Gas mixtures – specifically for your application

The number of possible gas mixtures is,assuming technical feasibility, virtually unlimited,just like the individual applications. Messer’smany years of experience and its employees’high level of expertise in development,production and analysis, ensure that we canalways offer our customers the high standardsof quality they expect.

Types of gas mixturesFor routine applications in a wide range of areas– from banana ripening and laser applicationsthrough to the operation of ionization chambers– we provide standard gas mixtures. Thanks totheir constant composition, these mixtures canbe produced in batches and delivered fromstock. Details of the different standard gasmixtures can be found in the relevant datasheets.

Individual gas mixtures are required for manyapplications, for instance in order to check orcalibrate measuring instruments. The intendeduse determines the composition and number ofcomponents. The mixtures are produced onuser’s request provided the physical andchemical possibility and the compliance with therelevant safety regulations.

Messer has its main European filling plants forspecialty gases and gas mixtures in Machelen(Belgium), Mitry-Mory (France), Lenzburg(Switzerland), Gumpoldskirchen (Austria) andBudapest (Hungary).

1) Information on the description of the gas    mixtures and the cylinder identification2) Main properties of the gas mixtures3) Valve connection and recommended fittings4) Product specifications and standard delivery forms    available

DefinitionsGas mixtures are homogenous mixtures ofdifferent gases or vapors. The multitude ofavailable substances gives rise to an almostunlimited number of possible combinations.However, the producibility of a gas mixture islimited by chemical, physical and safetyrestrictions. The components are the gases andvapors making up a mixture. The carrier orbalance gas is the main component of a mixture.

The concentration can be expressed in differentmeasurement units. The amount of substance(ppm) is often used, as this unit is pressure andtemperature independent. Also widespread arevolume content and mass concentration. Thesepressure and temperature dependent units areusually based on standard conditions of 0°C and1013 mbar.

Contentexpressed

Substancecontent

Volumecontent

Massconcentration

Masscontent

SI-Unit mol/mol m³/m³ kg/m³ kg/kgMol% Vol% g/m³ Wt%

Usual measures ppm vpm (ppmv) mg/m³ g/kgµ mol/mol (ppb) µ l/m³ (ppbv) µ g/m³ mg/kg (ppmw) ni Vi mi mi

Formulas xi = ----------------- ϕ i = ----------------- β i = ----------------- wi = ---------------- nG VG VG mG

with: ni, Vi, mi, = quantity of material, volume, mass of component inG, VG, mG = quantity of material, volume, mass of mixture

The tolerance describes the permitted deviationof the actual concentration (actual value) of acomponent from the required concentration(target value). Depending on the process, theproduction tolerance is normally about 3%-10%relative, depending on the concentration range aswell as type and number of components.

The actual value of a component can only bestated with a certain accuracy. In mathematicalterms, the uncertainty is defined with the formulaU = k x s, where s is the standard deviation and kis the factor for the “expanded uncertainty”.Messer uses the value k = 2 for determining theexpanded uncertainty. The necessary analyticalprocess is chosen depending on the type andquantity of a component. The achievableaccuracies range between 1-10% relative,depending on the process. With correspondinglyelaborate production processes, accuraciesbelow 1% relative (Topline) can be achieved.

Test gases are used for calibrating measurementinstruments. The content of a gas cylinder oftenlasts for many months. Therefore the stabilityperiod specifies the time from the date ofmanufacture, for which the actual value in thecertificate applies. Usually this period is 12months, although longer stability periods are, byall means, possible (Longlife Option).

In this context, the internal treatment of the gascylinders plays a crucial role. The production ofstable test gases is only possible throughthorough and consistent cylinder pre-treatmentwith extensive purging and evacuation cycles athigh temperatures as well as appropriateconditioning procedures.

Tolerance and accuracy of gas mixtures based onexample of 90 ppm NO Topline (tolerance +/-5%,accuracy +/-1%).

Target value (90 ppm) Tolerance +/- 5%

Actual value (88,5 ppm)Accuracy +/- 1%

Concentration(ppm)

Mixture categoriesIn accordance with the different gas mixturerequirements, Messer offers the products in avariety of mixture categories which define thetolerance, accuracy and stability period:

TypAccuracy(% rel.)

Tolerance(% rel.)

ConcentrationStability(months)

Tecline no certificate 2-10 % 1-100%Traceline 5 % 10 % 5-1000 ppb < 12Labline 2 % 5 % 1 ppm-100 % 12Topline <1 % <5 % 10 ppm-100% 12Longlife option 24/36/60: prolonged stability period (24/36/60 months)Accredited option: with calibration certificate from an ISO 17025 accredited laboratory

Tecline mixtures are supplied in accordance witha standard specification without a certificate.Typically, Tecline mixtures are used as operatingor process gases. The Labline category consistsof test gases with a certificate. The tolerance is5% and the accuracy is usually 2%. For highprecision measuring work, we recommendcalibration with Topline mixtures with anaccuracy of better than 1%. For trace and ultra-trace analysis, we offer the Traceline categorywith concentrations in the ppb range.

