Bob Bolland's Okinawa Rebreather Site

Miscellaneous Rebreather-associated Terms

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 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 X .0303030) + 1 = 2.3939 ATA. to convert ATA to FSW. ATA - 1 X 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 coffee 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, as well as the divers lungs.

Closed Circuit Rebreather. This type of rebreather does not release any gases from the unit except under 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 partial pressures of Oxygen (ppO2) and to inject the proper amounts of diluent and O2 into the breathing loop.

CEDU (Crestline Experimental Dive Unit):
See FEOR (Field Expedient Oxygen Rebreather) / Dr. Bob

CON-VENTID (acronym):
An acronym used to remember the central nervous system's signs and symptoms of oxygen toxicity:
  • CON: CONvulsions
  • V: Visual disturbances
  • E: Ear ringing
  • N: Nausea
  • T: Twitching, tingling
  • I: Irritability
  • D: Dizziness
  • 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.

    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 (e.g., air, nitrox, trimix or heliox)in the breathing loop are compressed.

    The "Doing It Right" (DIR) system evolved out of the exacting demands of the world's most extreme exploration diving, yet the approach is rapidly gaining favor among all levels of divers. Everyone benefits from a system that makes the underwater experience safer and more comfortable. The DIR system is much more than an equipment configuration. It is a diving style that ensures every aspect of each dive represents safety, fun and efficiency.

    DEADS (Diving Euthanasia Assistance Devices):
    Some home-made RBs (thanks to Tom Rose for this one)

    Dräger P-Connector:
    Also known as an Oxygauge-Port. Frequently used by RB builders for installation of an O2 sensor probe into the counterlung in order to accurately read O2 levels.

    DSV (Dive/Surface Valves):
    The divers mouthpiece. It has two one way valves in it, one in each end. These allow breathing gas to flow in only one direction. The DSV will close off the mouthpiece from water with a lever located in the bottom of it.

    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 X (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. 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.

    Fastex connectors: These are the plastic quick disconnect fitting as per many BCs, etc.

    FEOR (Field Expedient Oxygen Rebreather): From the Crestline Experimental Dive Unit

    Fraction of gas (f[x]):
    The percent of a particular gas in a gas mix. Air contains, 21% O2 and 79% N2. In Air the f02 = .21 (21%) and the fN2 = .79 (79%). You may also hear the term "fraction of inspired gas" which means the fraction actually inspired, or "breathed in".

    Frenzel maneuver: Move the mouth muscles forward and side-to-side to open the Eustachian tubes, then use the tongue as a piston to push air into the middle ear. (See: Toynebe Maneuver & Valsalva Maneuver)

    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.

    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.

    GUE: Global Underwater Explorers

    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. Some divers are known to feel jittery when using heliox, and as divers go beyond about 300 FSW, classic symptoms of HPNS can begin to appear.

    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.

    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 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.

    KISS: Keep It Simple Stupid
    The KISS ("Keep It Simple Stupid!") Rebreather from Jetsam technologies is an example of rebreather technology in a constant state of evolution. In terms of oxygen delivery the KISS system uses a static inter-stage pressure constant flow delivery system, similar to that utilized in Semi Closed Rebreathers to deliver a pre-set flow of oxygen into the loop keyed to the divers O2 metabolism rate. This flow rate roughly equates to the O2 being used by the divers body, any extra required is simply added by a press of the manual add button. Equipped with both an Automatic Diluent Addition Valve and OC/CC bailout mouthpiece the unit is easy to operate. Simple to configure and adaptable to the divers changing needs the unit can be used as a simple Nitrox rebreather or with suitable training for advanced Tri-Mix diving.

    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.

    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. The valves are very important to ensure that the gases are scrubbed of CO2 and that there is enough O2 in the breathing loop. The valve is called a mushroom valve because the diaphragm of the valve resembles 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.

    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.
    Hypoxic Levels:
  • normoxic: .21
  • mild symptoms: .16
  • hypox symptoms: .14
  • helpless: .11
  • death: .10

    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.

    Open-circuit scuba:
    First introduced to recreational divers by Cousteau and employs a compressed gas supply and a demand regulator from which the diver breathes. The exhaust gas is discarded in the form of bubbles with each breath, therefore the term "open-circuit". Open-circuit scuba is inherently inefficient: because only a small fraction of each inhaled breath is actually used by the diver for metabolism, there is a tremendous waste of useable Oxygen (O2) with each breath. Furthermore, the quantity of O2 lost in this manner increases with increasing depth.

    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. 1 ata for 1 minute = 1 OTU.

    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 "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 "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

    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 X 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.

    Paul Bert Effect:
    (see Oxygen Toxicity)

    PFO (Patent Foramen Ovale):
    The condition is a heart defect called patent foramen ovale, or PFO. This is a foramen (a small oval opening) which is located the interatrial septum, the tissue between the atria, two of the four chambers of the heart. Everyone has this open hole in the heart prior to birth when it is needed in order for fetal circulation to bypass the lungs. A baby’s first breath should cause pressure in the left atrium to increase, so that the flap covering this opening closes and it normally eventually seals. Yet this valve flap stays loose in some individuals, causing an atrial septal defect or interatrial shunt called PFO.

    The chamber in which all associated breathing gases are allowed to mix

    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.

    Passive Variable Ratio. Used w/ SCR-Calculations See: Karl's Rebreather Page

    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 coffee 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 scrubber".

    Reality Check:
    Panic prevention starts before you get wet. Are you anxious about this dive? Is the current stronger, the water colder, the visibility worse than you've experienced before? Or is it deep? Nitrogen narcosis is almost inevitable below 100 feet, and when it strikes it may well increase your existing anxiety. If you look at the situation and realize you just don't feel right about this dive, don't do it.

