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Glossary of Pressure Regulator Terminologies (A-O)


Absolute Filtration – A filter which will pass particles of no larger than a prescribed size. To achieve filtration, an infinitely thin membrane with exact pore size is required. Most filters do not operate on the principle of absolute filtration and a rating of this type is mostly theoretical.

Absolute Pressure – A quantity of pressure measured with respect to total vacuum. Equal to the sum of a pressure gauge reading and atmospheric pressure (14.696 psia at 60 deg. F at sea level).

Absolute Zero – The minimum point in thermodynamic temperature scale (-273.15 deg. C or –459.7 deg. F).

Accuracy – The degree of agreement of a measured value with the true value of a universally accepted standard.

Adsorption – Adherence of the atoms, ions or molecules of a gas or liquid to the surface of another substance, called the adsorbent. Molecular Sieves are adsorbents.

Anydrous – A descriptive term meaning without water. An inorganic compound that does not contain water.

Argon Oxygen Decarburization (AOD) - A melt process in the manufacturing of stainless steel; usually the first melt in a two-melt process.

Areas of Entrapment – Any wetted area in a valve, regulator, etc. that can hold particles.

Asphyxiant Gas - A gas which has little or no positive toxic effect but can bring about unconsciousness and death by replacing air and thus depriving an organism of oxygen. Nitrous oxide is an example of such a gas.

Atomic Weight – The relative weight of an atom of an element, compared to carbon-12. Equivalent to the sum of protons and neutrons in the nucleus.

Attached Poppet (Tied-Seat; Tied Diaphragm) – A feature of certain regulators whereby the stem (poppet) is physically attached to the diaphragm.

Azeotropic Mixture (Azeotrope) - A liquid mixture of two or more substances which behaves like a single substance in that the vapor produced by partial evaporation of liquid has the same composition as the liquid. The constant boiling mixture exhibits either a maximum or minimum boiling point as compared with that of other mixtures of the same substances.


Back Pressure Regulator – A device used to controlling inlet pressure (upstream pressure) rather than reducing pressure. Normally used for pressure relief applications. Similar in function to a relief valve.

Balanced Poppet (Balanced Valve; Balanced Stem) – This is a design description of the poppet assembly wherein the forces exerted on the top and the bottom of the poppet are equal. In the case of regulator description the unit is sometimes referred to as a balanced pressure regulator or pressure compensated regulator. The valve spring provides the shutoff force. Used essentially to reduce or minimize decaying inlet pressure effect.

Bellows Valve – A valve design in which the shutoff is created by forcing a plunger covered by an "accordion-like" membrane (a "bellows") against a seat to form a seal. This design has many areas of entrapment and, due to the flexing of the bellows, can cause particle generation.

Boiling Point – Temperature at which the vapor pressure of a pure liquid becomes equal to the external pressure.

Bonnet (Spring Housing) – The part of a regulator which houses the control or range spring.

Bourdon Tube – A curved metal tube, sealed at one end, which flexes to a known degree when pressurized internally; used in pressure gauges to drive the pressure indicator pointer.

Brass – Copper/zinc alloys of varying composition. Some brass also contains low percentages of other elements such as manganese, aluminum, silicon, lead and tin.

British Thermal Unit (BTU) – The quantity of heat required to raise the temperature of one pound of water one degree F at or near its point of maximum density (39.1 deg. F).

Bubble Tight Shutoff – A seal that will not allow the visible passage of "bubbles." This is usually determined by connecting the outlet of a regulator with a piece of tubing and submerging the end under an inch or two of water. With inlet pressure applied and the regulator in an off condition, there should be an absence of bubbles.

Burst Pressure – A design test pressure that allows for permanent deformation and leakage, but parts must remain assembled (i.e., no sudden ruptures). Normal industry standard is 4 times (400%) of maximum operating pressure. See also "Proof Pressure" and "Maximum Operating Pressure."

Burst Disk (Frangible Disk) – A metal disk which is used as a safety device, intended to burst and allow gas to escape within predetermined pressure limits to prevent rupture of the device it is installed on. Similar in function to a safety relief valve, except it has no reseal capability.

Butt Weld – A welding of two materials without filler materials, formed by placing the two ends flush together.


Calibration – Comparison of a measurement standard or instrument with another standard or instrument to report or eliminate by adjustment any variation (deviation) in the accuracy of the item being compared.

Calibration Gas – A gas or gas mixture of accurately known composition used as a comparative standard in analytical instrumentation.

Captured Vent – A feature offered on some regulators which allows for the gases within the bonnet cavity to be piped away.

