Scuba Glossary: H - I - J


Haldanian  –  Related to Haldane’s theory that nitrogen is absorbed up and released in an exponential manner during a dive, and that there is some safe ratio of pressure change for ascent.

Half Time  –  Half the time it takes for a dissolved gas in a tissue (such as nitrogen) to equilibrate to a new pressure, or to reach full saturation at a new pressure. Theoretical tissue half times are used in designing dive tables and algorithms for dive computers.

Halocline  –  A Halocline is a change in the salt content of the water, but is not often apparent to divers unless severe enough to cause buoyancy changes.

Hand Signals  –  Method that divers use to communicate underwater.

Hang Tank  –  An extra tank, ideally of deco mix, staged at the decompression stop. Normally found at the rear of a dive boat in case of emergencies.

Head  –  Restroom on a boat.

Heliox  –  Mixture of helium and oxygen, usually reserved for very deep diving.

Helium  –  Second lightest gas; does not cause problems of narcosis to the same extent as seen with nitrogen, and is therefore used for very deep diving.

Henry’s Law  –  The amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas in contact with the liquid and the solubility coeffient of the gas in the liquid.

High Altitude Diving  –  Is done in mountain lakes or other high altitude waters at or greater than 1,000 feet (300 meters) above sea level with increased risk of decompression sickness because of lower-than-sea-level atmospheric pressure at the surface; regular dive tables and some dive computers and depth gauges are inaccurate above sea level; special high altitude dive tables and recalibration of gauges and computer are required; specialty courses are available due to the complexity and added hazards of this activity.

High pressure nervous syndrome  –  Abbr. HPNS; 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.

Hogarthian Principles  –  The Hogarthian configuration is named after Bill ‘Hogarth’ Main. It is based on reducing equipment to a minimum streamlined configuration that nevertheless includes sufficient redundancy for extended decompression dives.

Hoods  –  Garment worn over the head to reduce thermal loss.

Hookah  –  A surface-supplied compressed air apparatus, for use in shallow diving in calm waters. The air is delivered to one or more divers through a long hose.

Hydro  –  The common term for the hydrostatic test required on scuba cylinders every five years to determine whether the tank walls are still strong enough for safe usage.

Hydrogen  –  An inert gas, and lightest of all the elements, has been used in experimental diving situations.

Hydrostatic Test  –  Pressure test in which the tank is filled with water instead of air and raised to five thirds the maximum working pressure, causing the water to expand and be displaced.

Hyperbaric Chamber  –  Air-tight chamber that can simulate the ambient pressure at altitude or at depth, is used for treating decompression illness.

Hypercapnia  –  A higher than normal P02level in the blood.

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.

Hyperthermia  –  A body temperature warmer than normal, less common in diving than Hypothermia, but can occur from overheating in a wet suit.

Hyperventilation  –  Over breathing to the extent that the blood carbon dioxide level is lowered, may lead to tingling in fingers and dizziness.

Hypothermia  –  A subnormal chilling of the body.

Hypoventilation  –  Under breathing to the extent that the blood carbon dioxide level is elevated, may be manifested by carbon dioxide narcosis.

Hypoxia  –  Lower than normal PO2 level in the blood, insufficient oxygen in the blood.

Hyprthermia  –  A body temperature colder than normal (98.6F), severe problems start to manifest when body temperature reaches about 95’F.


 IANTD  –  International Association of Nitrox and Technical Divers. IANTD is the only EANx agency that offers training in all aspects of EANx through continuing education programs. This allows you to expand your knowledge and training with the top professionals in the field. IANTD instructors are well trained, highly experienced and extremely qualified. IANTD Standards and Procedures provide for the highest level of EANx education and training available. IANTD (IAND, Inc.) was founded by Dick Rutkowski, the former dive supervisor for the National Oceanic and Atmospheric Administration (NOAA). Mr. Rutkowski introduced the recreational diving community to the technology of EANx. This program was developed through NOAA during his tenure. Today the board of directors and the membership of IANTD are composed of many of the most experienced divers in the world pertaining to the use of breathing gases other than air.

Immersion Hypothermia  –  The lowering of body temperature when a diver is fully immersed in cold water without proper thermal protection.

Inflator Valve  –  A manually operated valve that puts compressed air into the buoyancy bladders of a BCD or into a dry suit.

Invertebrate  –  An animal without a backbone, (vertebrate)


 Jellyfish  –  Jellyfish are members of the cnidarians group of invertebrates. They are transparent and bell shaped with tentacles hanging down from their body. They are free-floating and seem serene, but in their tentacles there are stinging mechanisms that are used to catch prey and for defense.

Jetty –  A structure, usually made with rocks, extended into a sea, lake or river to influence the current or tide in order to protect a harbor.

Jon Line  –  A line with a hook device that is designed to attache to any thickness anchor or down line that allows the diver to stay in the water column regardless of wave action during decompression stop.

J-Valve  –  Contains a spring loaded valve that shuts off a divers air supply at approx. 300psi.

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