The same effect can result from a large and rapid increase in depth if the air supply is insufficient to keep up with the increase in ambient pressure. On a helmet with a neck dam, the neck dam will allow water to flood the helmet before serious barotrauma can occur. This can happen with helium reclaim helmets if the reclaim regulator system fails, so there is a manual bypass valve, which allows the helmet to be purged so breathing can continue on open circuit. Pulmonary barotrauma
Lung over-pressure injury in ambient pressure divers using underwater breathing apparatus is usually caused by breath-holding on ascent. The compressed gas in the lungs expands as the ambient pressure decreases causing the lungs to over-expand and rupture unless the diver allows the gas to escape by maintaining an open airway, as in normal breathing. The lungs do not sense pain when over-expanded giving the diver little warning to prevent the injury. This does not affect breath-hold divers as they bring a lungful of air with them from the surface, which merely re-expands safely to near its original volume on ascent. The problem only arises if a breath of ambient pressure gas is taken at depth, which may then expand on ascent to more than the lung volume. Pulmonary barotrauma may also be caused by explosive decompression of a pressurised aircraft, as occurred on 1 February 2003 to the crew in the Space Shuttle Columbia disaster.
A craft or trade is a pastime or an occupation that requires particular skills and knowledge of skilled work. In a historical sense, particularly the Middle Ages and earlier, the term is usually applied to people occupied in small scale production of goods, or their maintenance, for example by tinkers. The traditional term craftsman is nowadays often replaced by artisan and by craftsperson (craftspeople). Historically, the more specialized crafts with high-value products tended to concentrate in urban centers and formed guilds. The skill required by their professions and the need to be permanently involved in the exchange of goods often demanded a generally higher level of education, and craftsmen were usually in a more privileged position than the peasantry in societal hierarchy. The households of craftsmen were not as self-sufficient as those of people engaged in agricultural work, and therefore had to rely on the exchange of goods. Some crafts, especially in areas such as pottery, woodworking, and various stages of textile production, could be practiced on a part-time basis by those also working in agriculture, and often formed part of village life. When an apprentice finished his apprenticeship, he became a journeyman searching for a place to set up his own shop and make a living. After he set up his own shop, he could then call himself a master of his craft. This stepwise approach to mastery of a craft, which includes the attainment of some education and skill, has survived in some countries until today.