Unloading

The release of contaminant that was initially captured by the filter medium.

Vacuum separator

A separator that utilizes sub atmospheric pressure to remove certain gases and liquids from another liquid because of their difference in vapour 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, and 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, and relief

A pressure control valve whose primary function is to limit system pressure.

Valve, relief, and 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.

Vapour pressure

Pressure of a confined vapour in equilibrium with its liquid at specified temperature thus, a measure of a liquid's volatility.

Vapour Pressure-Reid (RVP)

Measure of the pressure of vapour accumulated above a sample of gasoline or other volatile fuel in a standard bomb at 100°F (37.8°C). Used to predict the vapour 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, non-wipeable film deposit occurring on interior parts, resulting from the oxidation and polymerisation 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, which is defined as the force in dynes required to move a surface one square centimetre in area past a parallel surface at a speed of one centimetre per second, with the surfaces separated by a fluid film one centimetre 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 millilitres 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.