Technical Data > Glossary Terms

GLOSSARY TERMS

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Absolute Pressure
Pressure measured with respect to total vacuum abbreviated as "psia" or "bar." Absolute pressure is equal to the sum of a pressure gauge reading and atmospheric pressure (14.69 psia or 1 bar at sea level).

Absolute Zero
The minimum point on the thermodynamic temperature scale (-459.69°F or -273.16°C).

Accuracy
The difference between the "measured" and "true" values. The degree of statistical agreement (usually at the 95% confidence level) between a measured value and the true value, or the certainty or "sureness" with which a measured value is known.

ADR
Accord Europeen Relatif au Transport des Marchandises Dangereux par Route. European agreement concerning the international transportation of dangerous goods via road. Each participating country may have additional national agreements.

Adsorption
The condensation of gases, liquids or dissolved substances on the surface of solids.

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.

American Conference of Governmental Industrial Hygienists (ACGIH)
This conference is a professional society, not an official government agency. It is an organization devoted to the development of administrative and technical aspects of worker protection.

Analysis
Instrumental analysis in the lab after blending. A process based on statistics used to estimate the true concentration for each component of a gas mixture and to assign the analyzed concentration. A further objective of analysis is to determine the analysis accuracy for each component. Analytical measurements have sufficient work content to assure that component concentrations and accuracies meet customer and Scott specifications. Example: A Certificate of Accuracy stating Component X: 100 ppm +/2% (mol/mol)" means "Scott has analyzed this gas mixture and determined that the most probable value for Component X is 100 ppm and the true value is between 98 and 102 ppm, on a mole basis, at the 95% confidence level."

Analytical Accuracy
The difference between the analyzed concentration and the true concentration. The degree of statistical agreement between the analyzed and the true concentration for a component of a gas mixture (usually at the 95% confidence level), or the certainty or "sureness" with which the analyzed concentration is known. Analytical measurements have sufficient anchoring to reference standards, calibration, precision, resolution and traceability to assure that the true concentration for a component can be estimated to the claimed accuracy. Analytical accuracy can be calculated because Scott analytical measurements require actual data for reference standards, calibration, precision, resolution and traceability.

Analytical Tolerance
The difference between the analyzed concentration and the requested concentration. The degree of agreement between the analyzed concentration and the requested concentration for a component. Analytical tolerance is not determined by statistical means (whereas analytical accuracy is).

Analyzed Concentration
The concentration measured by lab analysis. The numerical concentration assigned to a component based on the results of analysis. the analyzed concentration is expressed in both concentration units i.e. "percent," "parts per million" and concentration basis (i.e. "mol/mol," "weight/weight"). An analyzed concentration is also accompanied by an analysis accuracy value (i.e.+/-2%).

Anhydrous
A descriptive term meaning "without water" or "dry."

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

Back Pressure Regulator
A gas pressure regulator which controls upstream (inlet) pressure. Similar in function to a relief valve.

Balance (Balance Gas)
The material ("component") making up the majority of a gas mixture. The balance may be either gas or liquid phase. The balance may also be a mixture (i.e. air). If the mixture is entirely gas phase, the balance is referred to as the balance gas.

Balanced Poppet (Balanced Valve or Balanced Stem)
A valve which has been designed to be pressure balanced; hence the valve spring provides the shutoff force. Used essentially to reduce or minimize decaying inlet pressure effect.

Bias Error
A systematic error in a process that causes all measured values to deviate from the true value by a consistent amount in a consistent direction (higher or lower than).

Blend Accuracy (Process Accuracy)
The difference between the blended concentration and the true concentration. The degree of statistical agreement between the blended and the true concentration for a component of a gas mixture (usually at the 95% confidence level), or the certainty or "sureness" with which the blended concentration is known. Blend accuracy can be calculated because Scott gravimetric measurement requires actual data for scale calibration, resolution and weight uncertainties.

