A liquefied compressed gas can be defined as a gas, when compressed in a cylinder (container), that becomes liquid at ordinary temperatures. Liquefied gases have boiling points that range from –130°F to 30°F (–90°C to –1°C). At 70°F (21°C), the cylinder contains liquid and gas. Cylinder pressure, or the “vapor pressure’’ of the gas, is directly affected by ambient temperature. Increases or decreases in the temperature will cause the vapor pressure to increase or decrease, respectively.

For example, at 70°F (21°C), a cylinder of hydrogen chloride has a vapor pressure of 613 psig. At 30°F (11.1°C), that same cylinder would have a pressure of 335 psig. Since both cylinders would contain the same amount of product (by weight), the amount of product in a cylinder cannot be determined by a pressure reading. With liquefied compressed gases, cylinder content can only be determined by product weight.