Last Updated: 4^th June 2007

10l, 12l, 15l: Refers to the size of air cylinders used in Europe, the number measures in litres the capacity of the cylinder (this is technically known as the water capacity). So an empty 15l cylinder contains 15 litres of air, double the air pressure (2 BAR) and it contains 30 litres of air. In the US they use cubic feet of air, rather than litres.
10m, 30m: The depth in metres. In the US they use feet.
1st Stage: Part of a Regulator, the central part that attaches to the diving cylinder. Apart from serving as the cylinder attachment the 1st Stage is a pressure reducing valve that reduces the cylinder pressure (200+ BAR at the start of a dive) to a lower 10 to 15 BAR pressure. There are ports (screw fittings) on the 1st stage that hoses to feed other equipment attach to, there are normally four low pressure ports (LP Feed) and two high pressure (HP Feed) ports.
2nd Stage: The part of the regulator you breathe from, also known as a primary, octopus or demand valve.

A
A-Clamp: A type of fitting for connecting the regulator 1st stage to the cylinder pillar valve. The pillar valve is clamped into a frame on the regulator 1st stage, the assembled item resembles the letter 'A'. The A-Clamp is the 'traditional' way of connecting the 1st stage to a cylinder and is used throughout the world.
AA: Assisted Ascent
AAS: Alternative Air Source. A 'spare' secondary demand valve (often referred to as an octopus) carried by a diver for their own use or their buddy's use should something go wrong with their normal equipment. First brought into diving by cave divers and finally these pieces of equipment are now an essential part of diving equipment.
ABLJ: Adjustable Buoyancy Life Jacket. An outdated style of BCD that is still used by some divers. Looks like a standard 'over the head' life jacket (it is sometimes called a horse collar) but the amount of air in it can't be controlled. Now replaced by the BCD or Wings.
Absolute Pressure: The pressure currently acting on a divers body, as you read this you should be experiencing an absolute pressure of 1 BAR, i.e Atmospheric pressure. Each 10m of water above you adds 1 BAR, so a diver at 30m is at 4 BAR absolute pressure - 4 times the normal atmospheric pressure. When we breath at depth we breath at absolute pressure (the demand valve makes sure of this), so at 30m (4 BAR) we use twice as much air as at 10m (2 BAR).
ADV: Automatic Dilutant Valve
AGA: Full face mask, trade name (FFM)
AGE: Arterial Gas Embolism
AISE: Association of Independent Scuba Educators
Air Hog: Diver who sucks tank dry in record time
ANDI: American Nitrox Divers Incorporated ANDI website
AOW: Advanced Open Water. A PADI Advanced diver qualification. Some people think this is a course for "advanced divers", but it is a course for Advanced Diving techniques for Open Water divers.
AP: Absolute Pressure or Ambient Pressure
APD: Ambient Pressure Diving (home of the Inspiration/Evolution, Yellow Turtle or YBOD!)
ASD: Atrial Septal Defect, 'Hole in Heart'
ASF: Australian Speleological Federation
ASL: American Sign Language
Argon: An inert gas sometimes used by divers to inflate their dry suits, argon is used because it is a much better insulator from the cold than normal air (some say as much as 50% better).
ATA: Atmospheres Absolute
ATM: Atmosphere
AUF: Australian Underwater Federation
Auto Air/Air II: Another form of AAS. A demand valve that is built into the inflator/deflator control of a BCD.