Blend Tolerance
The difference between the blended concentration and the requested concentration. The degree of agreement between the blended concentration and the requested concentration for a component. Blend tolerance is not determined by statistical means (whereas blend accuracy is).

Blended Concentration
The concentration measured by the blending process. The numerical concentration assigned to a component based on the results of blending. A blended concentration is expressed in both concentration units (i.e. "percent," "parts per million") and concentration basis (i.e. "mol/mol," "weight/weight). A blended concentration may also be accompanied by a blend accuracy value (i.e. +/-2%).

Blending
Preparation of a gas mixture by measuring and mixing components. A process by which component and balance gases of a gas mixture are metered into a container. Blending process measurements (i.e. scale resolution, actual weight data, partial pressures, Acublend® response data) have sufficient resolution and accuracy to assure that components are in the required ratio to meet customer and Scott specifications.

Bonnet (Spring Housing)
The part of a gas pressure regulator that houses the control spring.

Bourdon Tube
A curved metal tube (sealed at one end) that flexes to a known degree when pressurized internally.

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

BS (British Standard)
A standard from the British Standards Institution. BS 341 recommends cylinder valve outlet connections for specific gas services based upon safety considerations.

Bubble Point
The temperature at which the first bubble of gas forms from a liquid material. The bubble point temperature is usually lower than the dew point temperature for a given mixture at a given pressure.

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 four (4) times or 400% of maximum operating pressure. See also PROOF PRESSURE and MAXIMUM OPERATING PRESSURE.

Bursting Disk (Frangible Disk)
A metal disk that is part of a safety device and is intended to burst and allow gas to escape within predetermined pressure limits to prevent rupture of the device on which it is installed. It is similar in function to a safety relief valve, however, it has no reseat capability.

Calibration
Setting up an instrument with standards. A process designed to determine the relationship between detector response and concentration for a given component on an instrument. During calibration, one or more samples of known concentration (i.e. "standards") are introduced to the detector and detector response is recorded for each sample. Calibration may be performed with reference standards, or with calibration standards accompanied by reference standard verification.

Calibration Curve
The line determined by the calibration standard response data for an instrument. A mathematical function produced by regression of the detector responses recorded during calibration of an instrument. The function describes detector responses over a range of concentrations and is used to predict the concentration of an unknown sample based on its detector response.

Calibration Gas
A gas with an accurately known concentration that is used as a comparative standard in analytical instrumentation.

Calibration Range
A range between the highest and lowest concentrations of standards used for calibrations, from which the concentrations of unknown samples can be determined. A concentration range may be further limited by precision and accuracy requirements.

CAS (Chemical Abstract Service)
CAS numbers represent chemical substances recorded in the CAS Chemical Registry System. This numbering system identifies chemical substances by an unambiguous computer language description of its molecular structure, including all stereo-chemical detail. The CAS number, which has no chemical significance, is simply a number assigned in sequential order to each substance as it enters the registry system. All specific substances reported in the world's scientific and technical literature and indexed in Chemical Abstracts (CA) since 1965 (when the registry system began) are included in the master file.

Certificate of Accuracy
A document that represents the certified concentration of a component and its associated accuracy to the customer. Scott Certificates of Accuracy report either the blended or analyzed concentration and require backup data to substantiate concentration and accuracy claims.

Certificate of Analysis
A document that represents the certified concentration of a component of a gas mixture to the customer. Scott Certificates of Analysis require actual analysis and backup data to substantiate concentration and accuracy claims.

Certified Concentration
The concentration printed on the certificate. The numerical concentration assigned to a component that is printed on the Certificate of Analysis or Accuracy. This is the concentration represented to the customer for that component and the concentration the customer is supposed to use. The certified concentration may be either the blended or analyzed concentration.

CFC's
Chlorofluorocarbons; Chloro/fluoro-substituted hydrocarbons known to deplete the Earth's ozone layer when released into the environment.

CGA Number
Cylinder/container valve outlet connection number assigned by the Compressed Gas Association. CGA numbers are detailed in CGA Standard V-1.

