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Pneumatic Glossary of Terms


Glossary of Pneumatic Terms


Accumulator.  A container in which fluid is stored under pressure as a source of fluid power.

Accumulator, Hydropneumatic Bladder.  A hydro pneumatic accumulator in which the liquid and gas are separated by an elastic bag or bladder.

Acetal Plastic.  Tough, stable engineering thermoplastic with a high modulus of elasticity, high strength, good rigidity, dimensional stability, resistance to moisture, solvents and other chemicals.

Active Device.  A device that has an input(s) that controls a power supply to the device.

Actuator.  A device in which power is transferred from one pressurized medium to another without intensification.

Additive.  A chemical added to fluid to impart new properties or to enhance those that already exist.

Air Motor.  A device that converts pneumatic fluid power into mechanical torque and motion.  It usually provides rotary mechanical motion.

Air, Compressed (pressure).  Air at any pressure greater than atmosphere.

Air, Dried.  Air with moisture content lower than the maximum allowed for a given application.

Air, Free.  Air at ambient temperature, pressure, relative humidity and density.

Air, Saturated.  Air at 100% relative humidity with a dew point equal to temperature.

Air, Standard.  Air at a temperature of 68.8° F, a pressure of 14.7 pounds per square inch absolute, and a relative humidity of 36% (0.0750 pounds per cubic foot).  In gas industries the temperature of “standard air” is usually given as 60.8° F.

Air.  A gas mixture consisting of nitrogen, oxygen, argon, carbon dioxide, hydrogen and small quantities of neon, helium and other gases.

Air Bleeder.  A device for the removal of air.

Air Breather.  A device permitting air movement between atmosphere and the component in which it is installed

Air Motor.  A device which converts pneumatic power into mechanical force and motion which usually provides rotary mechanical motion.

Amplification.  The ration between the output signal variations and the control signal variations (for analog devices only).

Amplification, Power.  The ratio between the output power variation and the corresponding input (control) power variation (for analog devices only).

Amplification, Pressure.  Ratio between the outlet pressure and the inlet (control) pressure.

Avogadro's Law.  A gas law that states that, under the same condition of temperature and pressure, equal volumes of all gases contain the same number of molecules.


Bernoulli’s Law.  If no work is done on or by a flowing frictionless liquid, its energy due to pressure and velocity, remains constant at all points along the streamline.

Bleed Orifice.  Used in indirect or semi-direct acting solenoid valves, a bleed orifice is installed on the diaphragm enabling the valve to use the line pressure to open or close the valve.

Boyle’s Law.  The absolute pressure of a fixed mass of gas varies inversely as the volume, provided the temperature remains constant.

Breakout.  Force necessary to inaugurate sliding.  Expressed in same terms as friction.  An excessive breakout valve indicates the development of adhesion.

Breathing Capacity.  A measure of flow rate through an air breather.

Bulk Modulus.  The measure of resistance to compressibility of a fluid. It is the reciprocal of compressibility.


Cavitation.  A localized gaseous condition within a liquid stream that occurs where the pressure is reduced to the vapor pressure.

Charles’ Law.  The volume of a fixed mass of gas varies directly with absolute temperature, provided the pressure remains constant.

Circuit, Metered-In.  A speed control circuit in which the control is achieved by regulating the supply flow to the actuator.

Circuit, Metered-Out.  A speed control circuit in which the control is achieved by regulating the exhaust flow from the actuator.

Circuit, Sequence.  A circuit that established the order in which two or more phases of a circuit occur.

Compression Set.  The amount by which a rubber specimen fails to return to original shape after release of the compressive load.

Compressibility.  The change in volume of a unit volume of a fluid when subjected to a unit change in pressure.

Compressor.  A device that converts mechanical force and motion into pneumatic fluid power.

Condensation. The process of changing a vapor into a liquid condensate by the extraction of heat.

Conductor.  A component whose primary function is to contain and direct fluid.

Containment.  Any material of substance that is unwanted or adversely affects the fluid power system or components, or both.

Contaminant.  Any material or substance that is unwanted or adversely affects the system or components, or both.

Creep.  The progressive relaxation of a given rubber material while it is under stress.  This relaxation eventually results in permanent deformation or “set”.

Cushion.  A device that provides controlled resistance to motion.