B
BA: Breathing Apparatus
Back Plate: Part of a different type of diving rig, rigid (normally stainless steel or aluminium) that is designed to carry a twin set of cylinders. Used in conjunction with a wing BCD.
BAR: A measure of pressure, 1 bar = atmospheric pressure. The amount of air in a cylinder is measured in BAR, it is typically about 220 BAR at the start of a dive and 50 BAR or more at the end.
Barotrauma: A general term to describe a physical injury caused by a change in pressure.
BCD: Buoyancy Control Device. Normally the term used for the waistcoat like jacket that divers wear to which most of their equipment is attached. The BCD contains a bag that can be inflated and deflated, this controls the buoyancy of the diver.
Bends (The): The 'general public' term for what we call DCS or DCI. 'The Bends' is actually a mild form of DCS where bubbles form in joints such as elbows, to relive the pain and pressure people tend to continually bend their arms - hence the name 'The Bends'.
BP: Backplate, also Blood Pressure
BPM: Beats Per Minute (heart/pulse rate)
BSAC: The British Sub Aqua Club. An english diver training agency.
B52: One part Tia Maria, one part Bailey's and one part Cointreau :-)
Bradycardia: Heart beating too slow.
Braille Dive: Viz so bad you had to feel your way through the dive
Bubble Check: Used in technical diving, the two divers descend to about 6m and swim around their buddy looking for bubbles (leaks) before the descent.
Buddy: Your diving partner, we normally dive in twos - a buddy pair.
Buddy Check: A pre-dive check where two divers check each others dive equipment and explain how the essentials work and where they are located.
Buddy Monitoring: Keeping an eye on your buddy during a dive and checking that everything is OK.
Buoyancy: Whether something floats, sinks or hovers in the water. Negative Buoyancy = sinking, Neutral Buoyancy = hovering, Positive Buoyancy = rising or floating. Divers should be at (or near) neutral buoyancy at all times so that they are in total control of where they are in the water, this is achieved by injecting air into or dumping air from their dry suit or BCD.
BWRAF: Acronym used for your Buddy Check, stands for BCD, Weights, Releases, Air and Final ok. A couple of ways to remember this is: Being Wary Reduces All Failures; Beer, Wine Rum Are Fine; Big Women Really Are Fun; Big White Rabbits Are Fuzzy; Boogie With the Rich And Famous; Bruce Willis Ruins All Films

C
Cave: A (normally rock) tunnel with no immediate route up or out. Some underwater caves can go on for miles. Cave diving is a very specialised form of diving requiring special training, equipment and techniques. Without all three of these it is very dangerous!
Cavern: A depression in a rock face or the 'twilight zone' entrance to a cave. A cavern constitutes an overhead environment but for a cavern you can clearly see the light and the way out. When you reach the stage where you can't see the light any more you are in a cave!
CCR: Closed Circuit Rebreather. A type of rebreather that uses oxygen sensors and controls the level of oxygen being breathed. Makes very efficient use of gas and cuts down the time needed for deco. With regular SCUBA (open circuit) the exhaled gases are released into the water, whereas with a CCR the gases are recirculated and oxygen (or diluent) added as necessary according to the diver's depth and gas requirements.
Chamber: Short for decompression chamber. A medical facility where a diver can be pressurised to simulate diving pressures. Used to treat DCS amongst other things.
(Dive) Computer: A small piece of diving electronics worn on the wrist or included in the console. It constantly displays information such as current depth, maximum depth, dive time and remaining no stop time. It can calculate the decompression requirements for the dive. Most divers carry a dive computer. Some are quite basic "bottom timers", but you can choose some different models that show you your ascent rate, Oxygen Toxicity levels, even cylinder pressure. The higher end (i.e. more expensive) computers let you programme in different gasses to use and you can switch between them during the dive. Some even have games on them to use during your decompression stop!
Console: Part of a regulator, contains some of the instruments used by a diver. Typically there is a pressure gauge showing how much air is left in the cylinder, a depth gauge to show how deep you are and a compass to navigate with. The console attaches to one of the HP Feeds of the 1st Stage.
Cylinder (AKA tank\bottle): A steel or aluminium bottle or tank containing our breathing (normally...) gas. They come in various sizes up to 20l and two pressure ratings, the standard 232 BAR and 300 BAR. They can also be made of carbon fiber, but these are a lot more expensive.
CC: Closed Circuit
CCR: Closed Circuit Rebreather
CDG: Cave Diving Group
C-L: Cis-Lunar Development Laboratories
CL: Counterlung (on rebreathers)
CO: Carbon Monoxide
CO2: Carbon Dioxide

