Hardness
The resistance of a substance to surface abrasion.

Head
An end closure for the filter case or bowl which contains one or more ports.

Heat Transfer Oil
A medium used for the transfer of heat.

Heat exchanger
A device which transfers heat through a conducting wall from one fluid to another.

Herschel Demulsibility Number
A number which indicates the ability of an oil to separate from water under conditions specified by the Herschel Demulsibility Test.

Housing
A ported enclosure which directs the flow of fluid through the filter element.

Humidity Cabinet Test
A test used to evaluate the rust-preventing properties of metal preservatives under conditions of high humidity (ASTM Method D 1748).

Hydraulic Fluid
Fluid serving as the power transmission medium in a hydraulic system. The most commonly used fluids are petroleum oils, synthetic lubricants, oil-water emulsions, and water-glycol mixtures. The principal requirements of a premium hydraulic fluid are proper viscosity, high viscosity index, anti-wear protection (if needed), good oxidation stability, adequate pour point, good demulsibility, rust inhibition, resistance to foaming, and compatibility with seal materials. Anti-wear oils are frequently used in compact, high-pressure, and capacity pumps that require extra lubrication protection.

Hydraulic Oil
An oil specially suited for use as either the specific gravity or the API gravity of a liquid.

Hydraulics
Engineering science pertaining to liquid pressure and flow.

Hydrocarbons
Compounds containing only carbon and hydrogen. Petroleum consists chiefly of hydrocarbons.

Hydrodynamic lubrication
A system of lubrication in which the shape and relative motion of the sliding surfaces causes the formation of a fluid film having sufficient pressure to separate the surfaces.

Hydrofinishing
A process for treating raw extracted base stocks with hydrogen to saturate them for improved stability.

Hydrogenation
The chemical addition of hydrogen to a material. In non-destructive hydrogenation, hydrogen is added to a molecule only if, and where, unsaturation with respect to hydrogen exists. In destructive hydrogenation, the operation is carried out under conditions which result in rupture of some of the hydrocarbon chains (cracking); hydrogen is added where the chain breaks have occurred.

Hydrolysis
Breakdown process that occurs in anhydrous hydraulic fluids as a result of heat, water, and metal catalysts (iron, steel, copper, etc.)

Hydrolytic stability
ability of additives and certain synthetic lubricants to resist chemical decomposition (hydrolysis) in the presence of water.

Hydrometer
An instrument for determining either the specific gravity of a liquid or the API gravity.

Hydrostatic lubrication
A system of lubrication in which the lubricant is supplied under sufficient external pressure to separate the opposing surfaces by a fluid film.

Hypoid gear lubricant
A gear lubricant having extreme pressure characteristics for use with a hypoid type of gear as in the differential of an automobile.
 
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