Cv.  Flow coefficient or pneumatic conductance expresses the flow capability of any fixed orifice pneumatic device for a given fluid.

Cylinder Bore.  The internal diameter of the cylinder body.

Cylinder Cap.  A cylinder end closure that completely covers the bore area.

Cylinder Capacity.   Volume required for one full extension of a cylinder.

Cylinder, Double-Acting.  A cylinder in which fluid force can be applied to the moveable element in either direction.

Cylinder, Double Rod.  A cylinder with a single piston and a piston rod extending from each end.

Cylinder, Piston Type.  A cylinder in which the piston has a greater cross-sectional areas than the piston rod.

Cylinder, Rotating. A cylinder in which the piston and piston rod, plunger or ram, is permitted to rotate with reference to the cylinder housing.

Cylinder, Single-Acting.  A cylinder in which the fluid force can be applied to the movable element in only one direction.

Cylinder, Tandem.  Arrangement of at least two pistons on the same rod moving in separate chambers on the same cylinder body allowing the compounding of force on the piston rod.

Cylinder, Telescoping. Cylinder with two or more stages of extensions, achieved by hollow piston rods sliding one within the other (may be single- or double-acting).

Cylinder, Tie Rod.  A cylinder with head and cap end closures that are secured by tie rods.

Cylinder.  A device that converts fluid power into linear mechanical force and motion.  It usually consists of a movable element such as a piston and a piston rod, plunger or ram, operating within a cylindrical bore.


Desiccant.  Material that tends to remove moisture from compressed air.

Detented (maintained).  The design intention of a device to maintain the state of its last actuation after the operator force is removed.

Dew Point.  The temperature at which vapors in the gas condense. For practical purposed, it must be referred to a stated pressure.

Displacement, Volumetric. Volume absorbed or displaced per stroke of a cylinder or per cycle of a pump or motor.

Dither.  A low amplitude, relatively high frequency periodic electrical signal, sometimes superimposed on the servo valve input to improve system resolution. Dither is expressed by the dither frequency (Hz) and the peak-to-peak dither current amplitude.

Diverter (valve).  A device whose power source at one input port is diverted to one of two or more output ports.

Droop.  The deviation for reducing the moisture content of the working medium.

Durometer   1.  An instrument for measuring the hardness of rubber.  Measures the resistance to the penetration of an indentor point into the surface of rubber.  2.  Numerical scale of rubber hardness.


Filter.  1.  A device whose primary function is the removal by porous media of insoluble contaminants from a liquid or a gas.  2.  Chemically inert, finely divided material added to the elastomer to aid in process and improve physical properties.

Filter, By-Pass (reserve).  A filter that provides an alternative unfiltered flow path around the filter element when a pre-set differential pressure is reached.

Filtration Ratio.  The ration of the number of particles greater than a given size (u) in the influent fluid to the number of particles greater than the same size (u) in the effluent fluid.

FittingPneumatic fittings are connectors or closures for pneumatic lines and passages.

Fitting, Compression.  A fitting that seals and grips by manual lines and passages.

Fitting, Flange.  A fitting that utilizes a radially extending collar for sealing and connection.

Fitting, Flared.  A fitting that seals and grips by a pre-formed flare at the end of the tube.

Flash Point.  The temperature to which a liquid must be heated under specified conditions of the test method to give off sufficient vapor to form a mixture with air that can be ignited momentarily by a flame.

Flip Flop.  A digital component or circuit with two stable states and sufficient hysteresis so that it has “memory”.  Its state is changed with a control pulse; a continuous control signal is not necessary for it to remain in a given state.

Flow.  Movement of air or gas generated by pressure differences.

Flow Characteristic Curve.  The change in regulated (secondary) pressure occurring as a result of a change in the rate of airflow over the operating range of the regulator.

Flow Output.  Flow rate discharged at the outlet port.

Flow Fate.  The volume, mass or weight of a fluid passing through any conductor per unit of time.

Flow, Turbulent.  A flow situation in which the fluid particles move in a random fluctuating manner.

Fluid.  A substance which tends to conform to the outline of its container and is capable of flowing as a liquid or a gas.

Fluid, Aqueous.  A fluid that contains water as a major constituent besides the organic material. The fire-resistant properties are derived from the water content.