D
DCS: Decompression Sickness, the modern term for a whole host of very nasty things (the 'bends' is one minor form of DCS) that can happen if you break the rules or things go badly wrong. Sometimes even if you follow all the rules, you may still get a mild dose of DCS. During a dive nitrogen is forced into a divers body due to the pressure, when returning to the surface this nitrogen disperses again. If this happens too quickly (a bit like opening a pressurised can of fizzy drink) bubbles can form and cause DCS. A large part of diver training is on what causes and how to avoid DCS.
Deco or Decompression: The process of a diver 'returning to normal pressure' after a dive and allowing the nitrogen absorbed (see above) to disperse. For more advanced diving you have to stop on the way up and wait for your body to adjust to the new (reduced) pressure, this is called a deco stop. Most diving is what is known as No Stop diving, just coming up slowly is enough for the diver to decompress safely. Decompression continues when the diver is back on the surface, after a days diving it can take over 24 hours for all the extra nitrogen absorbed to fully disperse.
Deco Mix: A special mixture of gasses (usually Nitrox or Oxygen) to be breathed during a decompression stop. Breathing a high percentage of Oxygen, or pure Oxygen speeds up decompression. Special training is needed in order to use Nitrox in general or to use it or Oxygen for decompression!
Deep Stops: See our page on Pyle Stops
Delayed SMB: (or DSMB) See SMB
Demand Valve (DV): The part of a regulator that you put in your mouth and breathe from. It supplies air 'on demand' when you want it, the mechanics of a DV always supply you air at absolute pressure, so a breath of air at 30m is actually 4 times the amount of air you would breathe on the surface.
Diluent: A CCR (see above) uses two gas supplies. Oxygen and diluent (normally compressed air). When the Oxygen level is too low, Oxygen is added. The oxygen is metabolised into carbon dioxide and exhaled. The CO2 is then cleaned (scrubbed) by an absorbant material and, if necessary, more oxygen is added.
DIN Fitting: A type of fitting for connecting the regulator 1st stage to the cylinder pillar valve. There is a male screw thread on the 1st stage which screws into a female thread on the pillar valve. The DIN fitting is the modern way of connecting the 1st stage to a cylinder, it is used mainly on high pressure (300 BAR) cylinders, but used on most (if not all) cylinders used in Technical diving for added safety.
Dive Timer (DT): A simple electronic device for recording current depth, maximum depth and time of a dive. Similar to a dive computer but it does not calculate decompression.
Diving Suits: Different types of diving suits are used for different water temperatures and diving conditions, the following is a brief summary - Lycra or Skin: A thin body stocking used in tropical water, main purpose is to protect against abrasion and stings. Wet Suit: A thicker neoprene material that works by trapping a layer of water between the body and the suit, this forms and insulating layer. Semi-Dry Suit: An improvement on the wet suit, seals at the wrist and neck limit the amount of water that flows in and out of the suit. Dry Suit: Completely sealed from the water, dry inside. Diving dry suits have air valves that the diver uses to inflate/deflate the suit, as well as insulation the suit can also be used for buoyancy control. Most regular divers in cold waters use dry suits.
DM: Dive Master. A PADI diver qualification, the first Professional rating.
DPV: Diver Propulsion Vehicle, or scooter. Used a lot in cave diving and some larger wreck dives, but smaller DPVs are used in recreational diving to get to a reef or dive site out of swim distance.
Dump: A valve to allow the, or the action of, getting rid of (dumping) air from a BCD or a dry suit.
DAN: Diver's Alert Network (diving insurance)
DEMA: Diving Equipment Manufacturers Association
DIR: Do(ing) It Right
DO: Diving Officer
DV: Demand Valve

E

EAD: Equivalent Air Depth. When using Nitrox as a breathing gas it has the effect of 'simulating' a shallower dive made breathing air, for dive time calculation purpoes (see Tables) the EAD can be used for working out our no-stop time when using Nitrox.
EAN or EANX: Enahnced Air Nitrox. Technically, it means a breathing mix made up of Nitrogen and Oxygen, but with a higher than normal percentage of Oxygen (22-40% for recreational diving). Some people interchange this with the term "Nitrox", although Nitrox means ANY mixture of Nitrogen and Oxygen. Jargon for jargon's sake!
EC: Emergency Cylinder. A small cylinder (0.4l) connected to the BCD. Can be used to inflate the BCD and/or in conjunction with an Auto Air provide a very small redundant AAS. Not many people use them anymore, as if they stick open you shoot to the surface!.
END: Equivalent Narcotic Depth. This is mainly used when diving with Trimix for calculations.
Ex-Buddy: A Buddy that stuffs up a dive then insists its your round...

