Absolute Zero — The lowest temperature attainable. All molecular activity is considered to cease. Its value is -459.67°F (-273.15°C).

Absorption — The penetration of matter in bulk into other matter, as in the dissolving of a gas in liquid.

Adsorption — The surface retention of solid, liquid, or gas molecules, atoms, or ions by a solid or liquid.

Boiling Point — The temperature of a liquid at which the vapor pressure equals the surrounding pressure (normally specified at 1 atm.).

British Thermal Unit — The quantity of heat required to raise the temperature of 1 pound of water 1°F.

Calorie — The quantity of heat required to raise the temperature of 1 gram of water 1°C.

Catalyst — A substance that initiates a chemical reaction and allows it to proceed under different conditions than otherwise possible.

Coefficient of Viscosity — A measure of the tendency of a fluid to resist shear. The unit for viscosity is the poise, which is defined as the resistance (in dynes per square centimeter of its surface) of one layer of fluid to the motion of a parallel layer one centimeter away and with a relative velocity of one centimeter per second.

Critical Pressure — The pressure under which a substance may exist as a gas in equilibrium with the liquid at critical temperature.

Critical Temperature — The temperature above which a gas cannot be liquefied by pressure alone.

Cryogenic Fluid — A gas that liquefies at very low temperatures (from -459.67°F absolute zero to approximately -200°F).

Density — The weight of a substance per unit of volume.

Dew Point — The temperature at which liquid first condenses.

Diameter Index Safety System (DISS) — DISS outlet valves are generally used with high-purity products, toxics, and corrosives. Valves equipped with DISS outlet assignment provide a metal-to-metal seal that creates low particles, a permeation-free environment, and good leak integrity.

Dielectric Constant — The specific inductive capacitance of a material. It is equal to the ratio of the capacitances of two condensers of identical size, one using the particular dielectric, the other using air or a vacuum as the dielectric.

Eductor Tube — An eductor tube or dip tube permits the withdrawal of liquefied compressed gases in the liquid phase with the cylinder in the normal upright position, without having to invert the cylinder. Air Products offers two types of eductor tubes: gooseneck tubes and full-length tubes.

Expansion Engine — An apparatus for extracting work from a gas by allowing the gas to expand between two pressure levels. As the energy is extracted, the gas temperature is lowered.

Expansion Valve — A valve through which a fluid may be expanded from one pressure to a lower pressure at a controlled rate.

Flammable Limits — The concentration of flammable vapor in air, oxygen, or other oxidants that will propagate flame upon contact when provided with a source of ignition. The lower explosive limit (LEL) is the concentration below which a flame will not propagate; the upper explosive limit (UEL) is the concentration above which a flame will not propagate. A change in temperature or pressure may vary the flammable limits.

Fractional Distillation — A process used to separate the constituents of a liquid mixture due to differences in boiling points of the constituents.

Joule-Thomson Effect — The change in temperature resulting from expansion of a gas or vapor through an orifice or other restriction.

Kinetic Theory of Gases — Gases are considered to be made up of minute, perfectly elastic particles which are moving at random with high velocities, colliding with each other and with the walls of the containing vessel. The pressure exerted by a gas is due to the combined effect of the impacts of the moving molecules upon the walls of the containing vessel. The magnitude of the pressure is dependent upon the kinetic energy of the molecules and their number.

Latent Heat of Fusion — The heat required to convert a unit mass of a substance from a solid to the liquid state at the same temperature and pressure.

Latent Heat of Sublimation — The heat required to convert a unit mass of substance from the solid state to the gaseous state.

Latent Heat of Vaporization — The heat required to convert a unit mass of substance from the liquid state to the gaseous state at a given pressure (and temperature).

Molecular Weight — The average mass of a molecule calculated as the sum of the atomic weights of the constituent atoms.

Normal Boiling Point — The temperature at which a liquid boils when under a total pressure of one atmosphere.

Normal Sublimation Temperature — The temperature at which a solid sublimes under a total pressure of one atmosphere.

Pyrophoric Gas — A gas that can spontaneously self-ignite when exposed to normal atmospheric conditions.

Restrictive Flow Orifice (RFO) — A safety device placed in the outlet of a cylinder valve that is intended to limit the release rate of a hazardous gas to a maximum specified range in the event of the inadvertent opening of the valve, or failure of the system downstream of the valve outlet.

Specific Gravity — The ratio of weight of a substance at a given temperature to the weight of the same volume of another substance at a given temperature. For liquids, the comparison is to water; for gases, the comparison is to air.

Specific Heat — The ratio of the heat capacity of a body to the heat capacity of water at some reference temperature.

Specific Heat Ratio — Ratio of specific heat at constant pressure to the specific heat at constant volume at a particular temperature.

Specific Volume — The volume occupied by one unit mass of substance.

Superconductivity — The phenomenon by which some substances suddenly lose all electrical resistance when their temperature is reduced. These transitions occur at temperatures lower than that of liquid hydrogen.

Thermal Conductivity — The property of a material which describes the rate at which heat will be conducted through a unit area of material for a given driving force. It is dependent on the material and upon its temperature.

Triple Point — The temperature and pressure at which the solid, liquid, and vapor phases of a substance are in equilibrium with one another.

Vapor Pressure — The pressure exerted when a solid or liquid is in equilibrium with its own vapor. Vapor pressure is a function of the substance and the temperature.