Fluid Capacity.  The liquid volume coincident with the “high” mark of the level indicator.

Fluid, Friction.  Friction due to the viscosity of fluids.

Fluid Logic.  A branch of fluid power associated with digital signal sensing and information processing, using components with or without moving parts.

Fluid Power.  Energy transmitted and controlled through use of pressurized fluid.

Fluid, Pneumatic.  A fluid suitable for use in a pneumatic system, usually air.

Fluid, Stability.  Resistance of a fluid to permanent changes in properties.

Force Motor.  A type of electro-mechanical transducer having linear motion used in the input stages of servo valves.

Filter-Regulator-Lubricator (FRL).  FRLs condition and prepare compressed air for use in fluid power systems. Pneumatic applications with properly conditioned air will operate longer, cost less and improve system efficiency.


Gauge.  An instrument or device for measuring, indicating, or comparing a physical characteristic.

Gauge Damper (snubbed).  A device employing a fixed or variable restrictor inserted in the pipeline to a pressure gauge to prevent damage to the gauge mechanism caused by rapid fluctuations of fluid pressure.

Gauge, Diaphragm.  A gauge in which the sensing element is relatively thin and its inner portion is free to deflect with respect to its periphery.

Gauge, Pressure.  A gauge that indicates the pressure in the system to which it is connected.


Hose.  A flexible line or conductor whose nominal size is the inside diameter.

Hydro Pneumatics.  Pertaining to the combination of hydraulic and pneumatic fluid power.


Inhibitor.  Any substance which, when present in very small proportions, slows, prevents or modifies chemical reactions such as corrosion or oxidation.

Impedance.  Imaginary ration of pressure drop and transient mass flow in which pressure drop leads flow.

Heat Exchanger.  A device that transfers heat through a conducting wall from one fluid to another.

Ideal Gas Law.  The state of an amount of gas is determined by its pressure, volume, and temperature. The modern form of the equation relates these simply in two main forms. The temperature used in the equation of state is an absolute temperature: in the SI system of units, kelvin; in the Imperial system, degrees Rankin.


Joint.  A line positioning connector.

Joint, Rotary.  A joint connecting lines which have relative operational rotation.


Line.  A tube, pipe or hose for conducting fluid.

Line, Return.  A pipe (conductor) to return the working fluid to the reservoir.

Line, Working.  A line that conducts fluid power.

Lubricator.  A device that adds controlled or metered amounts of lubricants into a fluid power system.


Manifold.  Pneumatic manifolds are conductors that provide multiple connection ports.

Manifold Block.  A manifold that is open to the atmosphere and returns fluid to the reservoir.

Maximum Inlet Pressure.  The maximum rated gauge pressure applied to the inlet port of the regulator.

Memory.  Tendency of a material to return to original shape after deformation.

Momentary.  The design intention of a device to return to the normal punctuated state after the operator force is removed.

Muffler.  A device for reducing gas flow noise.  Noise is decreased by back pressure control of gas expansion.


Needle Valve. A type of valve featuring a needle-shaped plunger allowing for precise regulation of flow typically capable of relatively low flow rates.

Newt.  A unit of kinematic viscosity in the English system. It is expressed in square inches per second.

Non-Threaded.  A term applied to exhaust ports without internal threads to prevent connection.

NOR Device.  A control device which has its output in the logical 1 state if and only if all the control signals assume the logical 0 state.

Normally-Closed (electrical).  The state of the output or switch is ON (passing current) with no external influence.

Normally-Open (electrical).  The state of the output or switch is OFF (not passing current) with no external influence.

Normally-Closed (fluid power).  Often referred to as normally not passing   The state of the output or valve is OFF with no external influence.

Normally-Open (fluid power).  often referred to as normally passing   The state of the output or valve is ON with no external influence.

NOT Device.  A control device which has its output in the logical 1 state if and only if the control signal assume the logical 0 state.  The NOT device is a single input NOR device.


OR Device.  A control device which has its output in the logical 0 state if and only if all the control signals assume the logical 0 state, also known as a shuttle valve.

Operator.  A device that attaches to another assembly and applies force to the actuator of that assembly allowing it to act in the intended manner.


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.

Passive Device.  A device that does not have a dedicated supply source and operates solely on the input(s) alone.