F
Fenzy (Frenzy) Bottle: Another name for an EC.
FUBARed: Fuc*ed Up Beyond All Recognition.....(usually referring to a dive plan.)
FFM: Full Face Mask
FFW: Feet of Fresh Water
FSW: Feet of Salt Water

G
Gas: Breathing gas, usually air.
Goodman handle: U-shaped attachment to the light head of an umbilical torch for attaching the light head to your hand.
GPS: Global Positioning System. Electronic navigation aid that uses satellites to give a very precise read-out of your position. Originally designed for use by the US military.
GUE: Global Underwater Explorers. A technical training diving agency. See their website.

H
Halocline: Boundary between layers of water of different salinities
HB: Hard Boat, a term used to describe a larger dive boat, typically an in-shore trawler in the case of Irish diving.
Heliox: A special breathing gas mix containing helium and oxygen only, this is usually a comercial diving gas, TriMix is sometimes used for a very advanced form of diving.
HH: HammerHead electronics made by Juergensen Marine for Rebreathers
HID: High Intensity Discharge. A very energy efficient and expensive type of torch.
HP: High Pressure. Normally used when taking about regulators, in the case of regulator pressures high pressure is the current pressure in the cylinder, which could be up to 300 BAR.
HPNS: High Pressure Nervous Syndrome, only comes into play on very deep dives on Trimix. It is a side effect of breathing Helium at high pressures.
Hydro (test): A 5 yearly test to check the physical integrity of your tank
Hypercapnia: Too much Carbon Dioxide (CO2) in your bloodstream.
Hyperoxia: Too much Oxygen in your bloodstream.
Hypoxia: Too little Oxygen in your bloodstream.

I
IDC: PADI's Instructor Development Course, made up of the AI and OWSI courses
IE: PADI's dreadded Instructor Exam! Usually done immediately after the IDC or IOC
IOC: PADI's Instructor Orientation Course (for crossing over from another agency)
Inspiration: A popular rebreather model. See Ambient Pressure Diving
ITDA:A technical, recreational, commercial and medical diver training agency. See ITDA website
IANTD:A technical diver training agency. See IANTD website
ISC: Innerspace Systems Corporation (makers of the Megalodon rebreather)

K
KISS: A type of rebreather. It differs from most other CCRs in that while it senses oxygen levels the diver is left to control them. See their website

L
Lantern/Lamp: A large and powerful underwater torch. Not usually as expensive as a HID.
LP: Low Pressure. Normally used when taking about regulators, in the case of regulator pressures low pressure is about 8 - 10 BAR.
LIDS: London International Dive Show (usually in March)
Lycra: See Diving Suits

M
Manifold: A rigid tube that connects the two cylinders of a twin set together. The manifold connects between the pillar valves of the cylinders, it sometimes has its own 'isolation' valve in the middle - in which case it is an isolation manifold.
MDT: Maximum Dive Time
Megalodon: A CCR rebreather, as used in the film The Cave. See their website: Megalodon
MFW: Metres of Fresh Water
MGR: Mixed Gas Rebreather
Mix: A term used to describe a breathing gas that is NOT normal air. Apart from normal air Nitrox, TriMix, HeliAir and Heliox mixes are sometimes used.
MOD: Maximum Operating Depth
MSW: Metres of Salt Water

N
N2: Nitrogen
NACD: National Association for Cave Diving
NAUI: National Association of Underwater Instructors
Narcosis (Narked): An 'altered state of mind' (effects can be similar to alcohol) that effects divers at depth. Full name is Nitrogen Narcosis, it is caused by breathing the nitrogen component of your cylinder under pressure. This is why deeper divers use TriMix - less Nitrogen.
NDL: No Decompression Limit
Nitrox: A breathing gas mix of oxygen (about 21% in normal air) and Nitrogen (about 79% in normal air). Normally followed by a number that denotes the oxygen percentage - e.g. Nitrox 32 is a mix containing 32% oxygen, 68% nitrogen. For recreational diving, the minimum used is 22% and the maximum is 40%. Some people interchange this with the term "Enriched Air Nitrox", although Nitrox means ANY mixture of Nitrogen and Oxygen.
NOAA: National Oceanic and Atmospheric Administration
No Stop: A type of dive where a decompression stop is not required.
NSS-CDS: National Speleological Society - Cave Diving Section