CGA (Compressed Gas Association)
A trade organization for the gas industry, CGA is a nonprofit technical association whose membership includes many corporations active in all phases of compressed gas industry. Founded in 1913, the CGA uses the 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. The CGA also recommends cylinder valve outlet connections for specific gas services based on safety considerations.

CGA/DIN Fittings
CGA is the abbreviation for Compressed Gas Association which is the group that established standards in the gas industry for fittings which are used to attach to gas cylinders. The DIN system is European (established by Germany) and used by the majority of Europe.

Coefficient of Flow (C)
Defined as the actual flow performance in US gallons of water per minute at 60°F (16°C) when inlet pressure (P1) is 1 psig (0.1 bar) and outlet pressure (P2) is atmospheric 14.7 psia (1 bar).

Component
One of the gases in a mix. A constituent of a gas mixture, either "minor component" or "balance." A two-component gas mixture consists of one minor component and a balance material. There is no such thing as a one-component mixture -- this is referred to as a "pure" gas or material.

Compressed Gas
Any material or mixture having either an absolute pressure exceeding 40 psia (2.72 bar) at 70°F (21°C) or an absolute pressure exceeding 104 psia (7.07 bar) at 130°F (54°C).

Compressed Gas Association, Inc. (CGA)
A trade organization for the gas industry, CGA is a nonprofit technical association whose membership includes many corporations active in all phases of compressed gas industry. Founded in 1913, the CGA uses the 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. The CGA also recommends cylinder valve outlet connections for specific gas services based on safety considerations.

Concentration
The relative quantity of a component in a gas mixture. The ratio or proportion of a given component to the total quantity of gas mixture. The ratio is expressed in terms of concentration units (i.e. % or parts per hundreds, ppm or parts per million, ppb or parts per billion) and concentration basis (i.e. mol/mol, weight/weight). Scott Specialty Gases states concentrations on a mole basis, unless otherwise specified.
For example: 10 ppm nitric oxide in a balance of nitrogen means that there are 10 moles of nitric oxide per million moles of total mixture of nitric oxide and nitrogen.

Concentration Basis
The basis for the units of concentration. The basis or "counting" units for expressing the ratio of a component to the total gas mixture (i.e. mole per mole abbreviated as mol/mol, weight per weight abbreviated as w/w, or volume per volume abbreviated as v/v). Example: A Certificate of Analysis stating "Component X: 100 ppm +/-2% (mol/mol)" means "for every 1,000,000 total moles of mixture contained within this gas cylinder, 100 moles (no less than 98, no more than 102) are Component X."

Confirmation
A limited measurement process after blending to assure that blended concentrations are correct. A process designed to demonstrate that a component of a gas mixture is present at or near the blended concentration, and to provide sufficient information to support the claim that the blended concentration is correct. Confirmation is less rigorous than analysis because, by design, it does not have sufficient work content to estimate the true value of the component concentration or to determine the accuracy.

Contaminant
An undesired component in a pure gas or gas mixture.

Corrosive
Any gas that chemically attacks materials with which it comes in contact (i.e. metals or skin).

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

Creep
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 in the regulator seat causing the gas pressure regulator to remain slightly open henceforth additional outlet pressure.

Critical Pressure
The pressure exerted by a material at the critical temperature.

Critical Temperature
The temperature above which a gas cannot be liquefied by pressure alone.

CRM (Certified Reference Material)
A gas reference standard available from NMi. CRMs used to be available from US EPA/NIST, but have been phased out in favor of NTRMs.

Cryogenic
Refers to the field of low temperatures, usually -130°F (-90°C) or below, as defined by 173,300(f) of Title 9 of the Code of Federal Regulations.

Cryogenic Liquid Container
An insulated container designed to store, handle and transport liquids having boiling points below -130°F (-90°C).

Cylinder
A container designed to hold compressed gases or liquefied compressed gases. Cylinders are manufactured and tested according to US DOT specifications.

