNS (High Pressure Nervous
- HPNS is a condition which results from breathing Helium under high pressures. Early
symptoms of HPNS are somtimes seen as shallow as 300FSW but more commonly over 600FSW. The
severity also depends on the mix of breathing gases, Nitrogen can often moderate the
affects of HPNS. The early symptoms include muscle tremors, followed by changes in
electroencephalogram (EEG) readings, impaired motor and problem solving skills. Other
symptoms can include euphoria, nausea, vomiting, lack of appetite and drowsiness. Symptoms
sometimes moderate or entirely dissapear with continued exposure.
- Loraine Smith Effect (see Oxygen Toxicity)
- MOD (Maximum Operating Depth) -
The maximum operating depth of a breathing gas before reaching a predetermined
maximum partial pressure of Oxygen, usually 1.4 or higher.
This limit is to protect from Oxygen toxicity.
The mushroom valve on a rebreather is the one way valve on either side
of the mouthpiece which keeps the breathing gases traveling the loop in one direction.
The valves are very important to ensure that the gases are scrubed of CO2 and that
there is enough O2 in the breathing loop. The vavle is called a mushroom valve
because the diaphram of the valve resumbles a mushroom. The mushroom valve is one of
the places in the breathing loop that can have a very direct effect on the work of breathing.
- Normoxic -
This is the term used to describe the normal mixture of gases found in the
atmosphere. More specifically it refers to the percent of Oxygen in the mix. Since the
atmosphere has 21% Oxygen, a mixture of gas with 21% Oxygen would be called normoxic. This
is contrasted with hyperoxic and hypoxic.
- Nitrogen Narcosis -
- A narcotic effect produced by high partial pressures of Nitrogen. This can affect the
divers judgment and for most divers starts to be noticeable at a pN2 over 3.00
this corresponds to approximately 100 FSW when breathing air.
- Any mixture of Oxygen and Nitrogen which has more or less Oxygen than air. However, It
is usually used to describe those mixtures with a more than normal fraction of Oxygen. The standard Nitrox mixtures are EAN32, EAN36 and EAN50. Most basic nitrox courses assume a mixture of
EAN32 or EAN36.
- Oxygen Toxicity -
- Physiological damage resulting from higher than normal partial pressures of Oxygen.
There are two primary types of Oxygen toxicity. One results from long exposures of
elevated ppO2's and is called "The Loraine Smith
Effect" or "Pulmonary Oxygen Toxicity" as the primary damage is to the
lungs and airways. The other type of Oxygen toxicity results from short high ppO2
exposures and is called "The Paul Bert effect" or "CNS Toxicity"
(Central Nervous System Toxicity) and is characterized by convulsions with little or no
warning signs. CNS toxicity usually occurs with ppO2's above 1.6
- OTU's (Oxygen Toxicity Units)-
Also called UPTD (Unit Pulmonary Toxic Dose) OTU's are a rough measurement of
long term low ppO2 oxygen exposure. The units are only viewed as guidelines to
help gauge whole body oxygen toxicity. They are based on
the exposure to 1 ata of oxygen for 1 minute. 1ata for 1 minute = 1 OTU.
- Paul Bert Effect (see Oxygen
- Partial Pressure -
The pressure within a gas mix of a particular gas. In simpler terms it may be thought of
as the number of molecules per given volume of gas. More molecules per volume = higher
partial pressures. In more specific terms it is the Fraction of the gas (F[x]) multiplied
by the absolute atmospheres.
- i.e.- Air has a fraction of Oxygen equal to 21% (Fi02 = .21).
At a depth (pressure)of 33FSW the absolute atmospheric pressure is equal to 2
(The 1 atm at sea level plus a 2nd atm at 33 feet). So the partial pressure of a tank of
air at 33FSW is .21 * 2 = .42 pp02.
- Partial pressures are commonly represented as "pp" followed by the atomic
symbol of the gas. So that the partial pressure of Oxygen would be written as pp02
and the partial pressure of He would be written as ppHe
- ppO2 (pee-pee-oh-two) -
The partial pressure of Oxygen in a gas mix.
- ppN2 (pee-pee-en-two) -
The partial pressure of Nitrogen in a gas mix.
- Radial flow scrubber-
A radial flow scrubber is a scrubber
design in which the breathing gases move from the middle to the outside (or vise-versa)
through the scrubber. The most common example is the current military rebreather
built by Sherwood and Fullerton in Canada. An example of a simple radial flow
scrubber would be to start with a coffe can, then insert a tube into the middle of the
coffee can from the top. Punch holes in the middle tube and the outside of the
coffee can. The top and bottom of the can should be sealed (with the exception of
one end of the middle tube). Now the breathing gases must move "radially"
in the cannister. Contrast this with "Axial flow
- Rebreather -
A self-contained device used to recirculate and regulate breathing gases for
the purposes of extended diving times and quiet operation. On a fully-closed circuit rebreather this is
accomplished by scrubbing CO2, and
adding O2 as necessary to maintain a constant partial
pressure of Oxygen. On most semi-closed
systems a portion of each breath is released to the water and the same portion of new
breathing gases are injected into the system. The semi-closed system also uses a scrubber.
