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spontaneously releases volatile constituents in the form of vapors or gases. In rubber compounds, these constituents may include water vapor, plasticizers, air, inhibitors, etc. Output stage ? The final stage of hydraulic amplifications used in a servo valve. Oxidation inhibitor ? substance added in small quantities to petroleum product to increase its oxidation resistance, thereby lengthening its service or storage life; also called anti-oxidant. Ozone resistance ? Ability to withstand the deteriorating effect of ozone (which generally causes cracking). Packing ? A sealing device consisting of bulk deformable material of one or more mating deformable elements, reshaped by manually adjustable compression to obtain and maintain effectiveness. It usually uses axial compression to obtain radial sealing. Pascal's Law ? A pressure applied to a confined fluid at rest is transmitted with equal intensity throughout the fluid. Permanent set ? The deformation remaining after a specimen has been stressed in tension for a definite period and released for a definite period. Permeability ? The rate at which a liquid or gas under pressure passes through a solid material by diffusion and solution. In rubber terminology, it is the rate of gas flow ex pressed in atmospheric cubic centimeters per second through an elastomeric material one centimeter square and one centimeter thick (arm cmVcm2 *cm«sec) Petroleum fluid ? A fluid composed of petroleum oil which may contain additives and/or inhibitors. Pipe ? A conductor whose outside diameter is standardized for threading. Pipe is available in standard, extra strong, or double extra strong wall thickness. Piston rod ? The element transmitting mechanical force and motion from the piston. Plasticizer ? A substance, usually a heavy liquid, added to an elastomer to decrease stiffness, improve low temperature proper-ties, and improve processing. Pneumatics ? Engineering science pertaining to gaseous pressure and flow. Poise ? The standard unit of dynamic viscosity in the COS (centimeter gram second) sys-tem. It is the ratio of the shearing stress to the shear rate of fluid and is expressed in millipascal sec. (equals 1 centipoise). Polar compound ? a chemical corn-pound whose molecules exhibit electrically positive characteristics at one extremity and negative characteristics at the other. Polar compounds are used as additives in many petroleum products. Polymer ? A material formed by the joining together of many (poly) individual units (mer) of one or more monomers; synonymous with elastomers. Polytropic compression occurs when heat is transferred to or from the air at a precise rate during compression so that PVn is constant. Port ? A terminus of a passage in a component to which CHAPTER 4 ISO Symbols & Glossary conductors can be connected. Port, differential pressure ? A port that provides a passage to the upstream and downstream sides of a component. Post cure ? The second step in the vulcanization process for the more exotic elastomers. Provides stabilization of parts and drives off decomposition products resulting from the vulcanization process. Pour point ? lowest temperature at which an oil or distillate fuel is ob-served to flow, when cooled under conditions prescribed by test method ATM D 97. The pour point is 3°C (5° F) above the temperature at which the oil in a test vessel shows no movement Power unit ? A combination of pump, pump drive, reservoir, controls and conditioning components to supply hydraulic power to a system. Pressure, absolute ? The pressure above zero absolute, i.e., the sum of atmospheric and gage pressure. In vacuum related work it is usually expressed in millimeters of mercury (mm-Hg). Pressure, atmospheric ? Pressure exerted by the atmosphere at any specific location. (Sea level pressure is approximately 14.7 pounds per square inch absolute. 1 bar = 14.5 PSI). Pressure, back ? The pressure encountered on the return side of a system. Pressure, break loose (breakout) ? The minimum pressure which initiates movement. Pressure, burst ? The pressure which causes failure of and consequential loss of fluid through the product envelope. Pressure, charge ? The pressure at which replenishing is formed into a fluid power system. Pressure, control range ? The permissible limits between which system pressure may be set. Pressure, cracking ? The pressure at which a pressure-operated valve begins to pass fluid. Pressure, differential (pressure drop) ? The difference in pressure between any two points of a system or a component. Pressure, gage ? Pressure differential above or below ambient atmospheric pressure. Pressure, induced ? Pressure generated by an externally applied force. Pressure, inlet ? The pressure at the apparatus inlet port. Pressure, intensified ? In a fluid power cylinder, the outlet pressure required to slow the piston rod extending under regulated pressure introduced at the cap end. Pressure, maximum inlet ? The maximum rated gage pressure applied to the inlet. Pressure, nominal ? A pressure value assigned to a component or system for the purpose of convenient designation. Pressure, outlet ? Pressure at the apparatus outlet port. Pressure, override ? The difference between the crackiiv pressure of a valve and the pressure reached when the valve is 4-41