Cylinder Regulator
A cylinder regulator requires an inlet CGA that must be compatible with the mating fitting on the cylinder valve. A typical cylinder regulator is supplied with an inlet and an outlet pressure gauge.

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

Delayed (Chronic) Health Hazard
Chemicals that cause an adverse effect to a target organ which manifests itself after a long period of time following or during repeated contacts with the substance. Includes carcinogens and other hazardous chemicals.

Delivery Pressure
See "Outlet Pressure."

Density
The mass exerted by a given volume of a material. It is usually expressed in pounds per cubic foot (lbs/ft3). For gases, density is greatly affected by temperature and pressure. See also "SPECIFIC GRAVITY."

Dew Point
The temperature at which condensation or formation of a liquid phase begins. The term most commonly applies to condensation of moisture from water vapor in the atmosphere.

DIN (Deutsche Norm)
A standard from the Deutsche Institut fur Normung. DIN 477 recommends cylinder valve outlet connections for specific gas services based upon safety considerations.

DOT (Department of Transportation)
A US government body that regulates transportation of hazardous materials.

Droop
The decrease in outlet pressure of a gas pressure regulator which results from an increase in flow rate. Essentially the reverse of lockup (also see "LOCKUP").

Dual-Analysis
Scott dual-analyzed products are subjected to two full, independent analyses on two different occasions.

Dual-Certification
Both blend and analyzed concentrations are reported and agree with each other. Scott dual-certified gas mixtures are supplied with both the blended concentration (accompanied by the calculated blend process accuracy) and an analyzed concentration (also accompanied by the calculated analytical accuracy). For dual-certified gas mixtures, Scott also requires that the blended and analyzed concentrations agree with each other to within the bounds of the least restrictive accuracy (i.e. the concentrations are "interlocked"). By providing both values, gas mixtures effectively have two certifications ("dual-certified") and an extra assurance of mixture reliability is thereby provided to the customer.

EEC
European Economic Community

EIGA (European Industrial Gases Association)
A technically oriented organization representing European and non-European companies producing and distributing industrial gases. EIGA works toward achieving safety and environmental care in the handling of these industrial gases, and maintains communication with standardization, regulatory, trade and industrial organizations.

Expiration Period
The stated date after which a gas mixture is no longer valid or legal to use for calibration purposes.

Exposure Limits
Concentrations of substances (and conditions) under which it is believed that nearly all workers may be repeatedly exposed day after day without adverse health effects. ACGIH limits are called TLV and OSHA exposure limits are called PEL.

Fill Density
The relationship expressed in percent between the weight of a gas in a container to the weight of water that the container will hold at 60°F (16°C).

Fire Hazard
Including flammable, combustible, pyrophoric and oxidizer.

Flammable Limits
The upper and lower concentration limits for a flammable gas, above and below which flame propagation does not occur upon contact with a source of ignition. Flammable limits are calculated at ambient temperature and pressure in the air.

Flash Point
The temperature at which a flammable liquid evolves enough vapor to form an ignitable mixture with air.

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

Flow Rate
A quantity of gas or liquid passing through a controlled orifice during a specific time period. Units of measure include: SLPM (Standard Liters per Minute) and SCFH (Standard Cubic Feet per Hour).

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.

Gas Phase
Pure materials or gas mixtures in which all components remain in the vapor state within the conditions (i.e. temperature) recommended for the product.

GMIS (Gas Manufacturers Intermediate Standard)
A gas reference standard made by a gas supplier and certified according to the US EPA protocol rules for GMISs. This term is frequently used in the gas industry to mean any generic gas reference standard, but Scott restricts the meaning to only those standards (gas mixtures) made by the US. EPA GMIS procedure.

Gravimetric
A proces of manufacturing gas blend that uses the weight of each gas added to control the concentration of the final mixture.

HCFC's
Hydrochlorofluorocarbons; Partially chloro/fluoro-substituted hydrocarbons known to deplete the Earth's ozone layer when released into the environment.

