Scuba Diving Glossary

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

HALF TIME: The time it takes at a new partial pressure for the concentration of a dissolved gas, such as nitrogen, to either increase or decrease by 50%. The term is commonly used in many biological, chemical, and physical events, such as decompression theory. Theoretical tissue half times are used in designing dive tables and algorithms for dive computers.

HAWAIIAN SLING: A free shaft spear that is launched with a rubber sling that is bound to a wooden handle with a hole drilled in the center. In the hands of an experienced diver, it is a very effective way to collect dinner.

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: At a constant temperature, the amount of any given gas that will dissolve in a liquid is directly proportional to the partial pressure of that gas.

HOOD: A garment worn over the head to reduce thermal loss.

HYDROGEN: An inert gas, and lightest of all the elements, that has been used in experimental diving situations.

HYDROSTATIC TEST: A 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: An 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 PO2 level in the blood.

HYPERTHERMIA: A body temperature warmer than normal, less common in diving than hypothermia, but can occur from overheating in a wetsuit or drysuit.

HYPERVENTILATION: Over breathing to the extent that the blood carbon dioxide level is lowered; may lead to tingling in the fingers and dizziness.

HYPOTHERMIA: A body temperature colder than normal (98.6 F). Severe problems start to manifest when body temperature reaches about 95 F.

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.