The amount of heating power required to overcome the latent heat of fusion of one short ton of ice, or to melt one short ton of solid ice at 0C to liquid water at 0C, in 24 hours. Equal to 12,000 BTU/Hr.

A torr is a unit of pressure equal to 1/760 of a standard atmosphere, or approximately 1 mm Hg. It is typically used to define vacuum pressure, or pressure below atmospheric. When used to describe a vacuum, the units indicate the amount of remaining pressure. The lower the number, the stronger the vacuum. Rough vacuum pressures can be measured in millitorr, or 1/1000 Torr.

Variable air volume (VAV) is a type of HVAC system that uses variable airflow at constant temperature to maintain temperature in a space. A variable air volume system typically affords more precise control of temperature when compared to CAV systems.

A millimeter of mercury is a unit of pressure equal to that which is generated by a column of mercury one millimeter high. One Standard Atmosphere is about 760 mmHG.

Unit of pressure that specifically denote the use of a perfect vacuum as the reference point. A pressure vessel at 0 PSIA is holding a perfect vacuum. Absolute pressure is typically measured in different units, such as mmHg, but it’s important to check the reference point.

Unit of pressure that specifically denote the use of Gauge Pressure, or pressure using atmospheric pressure as a reference point. A pressure vessel at 0 PSIG actually contains fluid at atmospheric pressure, or about 14.7 PSIA. The “G” is frequently dropped when context is enough to indicate the basis point. For example, a compressed air pressure measurment of 10 PSI can be safely assumed to be PSIG, as 10 PSIA would actually be a coarse vacuum relative to ambient pressure.