A
Absolute atmospheres (or Atmospheres absolute, or ATA):
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 33 FSW. 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.
Adsorbent:
The substance that prevents carbon dioxide buildup in the breathing loop.
Axial flow canister:
An axial canister is a canister design in which the breathing gases move from top to bottom (or vise-versa) through the canister. Contrast this with "Radial canister"
B
Bail out:
A gas source the diver can switch to in an emergency in case of equipment failure. This can be either an external gas source with regulator, or it may be utilize a rebreather's diluent supply.
Breathing bag (Also called Counterlung):
A flexible bag in the loop that expands and contracts as the diver breathes, allowing for respiration and changes in the volume of the breathing loop.
Breathing loop:
The breathing loop in a rebreather is composed of all the internal areas within which the breathing gases flow. This includes the counter-lung, canister, breathing hoses and the divers lungs.
Bubblers:
Open-circuit scuba divers who unavoidably release vast, wasteful clouds of gas wherever they go, causing a deafening noise and scaring all the marine life within 10 meters.
C (back to top of page)
Canister:
The rebreather component that contains the CO2 adsorbent material.
Caustic Cocktail:
A highly alkaline (caustic) liquid producing a strong burning sensation, which can occur if water enters the canister canister and dissolves the adsorbent. It should be noted that condensation of warm moist gas on the colder canister is also a source of water which can cause a caustic cocktail. Also, the adsorbent reaction with CO2 also produces water vapor, which can then condense inside the canister.
Counter-lung: (Also called Breathing Bag):
A flexible bag in the loop that expands and contracts as the diver breathes, allowing for respiration and changes in the volume of the breathing loop.
CNS Toxicity (Central Nervous System Toxicity):
(see Oxygen Toxicity)
D (back to top of page)
Dams:
An anti channeling dam is a small rubber ring installed inside a granular canister canister to ensure that gas is properly and efficiently channeled through the granular adsorbent. Failure to properly install a dam into a granular canister canister can lead to a diver experiencing hypercapnia.
Diluent :
This is the gas used in a 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.
Dosage device (or demand unit):
A regulator used in a semi-closed rebreather which delivers a constant amount of pre-mixed gas to the breathing loop.
E (back to top of page)
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 = depth.
EAN(x) [Enriched Air Nitrox (percent of oxygen)]:
This is one of the naming conventions for a non-normal mixture of oxygen and nitrogen. Normal air has approximately 21% oxygen and 79% nitrogen. 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.
Exhalation Hose:
The exhalation hose is the breathing hose connected to the mouthpiece through which exhaled gases pass back from the diver to the rebreather.
Exhaust Bag:
A flexible bag which accepts exhausted respirations from the exhalation hose. In this bag, excess pressure is vented through the exhaust valve out of the system. The remaining gas is passed onto the canister canister.
Exhaust Valve:
This valve allows excessive gas to vent from the breathing loop. Expanding gas is vented upon ascent, or as gas accumulates in a semi-closed rebreather.
Exothermic:
A chemical reaction which produces heat. When the canister chemically reacts with the CO2 in the loop, it converts it to calcium carbonate, water and heat, thereby warming the breathing gas.
F-G (back to top of page)
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 (at 33 feet of depth the pressure is 33 FSW). However, if you are in fresh water, you can have a pressure of 33FSW, but you would not be 33 feet below the surface. Since fresh water is not as dense as salt water, 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 N2. In air, the fO2 = .21 (21 percent) and the fN2 = .79 (79 percent).
Fully-closed circuit rebreather:
This type of rebreather monitors the oxygen content of the breathing gas, and typically uses a solenoid to open a valve which then adds pure oxygen to the breathing loop as needed. This type of rebreather is the most efficient from a gas useage perspective, and can also minimize nitrogen uptake by optimizing oxygen content of the breathing mix. Its disadvantage is that of complexity and reliance on electronics for control.
H (back to top of page)
Heliox:
A breathing mixture of gases consisting entirely of helium and oxygen. This is used to eliminate nitrogen narcosis (by eliminating the nitrogen) and to control the affects of oxygen toxicity by reducing the oxygen in the breathing mix. Another benefit is reduced effort of breathing due to the lower density of helium.
Hypercapnia:
Hypercapnia is the physiological condition that results from too much C02 (carbon dioxide). 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 (using open circuit), 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.
