Vacuum separator -- a separator that utilizes subatmospheric pressure to remove certain gases and liquids from another liquid because of their difference in vapor pressure.

Valve -- a device which controls fluid flow direction, pressure, or flow rate.

Valve, by-pass -- a valve whose primary function is to provide an alternate flow path.

Valve, directional control -- a valve whose primary function is to direct or prevent flow through selected passages.

Valve, directional control, servo -- a directional control valve which modulates flow or pressure as a function of its input signal.

Valve, flow control -- a valve whose primary function is to control flow rate.

Valve lifter -- sometimes called a "cam follower," a component in engine designs that use a linkage system between a cam and the valve it operates. The lifter typically translates the rotational motion of the cam to a reciprocating linear motion in the linkage system.

Valve, pressure control, relief -- a pressure control valve whose primary function is to limit system pressure.

Valve, relief, differential pressure -- a valve whose primary function is to limit differential pressure.

Vapor pressure -- pressure of a confined vapor in equilibrium with its liquid at specified temperature thus, a measure of a liquid's volatility.

Vapor Pressure (Reid method or RVP) -- measure of the pressure of vapor accumulated above a sample of gasoline or other volatile fuel in a standard bomb at 100°F (37.8°C). Used to predict the vapor locking tendencies of the fuel in a vehicle's fuel system. Controlled by law in some areas to limit air pollution from hydrocarbon evaporation while dispensing.

Varnish -- a thin film deposit resulting from the oxidation and polymerization of fuels and lubricants. Similar to but softer than lacquer, and not removable by mechanical wiping.

Viscometer or Viscosimeter -- an apparatus for determining the viscosity of a fluid.

Viscosity -- measurement of a fluid's resistance to flow. The common metric unit of absolute viscosity is the poise, which is defined as the force in dynes required to move a surface one square centimeter in area past a parallel surface at a speed of one centimeter per second, with the surfaces separated by a fluid film one centimeter thick. In addition to kinematic viscosity, there are other methods for determining viscosity, including Saybolt Universal Viscosity (SUV), Saybolt Furol viscosity, Engier viscosity, and Redwood viscosity. Since viscosity varies in inversely with temperature, its value is meaningless until the temperature at which it is determined is reported.

Viscosity, absolute -- the ration of the shearing stress to the shear rate of a fluid. It is usually expressed in centipoise.

Viscosity, kinematic -- the absolute viscosity divided by the density of the fluid. It is usually expressed in centistokes.

Viscosity, SUS -- Saybolt Universal Seconds (SUS), which is the time in seconds for 60 milliliters of oil to flow through a standard orifice at a given temperature. This viscosity system is still in limited use, although it has been generally replaced by the ISO grading system.

Viscosity grade -- any of a number of systems which characterize lubricants according to viscosity for particular applications, such as industrial oils, gear oils, automotive engine oils, automotive gear oils, and aircraft piston engine oils. ISO and SAE are the two systems most often encountered in oil analysis applications.

Viscosity index (VI) -- empirical unitless number indicating the effect of temperature change on the kinematic viscosity of an oil. The higher the viscosity index, the smaller the relative change in viscosity with temperature.

Viscous -- possessing viscosity. Frequently used to imply high viscosity.

Volatility -- expression of evaporation tendency; the degree and rate at which a liquid vaporizes under set parameters of temperature and pressure. The more volatile a petroleum liquid, the lower its boiling point and the greater its flammability. Changes in liquid stability may result in reduced volatility.