When a balloon (not Mylar) is blown up and accidentally released into a room, we do not know where it will go. It will "fly" until all the internal pressure is gone. A balloon has approximately 1 psi (pound per square inch) of internal pressure. Compressed gas cylinders have internal pressures that can range from less than 100 psi to 6000 psi. Accidentally venting this pressure can also make a compressed gas cylinder "fly." And when this happens, people, equipment, buildings - anything and everything in its path can be destroyed. Should the gas be an oxidizer, toxic, corrosive, flammable, or poisonous, the catastrophic effects can be even worse.
Basically, two manufacturing methods are used to make cylinders that hold compressed gases or liquefied compressed gases, depending upon whether the cylinder is designed to hold a "high-pressure" or "low-pressure" product.
With a few exceptions, "low-pressure" cylinders are designed to hold compressed gases or liquefied compressed gases that have working pressures of not more than 250 psi at room temperature. "High-pressure" cylinders are designed to contain internal pressures higher than this.
These cylinders have seams, and are usually manufactured by forming the material (the most common ones are discussed below) cylindrically and welding to create a sealed sidewall. Cylinders made to stand upright usually have a ring stand welded to the base of the cylinder and a protective ring welded to the shoulder of the cylinder. This safeguards the valve should the cylinder fall. Often the top ring also has handgrips to ease movement.
The marking and permanent information, such as serial number and date of manufacture, are usually stamped on the shoulder base ring rather than on the cylinder itself.
Having welded or pressed seams in containers that hold higher pressures is not considered safe. This process does not offer the consistency needed engineering safety factor that must be assured for containing these products. Instead, high-pressure cylinders are made seamlessly. An extrusion or molding processes are usually used instead. In this way, the molten steel or aluminum (most common materials of construction) is formed so that the sides are uniform in thickness, the base is thicker and flat, and the shoulder is thicker than the sides. The shoulders of the cylinder are made thicker because the mandatory markings and information are stamped into the metal here. The process assures minimum thickness for the particular material being used so that, once again, the consistency needed engineering safety factor is assured for that particular cylinder size and pressure rating.
Because these are uniformly smooth on the surface, high-pressure cylinders have gently rounded shoulders while low-pressure cylinders have shoulders that are flatter or more pronounced. With very little time practicing, with a quick glance someone can tell whether a cylinder is manufactured for high or low-pressure materials.