Catalyst – Any substance, which by virtue of its presence, affects the rate of a chemical reaction, and remains unchanged at the end of the reaction.

Cation – An ion having a positive charge. Cations in liquid subjected to electric potential collect at the negative pole or cathode.

CGA – Compressed Gas Association.

CGA/DIN Fittings – Standards established by the Compressed Gas Association for fittings that are used to attach to gas cylinders. The DIN system is European and established by the Germans and generally used in Europe.

CGA Number – Cylinder/container valve outlet connection number assigned by the Compressed Gas Association. Relates to specific gases.

Class 100 Clean Room – Federal Standard 2098 defines the specific particle count of a clean room based on the maximum number of 0.5 micron sized particles or larger per cubic foot of air. Class 100 means there is no more than 100 particles 0.5 micron or larger present in the Clean Room specified as Class 100.

CNC – Computer Numerically Controlled. Modern machine tools controlled by computers.

Coefficient of Flow (Cv) – Defined as the actual flow performance in U.S. gallons of water per minute at 60 deg. F when inlet pressure (P1) is 1 psig and the outlet pressure (P2) is atmospheric (14.696 psia). For regulators, this value should be used as a ranking rather than a means of determining actual flows. See "Flow Curves" to determine actual performance.

Compressed Gas – (1) A gas or mixture of gases having, in a container, an absolute pressure exceeding 40 psi at 70 deg. F; or (2) a gas or mixture of gases having, in a container, an absolute pressure exceeding 104 psi at 130 deg. F regardless of the pressure at 70 deg. F; or (3) a liquid flammable material having, vapor pressure exceeding 40 psi at 100 deg. F as determined by ASTM D-323-72.

Compressed Gas Association, Inc. (CGA) – This is a nonprofit technical association whose membership includes many corporations active in all phases of the compressed gas industries. Founded in 1913, the CGA uses experience and knowledge of its members to promote industry-wide standards and procedures for safety in the manufacture, storage, transport and use of compressed gases.

Corrosive – The ability of a chemical compound to attack and produce irreversible damage to human tissues, such as eyes, skin or mucous membranes, as well as rubber, metal and other substances.

Cracking Pressure – A term used in back pressure control only (e.g., back pressure regulators, relief valves), for determining the inlet pressure at which flow starts.

Creep (Regulators) – This is an increase in outlet pressure occurring after lockup. Creep normally appears as a gradual rise in outlet pressure over a period of time. The usual cause of creep is contamination or damage to the regulator seat causing the regulator to remain slightly open henceforth additional outlet pressure.

Critical Flow – This is also sometimes referred to as sonic flow and is the maximum flow that can pass through a regulator or an orifice.

Critical Point – The transition point at which the liquid and gaseous states of a substance merge into each other. It is the temperature above which a substance cannot exist in two phases, no matter how great the pressure. See also "Critical Temperature" and "Critical Pressure."

Critical Pressure – The pressure that is just sufficient to cause the appearance of two phases at the critical temperature. It is the vapor pressure of a liquid at the critical temperature.

Critical Temperature – The temperature above which a gas cannot be liquefied by pressure alone. At this temperature, there is no distinction between liquid and vapor, both having the same density and constituting one homogenous system.

Crude - A naturally occurring mixture, consisting of hydrocarbons, and/or sulfur nitrogen, and/or oxygen derivatives of hydrocarbons. It can be separated into fractions.

Cryogenic – Refers to the field of low temperatures, usually -130 deg. F.

Cryogenic Liquid Container – An insulated container designed to store, handle and transport liquids having boiling points below -130 deg. F.

Cv – See "Flow Coefficient."

Cylinder – A container designed to hold compressed gases or liquefied compressed gases.


Dehydration – Removal of one or more molecules of water from a chemical compound.

Delivery Pressure – See "Outlet Pressure."

Differential Pressure (Delta P) - Also shown in flow diagrams as "DP." This is the difference between the inlet pressure and the outlet pressure of any flow or pressure device.

Density – The ratio of the amount of anything per unit volume, e.g., mass of any substance per cubic volume at any definite temperature. It is usually expressed in pounds per cubic foot (lbs/ft3). See also "Specific Gravity."

Depth Filtration – A filter element that performs by providing long paths for particles to pass through. This type of filter also has a greater particulate holding capacity.

Dew Point – The temperature at which formation of vapor begins. The term usually applies to a condensation of moisture from water vapor in the atmosphere.

