Sailors check<unk>  devices at sea. A<unk>  is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common, and only natural,<unk> . But other mixtures of gases, or pure oxygen, are also used in<unk>  equipment and enclosed habitats such as scuba equipment, surface supplied diving equipment, recompression chambers, high-altitude mountaineering, high-flying aircraft, submarines, space suits, spacecraft, medical life support and first aid equipment, and anaesthetic machines. Oxygen is the essential component for any<unk> , at a partial pressure of between roughly 0.16 and 1.60 bar at the ambient pressure. The oxygen is usually the only metabolically active component unless the gas is an anaesthetic mixture. Some of the oxygen in the<unk>  is consumed by the metabolic processes, and the inert components are unchanged, and serve mainly to dilute the oxygen to an appropriate concentration, and are therefore also known as diluent gases. Most<unk> es therefore are a mixture of oxygen and one or more metabolically inert gases.<unk> es for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression stops, reducing nitrogen narcosis or allowing safer deep diving. A safe<unk>  for hyperbaric use has four essential features: * It must contain sufficient oxygen to support life, consciousness and work rate of the breather. * It must not contain harmful contaminants. Carbon monoxide and carbon dioxide are common poisons which may contaminate<unk> es. There are many other possibilities. * It must not become toxic when being breathed at high pressure such as when underwater. Oxygen and nitrogen are examples of gases that become toxic under pressure. * It must not be too dense to breathe. Work of<unk>  increases with density and viscosity. Maximum ventilation drops by about 50% when density is equivalent to air at 30 msw, and carbon dioxide levels rise unacceptably for moderate exercise with a gas density exceeding 6 g/litre.<unk>  density of 10 g/litre or more may cause runaway hypercapnia even at very low work levels, with potentially fatal effects. The techniques used to fill diving cylinders with gases other than air are called gas blending.<unk> es for use at ambient pressures below normal atmospheric pressure are usually pure oxygen or air enriched with oxygen to provide sufficient oxygen to maintain life and consciousness, or to allow higher levels of exertion than would be possible using air. It is common to provide the additional oxygen as a pure gas added to the<unk>  air at inhalation, or though a life-support system.
Breathing gas
Blood gas tension
Oxygen saturation
Oxygen saturation 
Hemodynamics
Respiratory sounds
Respiratory rate
Bottled gas