- RMV (Respiratory Minute Volume)
The amount of gas that you breathe out in one minute. The main controller of RMV is work
rate. The harder you work the higher the RMV. The increase in RMV is driven by the
increase in CO2 with higher work rates. Increasing levels of CO2 is
the primary 'flag' the body uses to tell us to breathe.
- Scrubber or (C02 scrubber) -
The part of a rebreather that removes excess CO2 from the breathing
loop. This is accomplished through the chemical bonding of the CO2 with a
reactive substance. In most current rebreathers the substance used is Soda
Lime. There are a number of designs of scrubbers, but the two most widely used
are axial and radial scrubbers.
- Semi-closed circuit rebreather -
A rebreather which vents part of the exhaled gases
from the breathing loop as a function of each breath, RMV, or some other method. CO2
produced by metabolic processes is absorbed by a scrubber.
Because most semi-closed rebreathers don't monitor ppO2,
they are primarily used with a premix of nitrox or trimix
which in turn is mixed for the planned MOD.
This may be contrasted with a fully-closed
- Skip breathing -
The practice of inhaling, holding the inhalation for a period of time and then
exhaling in order to attempt to extend the time underwater by using less air. This
practice can lead to a buildup of CO2 and symptoms of hypercapnia.
- Soda-Lime - (where
Soda lime (also called hydrated lime) is a chemical agent which reacts and bonds with CO2
and is commonly used in the scrubbers of
- The primary constituents of soda lime include Calcium Hydroxide - Ca(OH)2 (about
70-80%), Water - H2O (about 16 to 20%), Sodium Hydroxide - NaOH (about 1-2%),
and Potassium Hydroxide - KOH (about >0-1%).
- Water is an important part of the reaction which takes place to bind the CO2 .
The general description of the reaction is as follows: First the gaseous CO2
reacts with water to form carbonic acid - H2CO3. Then the NaOH reacts with the carbonic
acid to produce Na2CO2 and H2O. The Na2CO2
reacts with the Ca(OH)2 which has been disassociated into Calcium and Hydroxide
Ions. (Ca++ and OH-) to produce CaC02 (calcium carbonate, otherwise known as
limestone.) The CO2 is now in a relatively stable state. There is a net
production of three H2O molecules for every molecule of CO2 which is
- Apparently the complete reaction is still not completely understood, but it is true to
say that CO2 is absorbed and water and heat are generated.
- Lithium Hydroxide Li(OH) is actually a more reactive chemical agent for scrubbing CO2
but it has the disadvantage of being caustic when in contact with water, thus making
it less practical for diving uses. Another disadvantage of Li(OH) is that it often
produces a fine dust when handled which can irritate the repiratory tract.
- SodaSorb and SodaSorb HP -
A brand of soda-lime made by W.R. Grace in the USA.
- Sofnolime -
A brand of soda-lime made by Molecular Products, of
- Trimix -
A breathing mixture of gases most often composed of Oxygen, Nitrogen and Helium. This
proportions of each are changed according to the needs of the particular dive plan to help
limit Oxygen toxicity and Nitrogen
- UPTD (Unit Pulmonary Toxic Dose)
- Whitey valve -
A valve made by the Whitey Co.. This valve is O2 compatible and aircraft grade.
It allows a switch between two different inputs to one output. There is no crossover so
that there is no mixing of the inputs. This valve is a good choice to plumb a second
diluent bottle into a rebreather.
Work of breathing (abbr.- WOB)-
The phroase "Work of breathing" relates to the amount of effort required by the
diaphram to move the breathing gases in and out of the lungs. Work of breathing is
affected by many things on a rebreather including the hose diameters, mushroom valves,
scrubber design, counter-lung placement and design, and more. The work of breathing
is also affected by depth. As depth increases the breathing gases become more dense
which increases the work of breathing.
The primary problem with a high work of breathing is that it increases the build up
of CO2 in the body. If CO2 levels get too high you will blackout. This
isn't much fun at all while under water.
Work of breathing is not a subjective matter. There are very specific tests
designed to measure the work of breathing on a breathing machine. You cannot
subjectively ascertain WOB.
Copyright © Northwood Designs, Inc. All rights reserved.
Revised: September 07, 2005.