Scuba & Nitrox Glossary: U

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 — U

Unit Pulmonary Toxic Dose (UPTD)
A historical metric related to cumulative pulmonary oxygen exposure; often discussed alongside OTUs.
Unanalyzed Gas
Gas whose composition has not been personally verified; it must not be assumed to match its label or intended use.
Usable Oxygen Range
The oxygen-percentage range within which a particular analyzer is specified to operate accurately.
Unverified Breathing Gas
A breathing gas whose composition has not been personally verified and should not be assumed.
Uncompensated Analyzer
An analyzer whose reading may require correction for pressure, humidity, or conditions.
Unit Oxygen Dose
A measure used in some oxygen-exposure tracking methods.
Unknown Residual Gas
Cylinder contents that must be analyzed or removed before safe blending.
User-Selected PPO2
The oxygen partial-pressure limit chosen within training and operational guidance.

General Scuba Terms — U

Underwater Navigation
Maintaining awareness of direction and position using compass, natural features, time, and distance.
Up-Line
A line deployed from depth to provide an ascent and decompression reference.
Upwelling
The upward movement of deeper water, often colder and nutrient-rich.
Umbilical
A surface-supplied bundle carrying breathing gas, communications, strength member, and sometimes other services.
Surface-Supplied Umbilical
A bundle supplying breathing gas, communications, and other services to a surface-supplied diver.
Underwater Slate
A waterproof surface used for notes, plans, or communication.
Undertow
Subsurface return flow associated with breaking waves near shore.
Deployed Up-Line
A line sent upward to support ascent or mark a diver's location.
Utility Pocket
A suit or BCD pocket used to carry accessories and safety equipment.