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 maintain consciousness.
HPNS (High Pressure Nervous Syndrome):
HPNS is a condition which results from breathing helium under high pressures. Early symptoms of HPNS are sometimes seen as shallow as 300FSW but more commonly over 600FSW. 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.
I-J-K (back to top of page)
Inhalation Bag (also called Breathing Bag or Counterlung:
A flexible bag connected between the canister canister and the inhalation hose which takes scrubbed gas from the canister canister, adds supply gas, and supplies it via the inhalation hose to the divers mouthpiece.
Inhalation Hose:
The inhalation hose is a corrugated, large diameter hose that allows a one-way flow of enriched gas from the inhalation or breathing bag through a checkvalve into the mouthpiece of the unit. The hose contains adjustable weights for diver comfort.
L (back to top of page)
Loraine Smith Effect:
(see Oxygen Toxicity)
M (back to top of page)
MOD (Maximum Operating Depth):
The maximum operating depth of a breathing gas before reaching a predetermined maximum partial pressure of Oxygen, usually 1.6 or higher. This limit is to protect from oxygen toxicity. The formula for MOD is (((1.6 / O2%) -1) * 33) or ((46.2 / O2%) -33).
Mushroom Valve:
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.
N (back to top of page)
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 (a mixture of gas with 21% oxygen would be called normoxic).
Nitrogen Narcosis:
A narcotic effect produced by high partial pressures of nitrogen. This can affect judgment, and for most divers starts to be noticeable at a pN2 over 3.00 (approximately 100 FSW when breathing air).
Nitrox:
Any mixture of oxygen and nitrogen which has more or less oxygen than air. The standard Nitrox mixtures are EAN32, EAN36 and EAN50 (EAN: enriched air nitrox).
O (back to top of page)
Open-circuit:
an underwater breathing device that supplies air to the diver (at ambient pressure, i.e. equal to the diver’s depth) from a tank with the resulting exhaled air being vented to the surrounding environment. The breathing circuit is "open", and the gas supply is not conserved by "rebreathing" it.
Oxygen sensor:
Device to measure the PO2 in the loop, usually found in the inhalation bag.
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 (called "The Loraine Smith Effect" or "Pulmonary Oxygen Toxicity" as the primary damage is to the lungs and airways), and the other results from short high ppO2 exposures (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.
P-Q (back to top of page)
Paul Bert Effect:
(see Oxygen Toxicity)
Partial Pressure:
The pressure of a particular gas within a gas mixture. In more specific terms it is the Fraction of the gas (F[x]) multiplied by the absolute atmospheres. For example, 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). Therefore, the partial pressure of a tank of air at 33FSW is .21 * 2 = .42 pp02.
ppO2:
The partial pressure of Oxygen in a gas mix.
ppN2:
The partial pressure of Nitrogen in a gas mix.
R (back to top of page)
Radial flow canister:
A radial flow canister is a canister design in which the breathing gases move from the middle to the outside (or vise-versa) through the canister. Contrast this with "Axial flow canister".
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 canister.
RMV (Respiratory Minute Volume):
The amount of gas that you breathe out in one minute.
S (back to top of page)
Scrubber(or C02 canister):
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.
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.
Shutoff Valve on mouthpiece:
The shutoff valve on the mouthpiece is shut to prevent water from getting into the mouthpiece when the mouthpiece is not in the users mouth.
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. Skip breathing (which is not recommended) is used for open circuit diving only, and will have no effect on system duration of any rebreather.
Soda-Lime:
Soda lime (also called hydrated lime) is a chemical agent which reacts and bonds with CO2 and is commonly used in granular canisters of rebreathers. 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%).
Sonic Orifice:
Used as a dosage technique for some semi-closed rebreathers, the sonic orifice is designed to maintain the same gas flow, independent of the supply tank pressure. Sonic orifices are generally preset for a specific EAN mixture.
T (back to top of page)
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 narcosis.
U-V (back to top of page)
UPTD (Unit Pulmonary Toxic Dose):
see OTU's
W-X-Y-Z (back to top of page)
Work of breathing (WOB):
The phrase "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, valves, canister design, counter-lung placement and design, etc. The work of breathing is also affected by depth (depth increases the density which increases the work of breathing).