Permanent Set.  The deformation remaining after a specimen has been stressed in tension for a definite period and released for a defined period.

Pipe.  A conductor that’s outside diameter is standardized for threading.

Piston Rod.  The element transmitting mechanical force and motion from the piston.

Pneumatics.  Engineering science pertaining to gaseous pressure and flow.

Poppet.  A component of a valve that seals or opens an internal passage across its full area to allow or prevent flow.

Port.  A terminus of a passage in a component to which conductors can be connected.

Power Supply.  The component or group of components which supplies and processes the fluid for operating fluidic systems.

Port, Differential Pressure.  A port that provides a passage to the upstream and downstream sides of a component.

PPS (Polyphenylene Sulphide).  PPS offers good performance at high temperatures with strong resistance to alkalis and acids.

Pressure.  Force per unit area, usually expressed in pounds per square inch (bar).

Pressure, Absolute.  The pressure above zero, i.e., the sum of atmospheric and gauge 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 = 14.7 psi).

Pressure, Back.  The pressure encountered on the return side of a system.

Pressure, Breakloose (breakout).   The minimum pressure which initiates movement.

Pressure, Burst.  The pressure that causes failure of and consequential loss of fluid through the product envelope.

Pressure, Charge.  The pressure at which replenishing fluid is forced into a fluid power system.

Pressure, Control Range.  The permissible limits between which system pressure maybe 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, Gauge.  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, Maximum Inlet.  The maximum rated gauge pressure applied to the inlet.

Pressure, Nominal.  A pressure value assigned to a component or system for the purpose of convenient designation.

Pressure, Proof.  The non-destructive test pressure, in excess of the maximum rated operating pressure, which causes non-permanent deformation, excessive external leakage, or other resulting malfunction.

Pressure, Rated.  The qualified operating pressure which is recommended for a component or system by the manufacturer.

Pressure, Static.  The pressure in a fluid at rest.

Pressure, Surge.  The pressure resulting from surge conditions.

PTFE (Polytetrafluoroethylene).  PTFE is a thermoplastic fluoropolymer which is chemically-inert to most substances. The material offers hydrophobic properties making it suitable for ultra-clean and chemical applications.


Quick-Disconnect Fitting.  A component that can quickly join or separate a fluid line without the use of tools or special devices.


Rated Flow.  The maximum flow that the power supply system is capable of maintaining at a specific operating pressure.

Regulator, Airline Pressure. A regulator that transforms a fluctuating air pressure supply to provide a constant lower pressure outlet.

Reservoir.  A container for storage of liquid in a fluid power system.

Rein.  The standard unit of absolute viscosity in the English system. It is expressed in pound-seconds per square inch.

Ring, O.  A ring that has a round cross-section.

Rotation.  The direction of rotation is always quoted as viewed looking at the shaft end. In dubious cases, provide a sketch.


Seal, Cup.  A sealing device with a radial base integral with an axial cylindrical projection at its outer diameter.

Seal, Dynamic.  A sealing device used between parts that have relative motion.

Seal, Elastomer.  A material having rubber-like properties; i.e., having the capacity for large deformation and rapid, substantially complete, recovery on release from the deforming force.

Seal, Rod (shaft).  A sealing device that seals the periphery of a piston rod.

Seal, Static (gasket).  A sealing device used between parts that have no relative motion.

Selector (valve).   A device that selects from separate power sources at two input ports and directs the selected source to a single output port.

Sensor.  A device that detects and transmits changes in external conditions.

Silencer.  A device for reducing gas flow noise.  Noise is decreased by tuned resonant control of gas expansion.

Squeeze.  Cross section diametral compression of O-ring between surface of the groove bottom and surface of the other mating metal part in the gland assembly.

Stroke Ratio.  Each push-button valve selected has a given actuation stroke and force.  The thumb and Cam Operators will provide an increased stroke and decreased force in the ratios noted while providing a correct ergonomic or machine interface for various system applications.

Subplate, Sub-Base.  Mounting to which a simple valve is fitted and which includes external ports for connections.


Temperature, Ambient.  The temperature of the environment in which an apparatus is working.

Tie Rod.  An axial external cylinder element that traverses the length of the cylinder. It is pre-stressed at assembly to hold the ends of the cylinder against the tubing. Tie rod extensions can be a mounting device.

