Gage pressures:

we frequent deal in gage pressures, or absolute pressure is equivalent to atmospheric pressure (14.7 psi at sea level) plus the pressure which would be read on an ordinary gage, we could simplify the above definition. Accordingly, a compressed gas is one which provides a pressure reading of:

1. Either 25 psig (pounds per square inch gage) at 70ºF; or 125 psig at 130ºF; or

2. If the holds substance is flammable, 25 psig at 100ºF.

This simplified definition and the range of boiling points between gases categorized gases into two major groups which differ in physical state while contained.

1. Non-liquified gases that are gases which do not liquify in containers at ambient temperatures and under pressures attained within commercially used containers which range to 2000 to 2500 psig.

2. Liquified gases, that are gases which do become liquids to a extremely large extent in containers at ordinary ambient temperatures and at pressures from 25 to 2500 psig.

The first group, generally known as non-liquified gases, has associatively low boiling points, around -150ºF or lower.

Non-liquified gases do, therefore, become liquids if cooled to temperatures below their boiling points. Those which liquify at "cryogenic" temperatures (from absolute zero [-459.7ºF] to around -240ºF) are called as cryogenic fluids.

The second group, or liquified gases, has boiling points associative near atmospheric temperatures (from about -130ºF to 30ºF). A liquified gas solidifies at cryogenic temperatures.

Oxygen, helium and nitrogen are instances of gases in huge use both as non-liquified gases and cryogenic fluids.   Along With respective boiling points of -297ºF, -425ºF, and -320ºF, they are charged within high pressure steel cylinders at more than 2000 psig at 70 F for shipment and use as non-liquified gases.  Therefore, while shipped as cryogenic fluids, that are cooled down to liquid form and charged within special insulated containers which keep them below their boiling points and are hold at pressures generally less than 75 psig.

A third physical state in the container is represented through only one hugely used gas, acetylene. The Acetylene is many times referred to as a dissolved gas.  A dissolved gas is described as a gas which is dissolved within a solution.