Glossary of Terms - V
A B C D E F G H I J K L M N O P Q R S T U V W Z
vacuum breaker - A mechanical device which automatically vents a water line to the atmosphere when subjected to a partial vacuum, thus preventing back-flow. (See back-flow, air gap, back-flow preventer.)
valence - A whole number (positive or negative) representing the power of one element to combine with another. In general terms, the valence number represents the number of electrons in an atom or combined group of atoms which can be easily given up or accepted to react with or bond to another atom or group of atoms to form a molecule. Example: Two hydrogen ions (each with a valence of +1) must be present for each ion of oxygen (-2) to form a molecule of water (H2O).
venturi - A tube with a narrow throat (a constriction) that increases the velocity and decreases the pressure of the liquid passing through it, creating a partial vacuum immediately after the constriction in the tube. The vacuum created has a sucking effect (eduction), and a venturi is commonly used to introduce a liquid (such as a regenerant) or gas (such as air) into a flowing water stream.
viable - Alive and capable of continued life
virus - A parasitic infectious microbe, composed almost entirely of protein and nucleic acids, which can cause disease(s) in humans. Viruses can reproduce only within living cells. They are 0.004 to 0.1 microns in size, and about 100 times smaller than bacteria.
viscosity - The tendency of a fluid to resist flowing due to internal forces such as the attraction of the molecules for each other (cohesion) or the friction of the molecules during flow. Viscosity varies with water temperature.
VOCs - Volatile organic chemicals.
void volume - The volume occupied by the interstitial spaces between the particles of ion exchangers, filter media, or other granular materials in a bed or column. Often expressed as percent of the total volume occupied by the medium bed.
volatile - Capable of becoming vapor at a relatively low temperatures.
volatile liquids - Liquids which easily vaporize or evaporate at room temperatures.
volatile organic chemicals (VOCs) - Organic chemicals that turn into vapor at relatively low temperatures.
volatile solids - The term used in the laboratory analysis of the solids content of a substance (such as water) to define the portion of the total suspended and/or dissolved solids that become expulsed or driven off after heating or burning a given sample of the substance at a specified temperature and for a specified time.
volumetric - Referring to measurement by volume rather than weight. (See gravimetric.)