Absolute atmospheres (or Atmospheres absolute, or ATA):
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 33 FSW. 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.
Adsorbent:
The substance that prevents carbon dioxide buildup in the breathing loop.
Axial flow canister:
An axial canister is a canister design in which the breathing gases move from top to bottom (or vise-versa) through the canister. Contrast this with "Radial canister"
B
Bail out:
A gas source the diver can switch to in an emergency in case of equipment failure. This can be either an external gas source with regulator, or it may be utilize a rebreather's diluent supply.
Breathing bag (Also called Counterlung):
A flexible bag in the loop that expands and contracts as the diver breathes, allowing for respiration and changes in the volume of the breathing loop.
Breathing loop:
The breathing loop in a rebreather is composed of all the internal areas within which the breathing gases flow. This includes the counter-lung, canister, breathing hoses and the divers lungs.
Bubblers:
Open-circuit scuba divers who unavoidably release vast, wasteful clouds of gas wherever they go, causing a deafening noise and scaring all the marine life within 10 meters.
C (back to top of page)
Canister:
The rebreather component that contains the CO2 adsorbent material.
Caustic Cocktail:
A highly alkaline (caustic) liquid producing a strong burning sensation, which can occur if water enters the canister canister and dissolves the adsorbent. It should be noted that condensation of warm moist gas on the colder canister is also a source of water which can cause a caustic cocktail. Also, the adsorbent reaction with CO2 also produces water vapor, which can then condense inside the canister.
Counter-lung: (Also called Breathing Bag):
A flexible bag in the loop that expands and contracts as the diver breathes, allowing for respiration and changes in the volume of the breathing loop.
CNS Toxicity (Central Nervous System Toxicity):
(see Oxygen Toxicity)
D (back to top of page)
Dams:
An anti channeling dam is a small rubber ring installed inside a granular canister canister to ensure that gas is properly and efficiently channeled through the granular adsorbent. Failure to properly install a dam into a granular canister canister can lead to a diver experiencing hypercapnia.
Diluent :
This is the gas used in a 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.
Dosage device (or demand unit):
A regulator used in a semi-closed rebreather which delivers a constant amount of pre-mixed gas to the breathing loop.
E (back to top of page)
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 = depth.
EAN(x) [Enriched Air Nitrox (percent of oxygen)]:
This is one of the naming conventions for a non-normal mixture of oxygen and nitrogen. Normal air has approximately 21% oxygen and 79% nitrogen. 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.
Exhalation Hose:
The exhalation hose is the breathing hose connected to the mouthpiece through which exhaled gases pass back from the diver to the rebreather.
Exhaust Bag:
A flexible bag which accepts exhausted respirations from the exhalation hose. In this bag, excess pressure is vented through the exhaust valve out of the system. The remaining gas is passed onto the canister canister.
Exhaust Valve:
This valve allows excessive gas to vent from the breathing loop. Expanding gas is vented upon ascent, or as gas accumulates in a semi-closed rebreather.
Exothermic:
A chemical reaction which produces heat. When the canister chemically reacts with the CO2 in the loop, it converts it to calcium carbonate, water and heat, thereby warming the breathing gas.
F-G (back to top of page)
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 (at 33 feet of depth the pressure is 33 FSW). However, if you are in fresh water, you can have a pressure of 33FSW, but you would not be 33 feet below the surface. Since fresh water is not as dense as salt water, 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 N2. In air, the fO2 = .21 (21 percent) and the fN2 = .79 (79 percent).
Fully-closed circuit rebreather:
This type of rebreather monitors the oxygen content of the breathing gas, and typically uses a solenoid to open a valve which then adds pure oxygen to the breathing loop as needed. This type of rebreather is the most efficient from a gas useage perspective, and can also minimize nitrogen uptake by optimizing oxygen content of the breathing mix. Its disadvantage is that of complexity and reliance on electronics for control.
H (back to top of page)
Heliox:
A breathing mixture of gases consisting entirely of helium and oxygen. This is used to eliminate nitrogen narcosis (by eliminating the nitrogen) and to control the affects of oxygen toxicity by reducing the oxygen in the breathing mix. Another benefit is reduced effort of breathing due to the lower density of helium.
Hypercapnia:
Hypercapnia is the physiological condition that results from too much C02 (carbon dioxide). 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 (using open circuit), 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.