HFC's
Hydrofluorocarbons; Partially fluoro-substituted hydrocarbons known to deplete the Earth's ozone layer when released into the environment.

High Flow
The flow of media at high velocity. The discharge of a large amount of media over a period of time.

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

Immediate (Acute) Health Hazard
Including highly toxic, corrosive, toxic irritant, sensitized and other hazardous chemicals that cause an adverse effect to a target organ which manifests itself within a short period of time following a one-time, high exposure to the substance.

Impurity
An additional or extra component of a pure gas or gas mixture. Impurities are most commonly encountered in pure material used as the raw material source for a component of a gas mixture. An impurity may be removed by purification. Alternatively, the impurity may be measured and accounted for during blending, thereby preventing it from becoming a contaminant.

Inert
A component with no uncontrolled chemical incompatibility with other components or with the container materials of construction. Examples of inert compounds include nitrogen, helium, carbon dioxide and methane.

Inlet Pressure
Always the pressure at the inlet of the gas regulator from the cylinder.

Inlet Pressure (P1, Supply Pressure, Upstream Pressure)
The pressure of the fluid to the supply connection of a control element.

Interlocking
Interlocked concentration values (interlocking features). The requirement that the two certified concentration values provided by Scott's dual-certified Universal and Reference Classes agree within ±1% of each other. Dual-certified calibration standards (gas mixtures) receive two rigorous, independent certifications: one during blending using the GravStat™ process and the other using proprietary laboratory analytical procedures. The interlocking requirement assures that all dual-certified calibration gas mixtures are of the highest accuracy.

Isotope
A substance having a different mass to the naturally occurring element

Limited Use
Short term use.

Line Regulator
A line regulator, as the name implies, is installed in a gas line to provide a means to further reduce the gas line pressure prior to its end use point. The inlet connection supplied with the Scott Specialty Gases' regulator is a compression fitting, but some suppliers offer a line regulator with either male or female pipe fittings. A typical line regulator is supplied with an outlet gauge.

Liquefied Compressed Gas
A gas that is partially liquid at its charging pressure and a temperature of 70°F (21°C).

Liquid Phase
Pure materials or gas mixtures in which one or more components exist in a liquid state under the conditions (i.e. temperature) recommended for the product. Liquid phase materials can be stored in equilibrium with the gas phase (conventional cylinder) or forced to an all-liquid state (piston cylinder).

Lockup
The increase in outlet pressure of a gas pressure regulator that occurs when flow is stopped. Essentially, the reverse of droop. Also see "DROOP".

Low Flow
A reduced amount of volume over time. The flow of media at a low velocity.

Lower Explosive Limit (LEL)
The Lower Explosive Limit of a flammable gas or vapor (% by volume in air) in which explosion can occur upon ignition in a confined area.

Manifold
A series of connectors to a common outlet allowing several gas cylinders to be used simultaneously.

Matrix
For a given component, the matrix is the "background" of all gases present in the gas mixture. This term is most frequently used to contrast two gas mixtures with a common minor component but with different combinations of other components which affect blending or analysis. "Matrix matching" is one technique that can be used to overcome interferences caused by matrix effects. Example: Mixture A contains 500 ppm of NO in a nitrogen balance, and Mixture B 500 ppm of NO and 20% CO2 and a nitrogen balance. The matrix for Mixture A is nitrogen, whereas the matrix for Mixture B is nitrogen with 20% CO2. The analysis for NO in these two gas mixtures is radically different because of CO2 interference.

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

Mean
The average of a set of replicate measurements. The replicate measurements should be made under similar conditions so that the mean value is a representation of the true value.

Minor Component
A component making up less than half of a gas mixture and for which specifications have been requested by the customer.

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

Mole
For a given molecule, one mole is the mass numerically equal to its molecular weight. A gram mole is the mass in grams equal to the molecular weight. A pound-mole is the weight in pounds equal to the molecular weight.

