Technical diving has its own terminology that can be confusing for beginners at first. What does trimix mean? What’s the difference between backmount and sidemount? And what exactly is rock bottom?
In this glossary, you will find explanations of all the basic terms used in technical diving – from decompression algorithms and special gases through equipment to safety procedures. Each term is explained clearly and concisely so you can quickly grasp the essence.
Use the navigation above for quick access to the letter you’re looking for.
🤿 Quick alphabetical navigation
A
ADV (Auto Diluent Valve)
A valve on a CCR that automatically adds diluent to the breathing loop when gas volume decreases.
Anchor line
A line connecting the boat to the bottom or anchor, used as a reference during descent and ascent.
Argon
An inert gas used for inflating dry suits due to its low thermal conductivity. Provides better insulation than air.
Ascent line
A line used for controlled ascent during decompression stops, may be connected to a buoy.
B
Backplate
The structural component of technical diving gear, a solid metal or composite plate forming the basis of the system.
Backmount
A configuration where cylinders are mounted on the diver’s back (single cylinder or doubles).
Bailout gas
Emergency breathing gas or system allowing safe completion of the dive in case of primary system failure (especially with CCR).
Barotrauma
Pressure-related injuries (ear, sinuses, mask, lungs). In practice, it’s useful to distinguish: ear/sinus/mask/lung barotrauma.
Best mix
The breathing mixture with the highest safe oxygen content for a given maximum dive depth, minimizing decompression obligation.
Bottom gas
The primary breathing mixture used at maximum dive depth.
Buoyancy compensator (Wing / BCD)
An inflatable bladder for buoyancy control, used with technical diving harness (backplate and wing system).
C
Canister light
A high-powered diving light with external battery in a canister, standard in technical and cave diving.
Cave diving
Technical diving in underground aquatic systems requiring specialized training, equipment, and procedures.
CCR (Closed Circuit Rebreather)
A breathing system that recycles exhaled gas, removes carbon dioxide, and replenishes oxygen.
CNS oxygen toxicity
Damage to the central nervous system caused by high partial pressure of oxygen. Manifests as convulsions, loss of consciousness.
CO₂ absorbent (Scrubber)
The material in a rebreather (typically soda lime) that chemically removes carbon dioxide from exhaled gas.
Cookie / Line marker
A plastic or metal marker attached to a guideline for orientation in caves and wrecks.
D
Decompression algorithm
A mathematical model for calculating decompression stops (e.g., Bühlmann, VPM, RGBM).
Decompression ceiling
The shallowest depth to which a diver may safely ascend at any given moment without risk of decompression sickness.
Decompression diving
Diving where direct return to the surface is not possible without mandatory decompression stops.
Decompression gas (Deco gas)
A breathing mixture with higher oxygen content used during decompression stops to accelerate off-gassing.
Deep air
Controversial practice of diving on air at depths exceeding 40 m, where narcosis risk increases.
Deep stop
An optional stop performed before the first mandatory decompression stop, intended to reduce bubble formation.
DIR (Doing It Right)
A philosophy unifying equipment, procedures, and team cooperation in technical diving, originally developed for cave diving.
Double-ender
A clip with spring-loaded gates on both ends, used for securing equipment and stage bottles.
Doubles
Two interconnected pressure cylinders used in technical diving, typically with an isolation manifold.
DPV (Diver Propulsion Vehicle)
A motorized device for underwater propulsion, enables longer distances with lower gas consumption.
Dry suit
A diving suit providing thermal insulation through an air layer, used in cold waters.
Dry suit zipper
A specialized zipper on a dry suit requiring regular maintenance and waxing.
DSMB (Delayed Surface Marker Buoy)
An inflatable buoy used to mark the diver’s position during ascent, enhances safety in boat traffic.
E
EAN (Enriched Air Nitrox)
Air with higher oxygen content (22–40%), used to reduce decompression obligation.
END (Equivalent Narcotic Depth)
The depth corresponding to the narcotic effect of a given breathing mixture compared to diving on air.
F
Fail-safe principle
Design of a system or procedure so that upon failure, it transitions to a safer state or allows safe completion of the dive.
G
Gap reel
A small reel with line used for temporary connections or exploring short sections in caves.
Gas analyzer
An instrument for measuring oxygen and possibly helium content in breathing mixtures.
Gas management
Planning gas consumption including reserves and emergency scenarios (Rule of Thirds, Rock Bottom).
Gas switch
Changing breathing mixture during the dive (from bottom gas to deco gas or travel gas).
Gradient Factors (GF)
Decompression algorithm settings (GF Low/High) determining the conservativeness of the decompression profile.
H
Harness
A system of straps connecting the diver to the backplate and wing.
Heliair
Air mixture enriched with helium (without additional oxygen), used as a transition mixture.
Helium
An inert gas used to reduce narcosis and breathing mixture density in deep diving.
Heliox
A mixture of oxygen and helium without nitrogen content, used at very great depths (commercial diving).
Hypercapnia
Excessive accumulation of carbon dioxide in the body, can occur with high workload, poor scrubber (CCR), or skip breathing.