Production of gas mixturesDynamic processes allow to carry out massproduction and filling of standard gas mixtures.This involves controlling two or more volumeflows of components and carrier gas via massflow controllers, homogenizing them in a mixingchamber, if necessary analyzing the mixture afterthe mixing chamber and compressing it into thecylinders. The composition of the mixtures in abatch is identical within very low variances.Dynamic production is therefore the method ofchoice for standard mixtures (Tecline).

With the manometric method of filling gascylinders, the partial pressures of thecomponents are added together in accordancewith Raoult’s Law. This procedure involvesmeasuring the pressure increase in the cylinderduring and after the addition of each mixturecomponent at a defined temperature.

The production tolerance mainly depends on theaccuracy of the pressure gauges andtemperature measurement. The advantage ofthis method is the high level of flexibility: allmixture types can be produced when the partialpressure reaches a measurable magnitude. Thedisadvantage of this method is the systematicallylower process accuracy. A subsequent analysisof individual cylinders generally allows a muchmore accurate determination of the actual valueof the component. That is why the analysisvalues and their uncertainty are certified.

With the gravimetric method, the individualcomponents are weighed out. The masscontents are determined directly, which can thenbe converted into substance contents. Theweighing process is one of the most accuratephysical measuring processes that exist. That iswhy high precision gas mixtures can be producedby this method. Quantitative control analysisusually does not achieve these levels ofaccuracy. It is only used to confirm the processparameters. Certificates are issued for the valuedetermined by gravimetric weighing and itsuncertainty.

Manometric gas mixture production

If the required accuracy is not achievable bydirect dosing of the component (e.g. because oflow contents of light substances), then one ormore gravimetrically produced pre-mixtures withhigher contents of the required component areused in order to produce the final mixture.

HomogenizationImmediately after gravimetric or manometricfilling, it is possible that the individualcomponents in the gas cylinder will form intolayers. A homogenous mixing of the componentsis ensured by rolling the cylinders in an almosthorizontal position.

Analytical processesThe analytical tools for determining thecomposition of gas mixtures and test gasescover a broad spectrum of physical and chemicalanalytical methods. They range from monitoranalysis and infrared and ultraviolet spectrometryto gas chromatography and mass spectrometry.

For multi-component mixtures gaschromatography is used. With its manydifferent versions of separation columns anddetectors this method offers suitable solutionseven for very specific analyses. For chemicallyreactive gases conductivity measurement,potentiometry and the classical methods oftitrimetry are used.

In general, the achievable analytical accuracy is inthe range of ±2% relative. Over and above this,special, highly developed calibration methods canbe used to achieve measurement accuracies of±1%.Monitor analysis (non-dispersive IR or UVspectrometry, chemoluminescence, sum-FID)makes it easy to measure certain components,for example IR-active ones, very quickly andaccurately.

TraceabilityMost of the methods that are used forquantitative analysis are relative methods.Appropriate measures must be taken to ensurethat the results achieved can be traced back torecognized standards. With gravimetricquantitative analyses, this is done by calibratingthe scales used with certified weight standards.This means that the results obtained with thesescales, i.e. the quantities of gravimetricallyproduced gas mixtures, can be traced backdirectly to the national mass standard of therelevant producer country.

Analytical quantitative analysis is based on thecalibration gases used in our laboratories. Onlyhigh-precision, gravimetrically produced gasmixtures are used for this. If possible, thesemixtures are confirmed through standardreference material (SRM) from third-partyproduction, e.g. NIST (National Institute ofStandards and Technology, USA), VSL (VanSwinden Laboratorium, Netherlands), BAM(Bundesanstalt für Materialforschung und-prüfung, Germany). If quantitative analysis isperformed by means of a direct comparison withthis standard reference material, the products areregarded as directly traceable to this standard.

Gravimetric production

Analysis of gas mixtures

CertificatesAll individually produced gas mixtures aresupplied with a certificate. This contains all theimportant information regarding the mixturecomponents and the composition (content anduncertainty) as well as the cylinder. In reducedform, the certificate is attached as a label to eachmixed gas cylinder.The information on the certificate is inaccordance with the relevant national regulationsof the individual European countries or the ENISO standard.

Service and SupportThe range of applications is just as great as therange of possible gas mixtures. It is not alwayseasy to choose the right mixtures for a givenapplication. Technical feasibility and the potentialcosts are often a limiting factor.

Our customer consultants will be happy to helpyou choose the optimal solution for your specificrequirements. We look forward to hearing fromyou!

AccreditationThe reliability of the content information providedon a certificate of a calibration gas is a crucialquality criterion. The fact that our customers canrely on our information has been confirmed byindependent experts: Messer has several of itsanalytical laboratories accredited as testlaboratories by the relevant state authorities. InSwitzerland, for example, the relevant authority isMETAS (Metrology and AccreditationSwitzerland). An accreditation is a certification ofan analytical laboratory’s professionalcompetence by independent experts. The basisfor this is the internationally recognized qualitystandard EN ISO 17025. In addition to thestandard certificate from Messer, users can, onrequest, receive a standard-compliant calibrationcertificate from the accredited calibrationlaboratory for many gas mixtures (AccreditedOption).

Messer Group GmbHGahlingspfad 31

47803 KrefeldTel. +49 2151 7811-0

Fax +49 2151 7811-501info@messergroup.com

www.messergroup.com