Molecular Weight
The sum of the atomic weights of all the constituent atoms in the molecule.

MSDS (Material Safety Data Sheet)
MSDS provides users with technical and safety information on pure chemicals or mixtures of chemicals. The information contained in an MSDS includes: physical and health hazards, physical properties, emergency response, storage and handling precautions, and regulatory information. MSDSs are used on the job or in emergency situations when the hazardous properties or health effects of a chemical are needed.

NEN (Nederlandes Norm)
A standard from the Dutch Normalisation Institute. NEN 3260 recommends cylinder valve outlet connections for specific gas services based on safety considerations.

NIST (National Institute of Standards and Technology)
A government agency, part of the US Department of Commerce, that is responsible for establishing and providing standards of all types, including gas reference standards.

NMi (Netherlands Measurement Institute)
A Dutch government agency that is responsible for establishing and providing standards of all types, ncluding gas reference standards. NMi fulfills a role in Europe similar to NIST in the United States.

NPL (National Physical Laboratory)
A government agency that is responsible for establishing and maintaining measurement standards in the United Kingdom. NPL fulfills a role in Europe similar to that of NIST in the United States.

NTP (Normal or Ambient Temperature and Pressure)
A gas industry reference base. 68°F (20°C) and 1 atm (760 torr).

NTRM (NIST-Traceable Reference Material)
A gas reference material (gas mixture) manufactured by a commercial gas supplier but certified by NIST. NTRM gas standards are equivalent to SRMs and are used to analyze and certify commercial gas mixtures, such as EPA protocols.

ODP
Ozone depletion potential; a term used to indicate the relative rate of ozone depletion of CFC's, HCFC's and HFC's

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

Outlet Pressure
Always the pressure at the outlet of the gas regulator. Outlet Pressure (P2, Delivery Pressure, Downstream Pressure) The pressure of the fluid from the discharge connection of a control element.

Oxidant
A material that supports combustion.

Partial Pressure
The vapor pressure exerted by one component of a gas mixture. In any gas mixture, the total pressure is equal to the sum of the partial pressures that each gas would exert were it alone in the volume occupied by the gas mixture.

Phase Envelope
A graph of the dew point and bubble point temperatures for a gas mixture over a range of pressures.

Polar
Descriptive of a molecule in which the positive and negative electrical charges are permanently separated, as opposed to nonpolar molecules in which the charges coincide. Polar molecules ionize in solution and impart electrical conductivity.

Precision
The closeness of repeated measurements. The degree of variation of repetitive, equivalent readings of measured value. Precision is one of the critical contributors to overall analysis accuracy, and must be known and controlled during analysis. Precision is commonly expressed in terms of CV% (Coefficient of Variation %) or RSD% (Relative Standard Deviation %).

PRM (Primary Reference Material)
The highest pedigree gas reference material manufactured and certified by NMi. NIST and NMi have declared mutual equivalence for most PRMs and SRMs, and PRM gas standards (gas mixtures) can be used interchangeably with SRMs in certain applications.

Process Accuracy (Blend Accuracy)
The difference between the blended concentration and the true concentration. The degree of statistical agreement between the blended and the true concentration for a component of a gas mixture (usually at the 95% confidence level), or the certainty or "sureness" with which the blended concentration is known. Blend accuracy can be calculated because Scott gravimetric measurement requires actual data for scale calibration, resolution and weight uncertainties.

Proof Pressure
A test pressure applied to control devices to verify structural integrity. No deformation or excessive leakage is permitted at this pressure and the control element must function normally subsequent to this test. Normal industry standard is 1.5 times (150%) of working pressure. See also "BURSTING PRESSURE" and "MAXIMUM OPERATING PRESSURE."

PTB
Physikalisch-Technische Bundesanstalt. A government agency that is responsible for establishing and providing standards of all types in Germany, including gas reference standards. PTB filfills a role similar to NIST in the USA, NPL in the UK and NMi in the Netherlands.