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 maintain consciousness.
HPNS (High Pressure Nervous Syndrome):
HPNS is a condition which results from breathing helium under high pressures. Early symptoms of HPNS are sometimes seen as shallow as 300FSW but more commonly over 600FSW. 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.
I-J-K (back to top of page)
Inhalation Bag (also called Breathing Bag or Counterlung:
A flexible bag connected between the canister canister and the inhalation hose which takes scrubbed gas from the canister canister, adds supply gas, and supplies it via the inhalation hose to the divers mouthpiece.
Inhalation Hose:
The inhalation hose is a corrugated, large diameter hose that allows a one-way flow of enriched gas from the inhalation or breathing bag through a checkvalve into the mouthpiece of the unit. The hose contains adjustable weights for diver comfort.
L (back to top of page)
Loraine Smith Effect:
(see Oxygen Toxicity)
M (back to top of page)
MOD (Maximum Operating Depth):
The maximum operating depth of a breathing gas before reaching a predetermined maximum partial pressure of Oxygen, usually 1.6 or higher. This limit is to protect from oxygen toxicity. The formula for MOD is (((1.6 / O2%) -1) * 33) or ((46.2 / O2%) -33).
Mushroom Valve:
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.
N (back to top of page)
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 (a mixture of gas with 21% oxygen would be called normoxic).
Nitrogen Narcosis:
A narcotic effect produced by high partial pressures of nitrogen. This can affect judgment, and for most divers starts to be noticeable at a pN2 over 3.00 (approximately 100 FSW when breathing air).
Nitrox:
Any mixture of oxygen and nitrogen which has more or less oxygen than air. The standard Nitrox mixtures are EAN32, EAN36 and EAN50 (EAN: enriched air nitrox).
O (back to top of page)
Open-circuit:
an underwater breathing device that supplies air to the diver (at ambient pressure, i.e. equal to the diver’s depth) from a tank with the resulting exhaled air being vented to the surrounding environment. The breathing circuit is "open", and the gas supply is not conserved by "rebreathing" it.
Oxygen sensor:
Device to measure the PO2 in the loop, usually found in the inhalation bag.
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 (called "The Loraine Smith Effect" or "Pulmonary Oxygen Toxicity" as the primary damage is to the lungs and airways), and the other results from short high ppO2 exposures (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.
P-Q (back to top of page)
Paul Bert Effect:
(see Oxygen Toxicity)
Partial Pressure:
The pressure of a particular gas within a gas mixture. In more specific terms it is the Fraction of the gas (F[x]) multiplied by the absolute atmospheres. For example, 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). Therefore, the partial pressure of a tank of air at 33FSW is .21 * 2 = .42 pp02.
ppO2:
The partial pressure of Oxygen in a gas mix.
ppN2:
The partial pressure of Nitrogen in a gas mix.
R (back to top of page)
Radial flow canister:
A radial flow canister is a canister design in which the breathing gases move from the middle to the outside (or vise-versa) through the canister. Contrast this with "Axial flow canister".
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 canister.
RMV (Respiratory Minute Volume):
The amount of gas that you breathe out in one minute.
S (back to top of page)
Scrubber(or C02 canister):
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.
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.
Shutoff Valve on mouthpiece:
The shutoff valve on the mouthpiece is shut to prevent water from getting into the mouthpiece when the mouthpiece is not in the users mouth.
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. Skip breathing (which is not recommended) is used for open circuit diving only, and will have no effect on system duration of any rebreather.
Soda-Lime:
Soda lime (also called hydrated lime) is a chemical agent which reacts and bonds with CO2 and is commonly used in granular canisters of rebreathers. 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%).
Sonic Orifice:
Used as a dosage technique for some semi-closed rebreathers, the sonic orifice is designed to maintain the same gas flow, independent of the supply tank pressure. Sonic orifices are generally preset for a specific EAN mixture.
T (back to top of page)
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 narcosis.
U-V (back to top of page)
UPTD (Unit Pulmonary Toxic Dose):
see OTU's
W-X-Y-Z (back to top of page)
Work of breathing (WOB):
The phrase "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, valves, canister design, counter-lung placement and design, etc. The work of breathing is also affected by depth (depth increases the density which increases the work of breathing).
