Scuba & Nitrox Glossary: H
Nitrox and breathing-gas terminology is prioritized first. General scuba terms follow below. Definitions are educational summaries and do not replace formal training, manufacturer instructions, or an operational dive plan.
Nitrox & Gas Terms — H
- Heliair
- A trimix created by adding helium to air, producing oxygen and nitrogen fractions related to the remaining air.
- Heliox
- A breathing-gas mixture of helium and oxygen with little or no nitrogen.
- Hypoxic Mix
- A mixture with too little oxygen to support consciousness at the surface; specialized training and travel gas are required.
- Hyperoxic Mix
- A gas with a higher oxygen fraction than air; the term depends on context.
- Heliox Mixture
- A breathing mixture of helium and oxygen with no intentional nitrogen component.
- High-Oxygen Deco Gas
- A richer oxygen mixture selected for accelerated decompression within its MOD.
- Hyperoxic Loop
- A rebreather loop containing more oxygen than intended for the current depth.
- Hypoxic Diluent
- A diluent not breathable at the surface and requiring advanced procedures.
- Hypoxic Trimix
- A trimix with insufficient oxygen for safe surface breathing.
General Scuba Terms — H
- Half-Time
- A model value describing how quickly a theoretical tissue compartment approaches gas equilibrium.
- Hang Tank
- A cylinder suspended at a decompression or safety-stop depth for emergency or planned use.
- Helium
- A low-density inert gas used in technical and commercial diving mixtures.
- Henry’s Law
- At constant temperature, the amount of gas dissolved in a liquid is proportional to its partial pressure.
- Hood
- Thermal head protection, usually neoprene.
- Hyperbaric Chamber
- A pressure vessel used for medical treatment, training, or research.
- Hypothermia
- A dangerous reduction in core body temperature.
- Half Time
- The time required for a modeled tissue compartment to move halfway toward equilibrium.
- Hand Signal
- A standardized visual gesture used for underwater communication.
- Hang-Tank Supply
- An emergency breathing-gas cylinder suspended at a planned stop depth.
- Hood Squeeze
- Discomfort or injury caused by trapped gas failing to equalize in a hood.
- Hydrostatic Pressure
- Pressure exerted by the water column at a given depth.