Random Error
Normal variation in measurements. The natural variation within a measurement process that causes measured values to deviate from each other to some degree. Repetitive measurements that consist of only random errors (no bias errors) will usually form a normal (Gaussian or bell-shaped) distribution around the true value.

Reactive
A component with known chemical incompatibility with other components or with the container materials of construction. Examples of reactive components include sulfur dioxide, alcohols, hydrogen sulfide, ethylene oxide and chlorine.

Reactive Hazard
Including unstable reactive, organic peroxide, and water reactive.

Reference Standard
A material for which the true value is known. The origin of, or the transfer vehicle for, the true value of a measured quantity. Analysis can only be performed (and the true concentration for a component determined) using a reference standard (gas mixture) with an established traceability. The highest level of reference standards originate from standardization agencies (NMi, NIST) and include items like PRMs, SRMs and Class S Weights. Lower level reference standards include retained internal cylinders (i.e. GMIS cylinder) or temporary standards such as flasks and permeation tubes.

Relief Valve
A type of pressure relief device which is designed to relieve excessive pressure, and to reclose and reseal to prevent further flow of gas from the cylinder after reseating pressure has been achieved.

Reproducibility
The degree of agreement between the concentration of an old gas cylinder and a new cylinder. The degree of agreement between the concentration of an old cylinder, used as a standard, and the concentration of a new cylinder, analyzed as an unknown. Agreement between these two gas cylinders verifies that the calibration of the instrument used to make the comparison is still valid and no recalibration is required.

Requested Concentration (also Ordered Concentration)
The concentration of a component that the customer asked for or ordered.

Resolution
The extent to which an instrument or device is capable of distinguishing closely adjacent responses.

Safety Relief Device
A safety device is usually incorporated in a gas cylinder valve that is actuated by excessive pressure or temperature. The safety relief device fails at predetermined limits to avoid failure of the pressure vessel (cylinder).

scottgas.com
The specialty gas industry's most extensive website feature: online purchasing of all Scott products including pure gases, gas mixtures and gas handling equipment; Internet-based supply chain management; online access to Certificates of Accuracy and Material Safety Data Sheets; downloadable product information; technical and safety data information; and a Technical Forum hosted by Scott's Technical Services Group.

SCOTTY® Express
SCOTTY® Express same day shipping (for orders received by 2:00 pm EST ) is available on many SCOTTY nonreturnable cylinders, with a maximum of six (6) cylinders per gas type, via Federal Express or ground carrier.

Self-Relieving (Self-Venting)
A feature incorporated in certain pressure reducing regulators that enables the unit to relieve the outlet pressure when adjusted in the decrease direction.

Set Points
The control point desired for operation of a gas pressure regulator.

Shelf Life
The length of time a gas mixture is guaranteed. The length of time over which Scott will guarantee that components of a gas mixture remain at their certified concentrations within the specified tolerances. A shelf life claim is based upon Scott history and experience, as well as stability studies (an expiration date may be based upon Scott history/experience or is arbitrarily set by regulations, i.e. EPA protocols).

Single-Certification
Either the blend or the analyzed concentration is reported as the certified concentration. Scott single-certified gas mixtures are supplied with either the blended concentration (accompanied by the calculated blend process accuracy) or the analyzed concentration (accompanied by the calculated analytical accuracy). For single-certified gas mixtures, Scott requires that sufficient work is completed so that the blended or the analyzed concentration can be reported as a stand-alone value. There is no requirement for "interlocking" the blend and analyzed concentrations. Single-certified provides reliable and cost-effective gas mixtures for customers.

Single-Stage Pressure Reducing Regulator
A gas regulator that reduces high-pressure to low-pressure and controls the low or outlet pressure with one stage of pressure reduction.

Specific Gravity (Sp. Gr.)
The ratio of the weight of a given volume of material to the weight of an equal volume of another substance used as a standard. For solids or liquids, the standard is usually water (H2O = 1) and for gases, the standard is air (air = 1).

