Oxygen (atomic number, 8; atomic weight, 16) is essential for all living things and has the ability to combine with almost all other elements. When elements fuse with oxygen, they are labeled as being oxidized. Oxygen is the most plentiful element in the world, comprising about 90% of water (hydrogen makes up the other 10%) and 46% of the earth's crust (silicon, 28%; aluminum, 8%; and iron, 5%; among others). Oxygen's melting point is −360°F (−218°C) and its boiling point is −297°F (−183°C). In its free state, oxygen is odorless, colorless, and tasteless. At temperatures below −297°F (183°C) oxygen takes on a pale blue liquid form.
Two-thirds of the human body is composed of oxygen. In humans oxygen is taken in through the lungs and distributed via the blood stream to cells. In the cells, oxygen combines with other chemicals, making them oxidized. The oxidized cells are then distributed where they are needed, providing the body with energy. The waste products of respiration are water and carbon dioxide, which are removed through the lungs.
Pressurized oxygen therapy is used to treat numerous medical aliments such as emphysema, asthma, and pneumonia. This medicinal form of oxygen is typically kept in medium-sized aluminum canisters equipped with pressure regulators and release valves. Large amounts of oxygen are kept in large, insulated steel tanks pressurized at 2,000 lb/in 2 (141 kg/cm 2 ).
The discovery of oxygen has generally been attributed to Joseph Priestley, an English chemist. In 1767, Priestly believed that air mixed with carbon was able to produce electricity. He called this carbonized air, mephitic air. Priestly went on to conduct experiments concerning air, and in 1774 he used a burning glass and solar heat to heat mercuric oxide. While doing this, he noticed that the mercuric oxide broke down under the extreme temperature and formed beads of elemental mercury. The mercuric oxide also emitted a strange gas that facilitated flames and opened the respiratory tract, making it easier to breath when inhaled. This gas was named dephlogisticated air by Priestley, based on the popular thought of the time that phlogiston was needed for material to burn. The phlogiston theory was deemed false by Antoine-Laurent Lavoisier, a French chemist.
Lavoisier had been conducting his own experiments with combustion and air in the mid- to late-eighteenth century. It was in 1774, that he met Priestley who told Lavoisier of the discovery of dephlogisticated air. Lavoisier began to conduct his own experiments on Priestley's pure form of air. He observed that the element was part of several acids and made the assumption that it was needed to form all acids. Based on this incorrect thought, Lavoisier used the Greek words oxy (acid) and gene (forming) to coin the French word oxygene—translated to oxygen in English—sometime around 1779.
There is yet a third man who is credited for his involvement in the discovery of oxygen in about 1771. Carl Wilhelm Scheele, a Swedish pharmacist and chemist, discovered that a certain element (Scheele also thought it to be phlogiston) was needed in order for substances to burn. Scheele called this element "fire air" due to it being needed for combustion. During these experiments with fire air, Scheele also discovered "foul air," now known as nitrogen. Despite the fact that Scheele had isolated oxygen before Priestley, Priestley published his findings first.
The raw materials to produce an oxygen tank are liquid air and aluminum. The aluminum starting stock is cast 6061. The liquid air is condensed and heated until pure oxygen remains then distributed into the aluminum tanks. A compressible Teflon ring is used to form the o-ring, which is placed in the o-gland forming a seal between the valve and the cylinder. The o-ring gland is a precision depression machined in the top of the cylinder. When the valve is screwed in the cylinder and when completely seated, it compresses the o-ring and completes the airtight seal between the valve and the cylinder.
Oxygen tanks vary in size, weight, and function but the manufacturing process is very similar. The typical medicinal oxygen tank contains pure oxygen and has a green top with a brushed steel body.
During the manufacturing process, the cylinders are inspected and cleaned numerous times. After the tank is sold and put into service, it must be put through hydrostatic and visual retesting every five years. The testing is conducted in accordance to the Compressed Gas Associations requirements. If the tank is not damaged and wear is minimal, there is unlimited service life.
DOT-3AL is the marking identifying the specification in which the cylinder was manufactured in compliance. The Department of Transportation (DOT) regulates the transportation of all goods. The transportation of compressed gases falls into this category.
In the manufacturing process nearly 93% of the starting material (the cast billet) is used in the final product. There is less than 7% manufacturing scrap of the starting material. After production is complete, any cylinders that are damaged to the point of being condemned are stamped through the "DOT-3AL" marking on the crown. If the tank has been pressurized, it is depressurized, the valve is removed, and the cylinder is sawn in half and recycled. The condemned, sawn cylinders can and should be recycled.
As the medical use of oxygen tanks increases, the tanks are getting smaller and more maneuverable. The standard medical E tank holds 680 l and can provide up to 11.3 hours at 1 liter per minute (lpm). This tank weighs 7.9 lb (3.6 kg) empty. One of the smaller oxygen tanks is an M9 tank. This tank holds 240 l of oxygen lasting for four hours at 1 lpm or two hours of continuous flow. There are accessories such as carts or bags that allow the user to transport the full tank easily.
Catalina Cylinders Web Page. 8 November 2001. < http://www.catalinacylinders.com >.
Tri-Med, Inc. Web Page. 8 November 2001.< http://www.trimed.freeservers.com >.
Deirdre S. Blanchfield