Concept
A pressure suit is a protective suit worn by high-altitude pilots who may fly at altitudes where the air pressure is too low for an unprotected person to survive, even breathing pure oxygen at positive pressure. Such suits may be either full-pressure (e.g., a space suit) or partial-pressure (as used by aircrew). Partial-pressure suits work by providing mechanical counter-pressure to assist breathing at altitude. The region from sea level to around is known as the physiological-efficient zone. Oxygen levels are usually high enough for humans to function without supplemental oxygen and decompression sickness is rare. The physiological-deficient zone extends from to about . There is an increased risk of problems such as hypoxia, trapped-gas dysbarism (where gas trapped in the body expands), and evolved-gas dysbarism (where dissolved gases such as nitrogen may form in the tissues, i.e. decompression sickness). Above approximately oxygen-rich breathing mixture is required to approximate the oxygen available in the lower atmosphere, while above oxygen must be under positive pressure. Above , respiration is not possible because the pressure at which the lungs excrete carbon dioxide (approximately 87 mmHg) exceeds outside air pressure. Above , also known as the Armstrong limit, fluids in the throat and lungs will boil away. Generally, 100% oxygen is used to maintain an equivalent altitude of . Generally, pressure suits work by either indirectly compressing the human body, or directly compressing it. Indirect compression is typically done by enclosing the body in a gas envelope. For this type, design effort focuses on compressing and containing the gas, at an equal pressure around the body as the wearer moves, and not having the gas pressure or the enclosing suit envelope restricting body movement of the wearer. Maintaining constant gas pressure as the wearer moves is difficult, because the internal volume of a simple construction inflatable suit will change when body joints are flexed.