Hypoxic trimix
Trimix with oxygen content below 18%, cannot be breathed at the surface, used only at depth.
I
ICD (Isobaric Counterdiffusion)
A phenomenon associated with switching mixtures with different inert gases at the same depth, can cause bubble formation.
Inert gas narcosis
Impairment of the nervous system by inert gases (especially nitrogen) at high pressure, manifests as impaired judgment.
Isolation manifold
A valve system on doubles allowing separation of left and right cylinders in case of regulator or hose failure.
J
Jump
A short line used to connect two main guidelines in caves, must be marked.
L
Lift bag
An inflatable bag used for lifting objects from the bottom or as emergency signaling.
Line marker (Cookie)
A plastic or metal marker attached to a guideline for orientation in caves and wrecks.
Long hose
The primary regulator hose (typically 2 m) enabling safe gas sharing in confined spaces.
Lost diver drill
Practice of the procedure when losing visual contact with a team member.
Lost line drill
Practice of finding the guideline in a cave when contact with it is lost.
M
MOD (Maximum Operating Depth)
The greatest depth at which a given mixture can be safely breathed without risk of oxygen toxicity.
N
NDL (No-Decompression Limit)
The maximum time that can be spent at a given depth without the need for decompression stops.
Normoxic trimix
Trimix with oxygen content ≥18%, can be breathed from the surface.
O
O-ring
A sealing element used in valves, regulators, and hoses, requires regular inspection and maintenance.
OTU (Oxygen Tolerance Units)
A metric expressing cumulative oxygen exposure, used for monitoring long-term toxicity.
Overhead environment
An environment without direct vertical access to the surface (caves, wrecks, under ice).
Oxygen cleaning
The process of removing contaminants from equipment to make it safe for use with high oxygen concentrations (>40%).
P
Partial pressure
The pressure of an individual gas component in a mixture, determines the physiological effects of the gas.
Pony bottle
A small bailout cylinder (typically 3-6 liters) with independent regulator, used as an emergency air source.
PPO₂ (Partial Pressure of Oxygen)
Measured in bars or atmospheres, the safe range for diving is typically 1.2-1.6 bar.
Pressure gauge (SPG)
An instrument displaying gas pressure in the cylinder, typically calibrated in bars or PSI.
Primary reel
The main reel with line (100+ m) used for laying the main guideline in caves.
Profile (Dive profile)
The progression of depth over time during a dive, graphically displayed in a dive computer or plan.
R
Ratio Deco
A decompression system based on ratios and formulas instead of computers, requires training and understanding of principles.
Rebreather
A device enabling breathing gas recycling, we distinguish CCR (closed circuit) and SCR (semi-closed circuit).
Redundancy
Use of backup systems enabling safe completion of the dive (two lights, two computers, bailout, etc.).
RMV (Respiratory Minute Volume) / SAC rate (Surface Air Consumption)
The amount of gas consumed per minute, normalized to the surface, used for gas planning.
Rock Bottom
The minimum amount of gas needed for two divers to safely return to the surface from maximum depth in case of failure.
Rule of Thirds
Gas management method: 1/3 gas for the way in, 1/3 back, 1/3 reserve for emergencies.
S
S-drill
Standardized gas sharing drill where one diver simulates gas loss and takes the long hose from their buddy.
Safety reel
A reel with 30-50 m of line, used for ascent, wreck penetration, or as backup.
Scrubber (CO₂ absorbent)
The material in a rebreather (typically soda lime) that chemically removes carbon dioxide from exhaled gas.
Sidemount
A configuration where cylinders are mounted on the sides of the diver’s body, advantageous for tight spaces.
SMB (Surface Marker Buoy) / DSMB (Delayed SMB)
An inflatable buoy used to mark the diver’s position during ascent, enhances safety in boat traffic.
Stage bottle
An additional cylinder carried by the diver, containing decompression, travel, or bailout gas. Marked with tape.
T
Technical diving
Diving exceeding recreational limits – includes decompression, special gases, greater depths, and overhead environments.
Travel gas
A breathing mixture used during descent or ascent between the surface and working depth.
Trimix
A mixture of oxygen, nitrogen, and helium used to reduce narcosis and gas density in deep technical diving.
U
Underwater scooter (DPV – Diver Propulsion Vehicle)
A motorized device for underwater propulsion, enables longer distances with lower gas consumption.
V
Valve drill
Practice of operating valves on doubles during a dive, including closing and opening valves blindly.
Valve shutdown
A safety procedure in case of regulator failure – closing the appropriate valve on doubles to isolate the problem.
W
Wetnotes
A waterproof notepad used for dive planning and underwater communication.
Wing (BCD / Buoyancy Compensator)
An inflatable bladder for buoyancy control, used with technical diving harness (backplate and wing system).
Wreck diving
Technical diving on shipwrecks, requires special training for penetration and navigation in overhead environments.
Z
Zipper (Dry suit zipper)
A specialized zipper on a dry suit requiring regular maintenance and waxing.