ABSOLUTE PRESSURE: The total pressure measured from an absolute vacuum. It is the sum of gauge pressure and the prevailing atmospheric pressure measured on a barometer. The unit of pressure is the Pascal (Newton per square meter). Secondary units include dynes per square centimeter (metric), pounds per square inch (British), inches of water, millimeters of mercury.

BIDIRECTIONAL: A device designed for flow in both directions.

CONTROLLER: An instrument actuated by a control signal (pneumatic, electric, etc.) for controlling process variables.

DIFFERENTIAL: The difference between two values of a measured quantity, generally the difference between the highest and lowest values of the quantity. Also called the delta of the quantity.

EROSION: The wearing away of a valve seat due to high velocity flow.

FLASH POINT: The temperature at which a fluid first gives off enough vapor to ignite with a spark or flame.

HEAD: The height of a column of liquid above a specified point expressed in units such as feet of water, inches of mercury, etc. It is a measure of pressure exerted by the column of liquid.

LINEAR: A straight-line relationship between two variables.

NORMALLY CLOSED: A condition of no-flow through a valve or other system when there is no input signal.

NORMALLY OPEN: A valve or other device that allows fluid flow when there is no input signal. An input action must be applied to close the valve.

PACKING: A device used to seal a valve or other component. It consists of a deformable material or deformable mating element.

PROPORTIONAL: A relationship of one variable to another for which there is a constant ratio.

RESERVOIR: A container used to store the liquid in a fluid power system.

SET POINT: The target value, which an automatic control device attempts to reach or to hold.

THRUST: A force exerted on an object, which causes a linear movement.

VELOCITY: The rate of change of position with time. Common units are meters per second and feet per second.

The terms and definitions provided here were abstracted from a glossary compiled by DeZURIK Water Controls, a business unit of SARTELL Valves Inc. (www.sartellvalves.com). SARTELL provides valves and related equipment to the water and wastewater industries.