Diaphragm Valve – A valve design in which the shutoff is created by a piston (nonwetted) driving a diaphragm or membrane (wetted) against another material to form a seal. Diaphragm valves are typically the least particle generating type of valve because of lack of areas of entrapment and the minimal friction that causes particle generation.

Dome Loaded Pressure Regulator – A regulator where the range spring and associated component of the load is placed on the regulator diaphragm or piston are replaced by external means usually pneumatic although it may be loaded by bleeding in from the inlet pressure of the regulator. It may be either a pressure- reducing or a back pressure type. This is not a completely contained regulator.

Droop – (a) The decrease in outlet set pressure of a pressure regulator, resulting from an increase in flow rate. Essentially the reverse of lockup. See also "Lockup." (b) This is the amount of outlet pressure decrease with respect to increasing flow demand on a pressure-reducing regulator. It can be expressed in percentage change of the set point or can be shown as pounds per square inch change with respect to flow increases.


Eductor (Liquid Delivery) Tube – A tube inside a cylinder or container attached to the cylinder valve that allows liquid product withdrawal from the cylinder.

Electro-Polish – A process of reducing surface area and enhancing CrO2 on the surface by means of electrolysis. This procedure improves a material corrosion resistance.

Environmental Protection Agency (EPA) – This is a government agency that establishes environmental standards within the United States.

Extraction – The process of removing a component of a mixture by introduction of a solvent. The components having approximately the same boiling point, but possessing quite different solubility characteristics.


Face Seal – A high purity metal to metal mechanical connection formed by the joining of a male and female connector.

Flammable Gas – (1) A gas that, at ambient temperature and pressure, forms a flammable mixture with air at a concentration of 13% by volume or less; or (2) a gas that, at ambient temperature and pressure, forms a range of flammable mixtures with air wider than 2% by volume regardless of the lower limit.

Flammable Range – The range over which a gas at NTP will form a flammable mixture with air.

Flash Point – The temperature at which a liquid gives off vapor sufficient to form an ignitable mixture with air.

Flow Capacity – The maximum flow capability of a control device established at a specific set of conditions.

Flow Curve – A visual representation of the performance of a regulator. The curve typically shows the change in outlet pressure as flow increases (see "Droop") at a constant pressure.

Flow Rate – A quantity of gas or liquid passing through a controlled orifice during a specific time period. Units of measure include, SCFM Standard Cubic Feet Per Minute, SCFH Standard Cubic Feet Per Hour, and SLPM Standard Liters Per Minute.

Fluid – Any material or substance that changes shape uniformly in response to an external force imposed upon it. The term applies to liquids, gases and finely divided solids.

Fluid Catalytic Cracking (FCC) – The process whereby hydrocarbon is introduced into a flowing (fluid) catalyst for the purpose of molecular change brought about by the contact with the catalyst.

Fractions (Chem) – Separated hydrocarbon components sharing a specified boiling range.

Freezing Point – The temperature at which a liquid solidifies. It is the temperature, at which the liquid and solid states of a substance are in equilibrium at a given pressure (usually atmospheric). For pure substances, it is identical with the melting point of the solid form.


Hand Loaded Pressure Regulator – A spring loaded, manually controlled pressure reducing regulator. It can also be used to apply pressure to the dome of a dome loaded regulator.

Hastelloy – The trade name of material used for its corrosion resistance.

Heat of Adsorption – The total heat involved in the adsorption process from zero adsorbate loading to some final adsorbate loading at a constant temperature (also called isothermal integral heat of adsorption).

Heat of Fusion – the heat required to convert a substance from the liquid to the gaseous state with no temperature change (also called latent heat of fusion).

Heat of Vaporization – The heat required to convert a substance from the liquid to the gaseous state with no temperature change (also called latent heat of vaporization).

HEPA – High Efficiency Particulate Air-Filtering.

Hurricane Cleaning – Veriflo’s proprietary cleaning process that completely removes all unbonded particles, organic films and metallic ions from polished surfaces.

Hydration – The reaction of molecules of water with a substance in which the H-OH bond is not split.

Hydrodesulfurization (HDS) – The process whereby a hydrocarbon-sulfur stream and hydrogen are reacted in the presence of a catalyst for the purpose of removing the sulfur from the hydrocarbon stream as H2S.

Hydrogenation – The chemical addition of hydrogen to a hydrocarbon molecule.

Hysterisis – The maximum difference in output, at any measured value within the specified range, when the value is approached first when increasing and then decreasing pressure.