O
OC: Open Circuit. Normal SCUBA gear, as opposed to rebreathers (Closed Circuit or Semi-Closed Circuit).
Open Water Diver (OW): An entry-level diver qualification.
Octopus: In diving terms this is a spare demand valve that is used as an AAS.
Overhead Environment: Any diving situation where there is no clear way up to the surface, typically inside a wreck or a cave.
OWSI: Open Water SCUBA Instructor. An entry-level instructor qualification.
OPV: Over Pressure (Relieve) Valve, or Over Pressurisation Valve
OTT: Oxygen Tolerance Test
OTU: Oxygen Toxicity Unit

P
PADI: Professional Association of Diving Instructors. An American based diver training agency offering standard recreational and technical training throughout the world. Certifying approximately one million divers per year, it is by far the largest diver training agency in the world. See their website
Partial Pressure (PP): One of the gas laws and a bit of mathematics that divers have to put up with! It is to do with gasses in a mixture, put simply, it is the pressure of one of the component gasses. e.g. A cylinder contains 25% oxygen, 75% nitrogen. A pressure gauge shows that the cylinder is at 4 BAR, the partial pressure of the oxygen (ppO2) is 1 BAR, the pp of the nitrogen is 3 BAR.
PFO: Patent Foramen Ovale, openable flap between atria. See ASD. A shunt between the right and left side of the heart that allows some blood to circulate back through the body without going to the lungs first. This means that micro-bubbles don't get removed from the blood stream by the lungs efficiently, making DCS more likely.
Pillar Valve: The metal fitting in the neck of a cylinder. Consists of an on/off valve and a connection point for the regulator 1st stage.
Pony: A small (3ltr) cylinder with its own regulator that is carried as a redundant AAS.
Port: (LP or HP) An off-take from the 1st Stage of a regulator.
Pyle Stops: See our page on Pyle Stops

R
Rebreather: An alternative (expensive!) type of scuba set. The normal scuba gear that is used is known as 'open circuit', gases are not re-used. For normal scuba gear your body only actually uses about 4-5% (it uses part of the oxygen content) of what you breathe, the other 96% (plus the 4% carbon dioxide your body generates) you send to the surface as bubbles. A rebreather works by 'keeping' what you breathe out, it cleans out the carbon dioxide and tops up the 4% your body has used from a small cylinder. You then 'rebreathe' this cleaned and topped up gas ad-infinitum (well, until your gas supplies run out).
Redundancy: Having a piece of equipment that is totally redundant under normal circumstances, it is just there in case the main item of equipment fails. e.g. Many divers carry two torches, a main torch and a smaller spare in their pocket. To be truly redundant the equipment must not rely in any way on the piece of equipment that it is it is the back-up for. Put simply, if you don't need it - don't bring it, if you do - bring two.
Reel: A drum, line and winder (like a large sea fishing reel) carried by a diver and used for various things such as connecting to a SMB or as a 'back to where you started' navigation aid. Origanally used in cave diving, then adopted by technical divers and recreational divers.
Regulator: The collective term for the spider like collection of metal, tubes and bits of plastic that divers connect to their cylinder and then to various other bits and pieces of dive gear. It is made up of several parts, a 1st Stage, Demand Valve, Octopus, LP Inflators and Console.
Repetitive Dive: Any dive undertaken when there is still nitrogen in your body from a previous dive. See Decompression.
RIB: Rigid Inflatable Boat, rigid hull with large inflatable tubes down each side. A purpose built dive boat owned by clubs and some individuals.
Rig: The collective name for a complete set of diving equipment (BCD, cylinder, regulator...) used on a dive.
ROV: Remotely Operated Vehicle. These are used mainly in commercial and military operations. They have electric motors with propellors and cameras that are linked by cable (umbilical) to the operator on the surface. They can go far deeper that humans, and are a lot less expensive than a mini-sub. You may have seen them in the film Titanic, amongst others...
RMV: Respiratory Minute Volume
Rule of Thirds: Air - a third to get there, a third to get back, and last third is for emergencies.

