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, 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, 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.

Valve
A device which controls fluid flow direction, pressure, or 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.

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 (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
When applied to lubrication, a thin, insoluble, nonwipeable film deposit occurring on interior parts, resulting from the oxidation and polymerization of fuels and lubricants. Can cause sticking and malfunction of close-clearance moving parts. Similar to, but softer, than lacquer.

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. 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. (ASTM Designation D88-56.)

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.

Viscosity index (VI)
A commonly used measure of a fluid’s change of viscosity with temperature. The higher the viscosity index, the smaller the relative change in viscosity with temperature.

Viscosity index improvers
Additives that increase the viscosity of the fluid throughout its useful temperature range. Such additives are polymers that possess thickening power as a result of their high molecular weight and are necessary for formulation of multi-grade engine oils.

Viscosity modifier
Lubricant additive, usually a high molecular weight polymer, that reduces the tendency of an oil’s viscosity to change with temperature.

Viscous
Possessing viscosity. Frequently used to imply high viscosity.

Volatility
This property describes the degree and rate at which a liquid will vaporize under given conditions of temperature and pressure. When liquid stability changes, this property is often reduced in value.


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