Torque.  Rotary force transmitted by the driving shaft of the pump.

Torque Motor.  A type of electro-mechanical transducer having rotary motion used in the input stages of servo valves.

Transducer, Flow.  A device that converts fluid flow to an electrical signal.

Trunnion.  A mounting device consisting of a pair of opposite projecting cylindrical pivots. The cylindrical pivot pins are at right angle or normal to the piston rod centerline to permit the cylinder to swing in a plane.

Tubing.  A stiff material in the form of a tube used for conveying gases or liquids.


Vacuum.  Pressure less than ambient atmospheric pressure measured in inches of mercury (Hg”).

Vacuum Pump.  A device that uses mechanical force and motion to evacuate gas from a connected chamber to create sub-atmospheric pressure.

Valve.  A device that controls fluid flow direction, pressure or flow rate.

Valve, Actuator.  The valve part(s) through which force is applied to move or position flow-directing elements.

Valve, Air.  A valve for controlling air.

Valve, Cartridge.  A valve that working parts contained in a cylindrical body. The cylindrical body must be inserted into a housing for use. Ports through the body cooperate with ports in the containing housing.

Valve, Diaphragm.  A flexible material that isolates the solenoid assembly from the media designed to contain the pressure of the fluid.

Valve, Direct-Operated.  A valve in which the controlling forces acting on the element directly influence the movement of the control elements.

Valve, Directional Control.  A valve whose primary function is to direct or prevent flow through selected passages.

Valve, Directional Control, 3-Way.  A directional control valve whose primary function is to pressurize and exhaust a port.

Valve, Directional Control, 4-Way.  A directional control valve whose primary function is to pressurize and exhaust two ports.

Valve, Flow Control (flow metering).  A valve whose primary function is to control flow rate.

Valve, Gate.  A straight-through shut-off valve in which the valve element moves perpendicularly to the axis of the flow to control opening and closing.

Valve, Needle A flow control valve in which the adjustable control element is a tapered needle.  Its usual purpose is the bidirectional control of flow.

Valve, Pilot-Operated (indirect).  A valve in which a relatively small flow through an integral flow path (pilot) controls the movement of the main elements.

Valve, Pilot.  A valve applied to operate another valve.

Valve, Pinch.  A straightforward shut-off valve in which the valve element consists of a flexible sleeve that is distorted to control the flow.

Valve, Poppet.  A valve in which the flow paths are opened or closed as the valve element (poppet) is lifted or seated.

Valve, Pressure Sensing.  A device similar to an electrical pressure switch, in which a signal to be sensed enters a control point, and actuates a mechanism that, at the proper pressure level causes one or more flow passages to change condition. Removal of the signal allows the pressure sensing valve to reset.

Valve, Proportional. A valve that relies on the laws of fluid pressure to distribute input forces to one or more output lines. A proportioning valve can increase or decrease forces for each output, depending on the cross-sectional surface areas of those output lines.

Valve, Quick Exhaust.  Valve in which when air pressure falls at the inlet, the outlet is automatically opened to the exhaust.

Valve, Relay.  A logic device that receives control signals and changes flow conditions in one or more controlled flow passages.

Valve, Seat.  The seat is the orifice that presses on the disc when closing the valve depending on the design of the valve.

Valve, Shuttle.  A connective valve that selects one or two or more circuits because flow or pressure changes between the circuits.

Valve, Slide.  A valve in that the flow paths are connected or isolated by means of a flat movable sliding member. The movement may be axial, rotary or both.

Valve, Solenoid.  Valve is controlled by an electric current through a solenoid: in the case of a two-port valve the flow is switched on or off; in the case of a three-port valve, the outflow is switched between the two outlet ports.

Valve, Spool.  A shear action valve design that utilizes a spool that slides throughout the flow path.

Valve, Stem.  A self-contained valve that opens to admit gas to a chamber (such as air to inflate a tire), and is then automatically closed and kept sealed by the pressure in the chamber, or a spring, or both, to prevent the gas from escaping.

Valve, Two-Position.  A directional control valve having two positions to give two selections of flow conditions.

Vortex.  Spiral motion of a fluid resulting in a radial pressure gradient. The trajectories are curves that encircle a single line (axis).