S
SAA: Sub Aqua Association. A Diver Training agency.
SAR: Search And Rescue team
SCR: Semi Closed Rebreather. A rebreather that reuses most of the gas but lets some escape, replacing it with fresh gas that contains a higher level of oxygen.
Scrubber: The part of the rebreather that removes carbon dioxide. Contains sofnolime.
SCUBA: Self Contained Underwater Breathing Apparatus.
Safety Stop: A decompression stop, normally at about 5-6m, done just for safety. Not actually required by the dive tables but most divers do a safety stop.
SDI: A recreational diver training agency. See SDI website
Semi-Dry: See Diving Suits
Shot Line: A rope with a heavy weight at one end and a buoy at the other. Dropped onto a wreck or other sea bed feature and used to guide the diver to/from what they are diving on.
SI: Surface Interval
Silt out: Complete loss of vision caused by silt being disturbed, typically by careless finning and/or bad bouyancy
SMB: Surface Marker Buoy. A surface float, normally filled with air, that the diver controls via a line, it shows your cover boat (and anyone else) where a diver is. Divers often carry a delayed SMB, it is carried rolled up in a pocket and it's deployment is 'delayed' until the diver is preparing to come to the surface.
SMI: Steam Machines Incorporated (PRISM Rebreathers)
Sofnolime: The chemical in a rebreather that removes carbon dioxide. Common rebreathers contain enough to last three hours.
Spare Air: Trade name of a large 'aerosol' type device that is a complete cylinder and DV in one small unit, carried as a redundant AAS, AKA Suicide Bottle (see below...)
Stab Jacket: See BCD.
Stage Bottle: A cylinder containing a breathing mix to be used during a decompression stop. AKA Deco Bottle. Although the term "Stage Bottle" actually refers to leaving a bottle at a stage to come back to, not just a deco bottle.
Stop: A required (by the tables or the computer) decompression stop. Or a Safety Stop, which is not a decompression stop, but is done to offgas any excess nitrogen in the bloodstream.
Strobe: An underwater photographers camera flash light. Also a small flashing light carried on a BCD to let other divers know where you are.
Suicide bottle: Many think that emergency cylinders contain too little air to provide much benefit, and that the risk of accidental inflation and subsequent injury from surfacing too quickly is too high.
Suicide Clip: AKA Widow Maker - A type of brass clip with a springed gate notorious for trapping lines and killing divers
STROKE: Society of Technical Risk-Optimizing Karst Explorers
Stroke Diver: 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. Diver who refuses to learn even when shown a better way

T
Tables: Look-up tables that tell you how long and how deep you can safely go when you dive. They tell you how long you can stay at a depth and still do a no stop dive, some tables also tell you the decompression requirements if you stay longer than the no stop time. A dive computer also does these calculations for you and keeps you constantly updated on your decompression requirements during the dive.
Tachycardia: Heart rate too fast.
TDI: A technical diver training agency. See TDI website
Technical Diving: General term for an advanced form of diving involving special mixes and decompression stops. Generically used for any type of diving beyond "normal" or recrational diving.
Thermocline: Boundary between layers of water of different temperatures
Trimix: A specialised breathing mix consisting of oxygen, nitrogen and helium, used for a very advanced form of diving, from 45m down. Trimix is used on all the Open Circuit Deep Dive record attempts.
Twin Set (or twins): A diving rig comprising of two (equally sized) cylinders, both of which are intended to be used during a dive. They are generally manifolded together. Twin 12l tanks are normally used, but we have seen 7l, 10l, 15l and 18l tanks used also. 232 bar tanks are the norm, but 300 bar are sometimes used, although the weight is significantly greater.
Turtle: Another name for an Inspiration.

V
VIP: Visual test for your tank, essential every two and a half years
Vis: Short for visibility, the horizontal range of visibility when underwater. Normally between 5m and 15m in Irish waters, sometimes more, sometimes less.
VENTID: Vision Ears Nausea Twitching Irritation Dizziness (signs of the onset of Oxygen Toxicity)

W
Wet Suit: See Diving Suits
WKPP: Woodville Karst Plain Project (not to be confused with the Klu Klux Klan!)
WOB: Work of Breathing (how difficult it is to breath using your equipment.)
Wing: A type of BCD, most commonly used by technical divers.

Y
YBOD: Yellow Box Of Death. Yet another name for an Inspiration. Some point to several deaths of divers using this particular rebreather, some of which haven't been explained, and say that its unsafe. Others say that there are thousands of them being used successfully and that many of the deaths were clearly caused by operator error. Everyone agrees that even the most experienced OC divers are novices again when switching to rebreathers.
Y-Valve: A special type of cylinder pillar valve. Instead of the normal one connection point for a regulator 1st stage there are two, adding a second regulator gives some redundancy.