1、BRITISH STANDARD BS EN ISO 6145-8:2008 Gas analysis Preparation of calibration gas mixtures using dynamic volumetric methods Part 8: Diffusion method ICS 71.040.40 BS EN ISO 6145-8:2008 This British Standard was published under the authority of the Standards Policy and Strategy Committee on 5 Septem
2、ber 2005 National foreword This British Standard is the UK implementation of EN ISO 6145-8:2008. It is identical with ISO 6145-8:2005. It supersedes BS ISO 6145-8:2005 which is withdrawn. The UK participation in its preparation was entrusted to Technical Committee PTI/15, Natural gas and gas analysi
3、s. A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard cannot confer immunity
4、from legal obligations. Date Comments This corrigendum renumbers BS ISO 6145-8:2005 as BS EN ISO 6145-8:2008 ISBN 978 0 580 60361 7 BSI 2009 28 February 2009 Amendments/corrigenda issued since publicationEUROPEAN STANDARD NORME EUROPENNE EUROPISCHE NORM EN ISO 6145-8 August 2008 ICS 71.040.40 Englis
5、h Version Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods - Part 8: Diffusion method (ISO 6145-8:2005) Analyse des gaz - Prparation des mlanges de gaz pour talonnage laide de mthodes volumtriques dynamiques - Partie 8: Mthode par diffusion (ISO 6145- 8:2005) G
6、asanalyse - Herstellung von Kalibriergasgemischen mit Hilfe von dynamisch-volumetrischen Verfahren - Teil 8: Diffusionsverfahren (ISO 6145-8:2005) This European Standard was approved by CEN on 30 July 2008. CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the
7、 conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN Management Centre or to any CEN member. This European Standard exists i
8、n three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as the official versions. CEN members are the national standards bodie
9、s of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom. EURO
10、PEAN COMMITTEE FOR STANDARDIZATION COMIT EUROPEN DE NORMALISATION EUROPISCHES KOMITEE FR NORMUNG Management Centre: rue de Stassart, 36 B-1050 Brussels 2008 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. EN ISO 6145-8:2008: EForeword
11、 The text of ISO 6145-8:2005 has been prepared by Technical Committee ISO/TC 158 “Analysis of gases” of the International Organization for Standardization (ISO) and has been taken over as EN ISO 6145-8:2008 by Technical Committee CEN/SS N21 “Gaseous fuels and combustible gas” the secretariat of whic
12、h is held by CMC. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by February 2009, and conflicting national standards shall be withdrawn at the latest by February 2009. Attention is drawn to the pos
13、sibility that some of the elements of this document may be the subject of patent rights. CEN and/or CENELEC shall not be held responsible for identifying any or all such patent rights. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries
14、are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sw
15、eden, Switzerland and the United Kingdom. Endorsement notice The text of ISO 6145-8:2005 has been approved by CEN as a EN ISO 6145-8:2008 without any modification. BS EN ISO 6145-8:2008 iiiContents Page Introduction v 1 Scope 1 2 Normative references . 1 3 Principle . 1 4 Reagents and materials 2 5
16、Apparatus. 3 6 Procedure. 4 6.1 Preliminary checks and operating conditions . 4 6.2 Determination of mass loss . 5 7 Expression of results 6 7.1 Calculation . 6 7.2 Sources of uncertainty . 7 Annex A (informative) Practical example of a diffusion cell calibrator configured for evaluating speed of re
17、sponse in a hygrometer 10 Annex B (informative) Example of performances of diffusion cells for toluene and trichloromethane. 13 Annex C (informative) Example of uncertainty calculations for a periodic weighing system 15 Bibliography . 19 BS EN ISO 6145-8:2008blank vIntroduction This part of ISO 6145
18、 is one of a series of International Standards that present various dynamic volumetric methods used for the preparation of calibration gas mixtures. In the lower part of the mole fraction range considered, it is difficult to prepare and maintain gas mixtures for example of certain organic or reactiv
19、e components in cylinders. This dynamic method has the advantage of a practically unlimited supply of calibration component, whereby adsorption effects can be reduced or even eliminated. If the complementary gas flow is measured as a gas mass flow, the preparation of calibration gas mixtures using d
20、iffusion is a dynamic-gravimetric method which gives contents in mole fractions. Principles for the measurement of the complementary gas flow are given in ISO 6145-1. BS EN ISO 6145-8:2008blank 1Gas analysis Preparation of calibration gas mixtures using dynamic volumetric methods Part 8: Diffusion m
21、ethod 1 Scope This part of ISO 6145 specifies a dynamic method using diffusion for the preparation of calibration gas mixtures containing component mole fractions ranging from 10 9to 10 3 . A relative expanded uncertainty of measurement, U, obtained by multiplying the relative combined standard unce
22、rtainty by a coverage factor k = 2, of not greater than 2 % can be achieved by using this method. By keeping the path between the diffusion source and place of use as short as possible, the method can be applied for the generation of low-concentration calibration gases of organic components that are
