diameter than tubing and can allow greater oxygen flow. When a hose is used it may have a ribbed or corrugated design to allow bending of the hose while preventing twisting and cutting off the oxygen flow. The quantity of oxygen delivered from the storage tank to the oxygen mask is controlled by a valve called a regulator. Some types of oxygen masks have a breathing bag made of plastic or rubber attached to the mask or oxygen supply hose to store a supply of oxygen to allow deep breathing without waste of oxygen with use of simple fixed flow regulators. Oxygen masks for aviators History An early 1919 high-altitude oxygen system used a vacuum flask of liquid oxygen to supply two people for one hour at . The liquid passed through several warming stages before use, as expansion when it evaporated, and absorbed latent heat of vaporization, would make the gasified oxygen so cold that it could cause instant frostbite of the lungs. The first successful creation for the oxygen mask was by Armenian born Dr. Arthur Bulbulian, in the field of facial prosthetics, in 1941. Many designs of aviator's oxygen masks contain a microphone to transmit speech to other crew members and to the aircraft's radio. Military aviators' oxygen masks have face pieces that partially cover the sides of the face and protect the face against flash burns, flying particles, and effects of a high speed air stream hitting the face during emergency evacuation from the aircraft by ejection seat or parachute. They are often part of a pressure suit or intended for use with a flight helmet. Regulations Three main kinds of oxygen masks are used by pilots and crews who fly at high altitudes: continuous flow, diluter demand, and pressure demand. In a continuous-flow system, oxygen is provided to the user continuously. It does not matter if the user is exhaling or inhaling as oxygen is flowing from the time the system is activated. Below the oxygen mask is a rebreather bag that collects oxygen during exhalation and as a result allows a higher flow rate during the inhalation cycle. Diluter-demand and pressure-demand masks supply oxygen only when the user inhales. They each require a good seal between the mask and the user's face. In a diluter-demand system, as the altitude increases (ambient pressure, and therefore the partial pressure of ambient oxygen, decreases), the oxygen flow