Inboard Leakage – This refers to leakage of the atmosphere or the environment surrounding the regulator while it is in service at operating pressure. The test for this is usually done with a mass spectrometer leak detector, which is sensitive to helium and, for practical purposes, a vacuum is pulled on the internal of the regulator and helium surrounding the regulator. For this reason, the leak rate is usually expressed in std cc per second of helium and the numbers are usually small such as 1 x 10-8 cc/sec helium.

Inert – A material which under normal temperatures and pressures, does not react with other materials.

Inhibitor – A compound (usually organic) that retards or stops an undesired chemical reaction such as corrosion, oxidation or polymerization.

Inlet Pressure (P1, Supply Pressure, Upstream Pressure) – The pressure of the fluid to the supply connection of a control element regardless of whether you are speaking of a pressure reducing or back pressure types.

Irritant – The ability of a chemical, which is not corrosive, to cause a reversible inflammatory effect on living tissue, by chemical action at the site of contact.

Isomerization – The process whereby a straight chain hydrocarbon is changed to an isomeric form without changing the molecular weight.

Isotopes – Forms of an element that differ from one another in the mass of their atoms and in the properties dependent on that mass. Having the same atomic number and the same number of valence electrons isotopes occupy the same position in the periodic table and have identical properties. They are distinguishable only by the small differences in atomic weight or by radioactive transformations.


Linearity (Regulators) – The maximum deviation of any calibration point, on a specified straight line, during any one calibration cycle.

Liquefied Compressed Gas – A gas which is partially liquid at its changing pressure and a temperature of 70 deg. F.

Liquid Density – The ratio of the mass of a liquid per unit volume at any definite temperature. It is usually expressed in pounds per gallon or pounds per cubic foot.

Load – Any force applied to a body. The force of a spring on diaphragm is considered a load. A force applied to a bonnet or cap prior to setting the nut clamp is considered a load.

Lockup – The increase in outlet pressure beyond the set pressure with respect to decreasing flow demand on a pressure-reducing regulator. Essentially the reverse of droop.

Lower Explosive Limit (EL) – The minimum percent by volume of a gas which, when mixed with air at normal temperature and pressure, will form a flammable mixture. See "Flammable Gas."


Manifold – A series of connectors to a common outlet allowing several cylinders to be used simultaneously. A special design incorporating valves and/or regulators in a single body.

Material Safety Data Sheet (MSDS) – An MSDS is a substance fact sheet containing characteristics and hazards of specific hazardous industrial material. Also, these data sheets provide precautionary information on safe handling of the material as well as emergency and first aid procedures.

Maximum Operating Pressure – The maximum allowable use pressure for which a system is designed. Also referred to as "working pressure."

Mixture – Any combination of two or more chemicals if the combination is not, in whole or in part, the result of a chemical reaction.

Molecular Weight – The sum of the atomic weights of all the constituent atoms in the molecule of an element or a compound.

Monel – A nickel/copper-based alloy material used for its corrosion resistance.


Nominal Filtration - The most common of filter ratings. A filter of this type operates for a range of particle size, for example 2-5 microns. This filter will prevent 99.9 percent of the particles from passing that are from 2-5 microns in size. It does not mean that it will keep them from passing. This would be absolute filtration.

Normal Boiling Point (nbp) – The temperature at which the vapor pressure of a liquid reaches 760 mm of mercury.

Normal Evaporation Rate (NER)– The degree of product loss from a cryogenic liquid container due to heat leak into the container as designed. The NER is checked by measuring the amount of product loss over a specified time and serves to confirm whether the insulation is still good or not.

Normal Temperature and Pressure (NTP) – A gas industry reference base. Normal temperature is 70 deg. F. Normal pressure is one atmosphere or 14.696 psia.

Nozzle – An assembly that contains the poppet, a poppet spring and seal of a regulator. This assembly may also contain a screen.

NPT – National Pipe Threads. A threaded connection typically sealed with teflon tape.


Occupational Safety and Health Administration (OSHA) – An organization within the U.S. Department of Labor that sets standards for employers to ensure safe and healthful working conditions for employees.

Outboard Leakage – This refers to leakage of the regulator from the internal area to the atmosphere while at operating pressures.

Outlet Pressure – Always the pressure at the outlet of the regulator regardless of whether you are speaking of a pressure reducing or a back pressure type.

Outlet Pressure (P2; Delivery Pressure; Downstream Pressure) – The pressure of the fluid from the discharge connection of a control element.

Oxidant - A gas that supports combustion.

Oxidizing Agent – A chemical reagent that causes oxidation of other substances and is thereby reduced.

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