    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.

    RGBM (Reduced Gradient Bubble Model):
    [ From the Abyss site ]
    The Reduced Gradient Bubble Model (RGBM) is a dual phase (dissolved and free gas)algorithm for diving calculations. Incorporating and coupling historical Haldanian dissolved gas transport with bubble excitation and growth, the RGBM extends the range of computational applicability of traditional methods. The RGBM is correlated with diving and exposure data on more complete physical principles. Much is new in the RGBM algorithm, and troublesome multidiving profiles with higher incidence of DCS are a target here. Some highlighted extensions for the ABYSS implementation of the Buhlmann basic algorithm include:
  • 1. Standard Buhlmainiann no-stop time limits.
  • 2. Restricted repetitive exposures, particularly beyond 30m / 100 ft, based on reduction in permissible bubble diffusion gradients within 2 hour time spans.
  • 3. Restricted yo-yo and spike (multiple ascents and descents) dives based on excitation of new bubble seeds.
  • 4. Restricted deeper-than-previous divers based on excitation of very small bubble seeds over 2 hour time spans.
  • 5. Restricted multiday diving based on adaptation and regrowth of new bubble seeds;
  • 6. Smooth coalescence of bounce and saturation limit points using 32 tissue compartments.
  • 7. Consistent treatment of altitude diving, with proper zero point extrapolation of limiting tensions and permissible bubble gradients (through zero as pressure approaches zero).
  • 8. Algorithm linked to diving data (tests), Doppler bubble, and laboratory micronuclei experiments.
    Overall, parameters in RGBM / ABYSS are conservative, but flexible and easy to change or fit to new data.

    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.

    (Surface Air Consumption).

    SCR: 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 circuit rebreather.

    Schrader Valve:
    Standard automotive tire air valve.

    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.

    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 (also called hydrated lime) is a chemical agent which reacts and bonds with CO2 and is commonly used in the scrubbers of rebreathers. The primary constituents of soda lime include Calcium Hydroxide - Ca(OH)2 (about 70~80%), Water - H2O (about 16~20%), 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 taken in. 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 respiratory tract.

    SodaSorb and SodaSorb HP:
    A brand of soda-lime made by W.R. Grace in the USA.

    A brand of soda-lime made by Molecular Products, of Thaxton, England.

    Stroke mix:
  • stroke (1) diver who's so clueless that he/she is a danger to him/herself and anyone nearby. Often considered an accident looking for a place to happen.
  • stroke (2) Diver who refuses to learn even when shown a better way.
  • stroke (3) Favorite insult of Hogarthian divers at WKPP.
  • stroke mix. Some divers believe that 80/20 is a bad idea, and refer to it as "stroke mix" (80/20: 80% O2, 20% N2).
  • strokery. The practices of a stroke.

    Toynebe Maneuver:
    Swallow while the mouth and nose are closed, which pushes air into the Eustachian tubes and relieves middle ear pressure. (See: Frenzel Maneuver & Valsalva Maneuver)

    Trimix (tmx):
    A breathing mixture of gases most often composed of Oxygen, Nitrogen and Helium. The proportions of each are changed according to the needs of the particular dive plan to help limit Oxygen toxicity and Nitrogen narcosis.

    Valsalva Maneuver:
    The method divers are commonly taught to relieve middle air pressure [can lead to decompression sickness among persons with patent foramen ovale (PFO), especially when it is used forcefully to clear the ears on second dives while the body is still offgassing from a first dive]. (See: Toynebe Maneuver & Frenzel Maneuver)

    Variable volume mixed gas rebreather systems:
    Also known as closed circuit rebreathers (CCR, C2R) and fully closed oxygen rebreathers.

    Voting Logic:
    The Inspiration (formerly the "Buddy Inspiration") has been made in England for about four years by Ambient Pressure Diving. The company says more than 1,000 units have been sold worldwide, though only 50 or so have reached the U.S. as yet. It's a closed-circuit system, so no bubbles at all will issue (except on ascent) and an optimum PPO2 can be maintained for less nitrogen uptake and faster decompression. The design has considerable redundancy. Two identical computers independently read three oxygen sensors. The computer you start first becomes the "Master" and controls gas injection; the other is the "Slave" and sort of idles along watching the sensors unless the Master fails, in which case the Slave instantly takes over. Both dangle in front of you and are usually hooked together so it's easy to compare all six PPO2 readings. Each computer uses what's called "voting logic" to assess the three oxygen sensors. If all three read close together, the two closest are averaged. If one is far off from the other two it is disregarded and an alarm sounds so you know a sensor has failed. Two of three oxygen sensors would have to fail at once to fool the computer—an unlikely event. If both computers failed—also very unlikely as they have independent power sources—you could still operate the rebreather by watching the PPO2 readings and manually injecting oxygen or diluent. The computers can maintain the PPO2 at either of two adjustable setpoints. Their default settings are 0.7 bar and 1.3 bar. An alarm sounds if the PPO2 in the loop exceeds the high setpoint or falls below the low one. Normally you use the lower one near the surface, the higher one deeper. You switch between the two by pressing a button on the computer. A bailout regulator is connected to the diluent cylinder.

    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 phrase "Work of breathing" relates to the amount of effort required by the diaphragm 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.

    Yellow Box of Death (YBOD): Historically this is what some RB divers call the Inspiration dive unit. Some of my fellow divers here on the island are calling the IDA-72 by the same name.

  • NOTE: This page in re presentation with Bob Bolland's Okinawa Rebreather Site.

    Page Date: 26 Mar '07
    Page Modification Date: 26 Mar '07
    Copyright © 2007 Robert F. Bolland