Specific Heat
The amount of heat required to raise the unit weight of a material one degree of temperature at constant pressure.

Specific Volume (Sp. Vol.)
The volume of a unit weight of a material at a given temperature and pressure (usually expressed in cubic feet per pound or gallons per pound). For gases, specific volume is greatly affected by temperature and pressure.

Spring Housing
See "BONNET."

SRM (Standard Reference Material)
The highest pedigree gas reference material (gas mixture) manufactured and certified by NIST. The NIST SRM is equivalent in stature to the NMi PRM.

Stability
How long a gas mixture is actually good. The time over which components of a gas mixture will remain at their certified concentrations within specified tolerances.

Stainless Steel
Alloy steels containing high percentages of chromium from less than 10% to more than 25%.

Standard Deviation
The "average" deviation of measurements from the mean value. A measure of the amount of scatter of a group of replicate measurements around the mean value. The larger the standard deviation, the more scattered the individual measurements are around the mean value. A larger standard deviation also means that the precision of the set of measurements is lower (worse), and the accuracy of the overall measurement is worse.

Standard Industry Methods
For certain applications or at a customers request, an additional analysis may be performed that conforms to a standard, non-Scott procedure preferred or accepted by that application or customer. Example: By special arrangements with US EPA, the Scott reference standards used to certify Protocol 11 H2S mixtures are analyzed using EPA Method 11, the official EPA classical chemistry technique for H2S.

STP (Standard Temperature and Pressure)
An internationally accepted reference base. 32°F (0°C) and 1 atm (760 torr).

Sublimation
The process by which a material passes from a solid state directly to a gaseous state without becoming liquid.

Sudden Release of Pressure Hazard
Including explosive and compressed gas.

Tare Weight
The weight of an empty gas cylinder without cap and valve.

TechniMate®
Instrument support gases developed exclusively by Scott that are fully certified and guaranteed to meet instrument manufacturers' specifications.

Tied-Diaphragm
See "Attached Poppet"

Tied Seat
See "Attached Poppet"

TLV - TWA (Threshold Limit Value - Time Weighted Average)
The time weighted average concentration of an airborne substance that nearly all workers may be exposed in a normal eight hour day, five day work week, without suffering adverse side effects.

Tolerance
The difference between the "measured" and "specified." The difference between a measured value and a Scott or customer specified value, such as requested concentration.

Toxic
The property of a material that may chemically produce injurious or lethal effects to humans.

Traceability
The pathway (lineage) back to the true value. The relationship between a measured value and an established element of the National/International Measurement System. Also the basis for establishing the true value of a standard. For traceability to exist, there must be an unbroken chain of comparisons between the sample and the National/International Measurement System. Elements of the National/International Measurement System can be fundamental units of measurements (i.e. weight, temperature, pressure) or secondary/transfer standards (i.e. synthetic gas standards).

True Value
The "real" or perfectly accurate value. The value that would be obtained by measuring 100% of all available samples using a measurement process with no random or systematic (bias) variations at all. The true value is, therefore, a theoretical value because all measurement processes have some variations regardless of how small. Accuracy is used to state how close a given measurement process approaches the true value.

Two-Stage Pressure Reducing Regulator
A gas regulator which reduces high pressure to low pressure and controls the low or outlet pressure with two stages of pressure reduction. Used when more stability of operation is required.

Vapor Pressure
The pressure exerted when a solid or liquid is in equilibrium with its own vapor at a particular temperature.

Wetted Parts
This term refers to the area of the device which comes in contact with the flow stream. This does not normally include the cap, range spring knob, etc.

Working Pressure
See "MAXIMUM OPERATING PRESSURE."

Zero Blend Tolerance
The certified and requested concentrations are the same. The certified concentration and the requested concentration for a component are the same within claimed accuracy limits. In order for a gas mixture to be certified with the requested concentration, the certified concentration must be so close to the requested concentrations as to be within +/-50% of the allowed accuracy specification.