23、 liquid at room temperature, with boiling points ranging from about 40 C to 160 C. This part of ISO 6145 is applicable not only for the generation of calibration gas mixtures of a wide range of hydrocarbons at ambient and indoor air concentration levels, but also for the generation of low-concentrat
24、ion gas mixtures of water. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
25、 ISO 6145-7, Gas analysis Preparation of calibration gas mixtures using dynamic volumetric methods Part 7: Thermal mass-flow controllers 3 Principle The calibration component migrates by diffusion through a diffusion tube of suitable dimensions (length, diameter) into a flow of a complementary gas,
26、i.e. the complementary gas of the mixture prepared. The liquid calibration component, of a known high purity, is contained in a reservoir that acts as the source of the component vapour. The reservoir is provided with a vertically placed diffusion tube. This assembly (the diffusion cell) is placed i
27、n a temperature-controlled containment that is purged at a known and constant flow rate by a high-purity complementary gas (see Figure 1). The composition of the mixture is determined from the diffusion mass flow of the calibration component and the flow rate of the complementary gas. The diffusion
28、mass flow rate of the calibration component in principle depends on its diffusion coefficient in the complementary gas, its vapour pressure at the temperature of the containment, the dimensions of the diffusion tube. BS EN ISO 6145-8:20082 Accurate determination of the mass flow rate is achieved by
29、either continuous weighing, after mounting the tube in a suspension microbalance, or by periodic weighing. The method of determination affects the uncertainty of the (momentary) mass flow of the calibration component (see 7.2). Key 1 complementary gas inlet 2 containment 3 diffusion tube 4 liquid re
30、servoir 5 calibration gas outlet Figure 1 Schematic of diffusion apparatus 4 Reagents and materials 4.1 Liquid substances to be used as calibration component, of the highest possible purity so as to avoid any effects on the diffusion mass flow. If possible, the nature and quantities of the impuritie
31、s should be known and allowance made for their effects. 4.2 Complementary gas, of known purity, established by appropriate analytical techniques, e.g. Fourier- transform infrared spectrometry or gas chromatography. The nature of the complementary gas shall be adapted to the substance to be used as t
32、he calibration component. For example, air shall not be used as complementary gas for the preparation by diffusion of calibration gas mixtures of oxidizable substances. BS EN ISO 6145-8:2008 35 Apparatus 5.1 Diffusion apparatus 5.1.1 Materials The materials of the diffusion apparatus shall be chosen
33、 so as to avoid effects of physical or chemical sorption or desorption on the content of the calibration component. The smaller the desired content, the greater the effect of sorption/desorption phenomena. Diffusion reservoirs and tubes, as well as temperature containments and blending apparatus, sh
34、ould preferably be manufactured out of borosilicate glass. Choose chemically inert, flexible tube materials for the supply of complementary gas and transport of calibration gas mixture. Pay special attention to all junctions as possible sources of leaks. 5.1.2 Complementary gas flow configuration Be
35、fore the complementary gas reaches the diffusion cell, it is essential that its temperature be controlled to that of the diffusion cell containment. In order to achieve the uncertainty stated in Clause 1, the temperature in the containment should be controlled to within 0,15 K. The minimum flow rate
36、 of the complementary gas should be sufficient to remove all component vapour without saturation. The maximum allowable rate should be low enough to avoid convective transport of the calibration component vapour inside the diffusion tube. This maximum flow rate is dependent upon the geometry of the
37、diffusion apparatus. It is recommended to keep the Reynolds number of the complementary gas flow in the diffusion cell below 100. At a temperature of 25 C, the following condition should approximately be fulfilled: 3 1, 6 1 0 vd where v is the average linear velocity of the complementary gas, in met
38、res per second; d is the diameter, in metres, of the tubing through which the complementary gas flows. 5.1.3 Choice of temperature The choice of temperature depends on the diffusion cell characteristics and the diffusion mass flow rate required. To carry out temperature control, establish thermal eq
39、uilibrium within the diffusion cell at a value close to ambient temperature or at a temperature sufficiently above ambient so as to avoid effects of ambient conditions on temperature control. The use of a temperature slightly above ambient has two advantages: accurate control of temperature can more
40、 easily be achieved near ambient temperature, the temperature of the complementary gas can more easily be controlled. 5.2 Diffusion cells, consisting of a borosilicate glass reservoir capable of holding a sufficiently large quantity of the liquid calibration component, fitted with a diffusion tube.
41、Several design examples are given in Reference 1. In principle, Equation (1) can be applied for the prediction of diffusion volume flow rates 1and, conversely, for the calculation of approximate dimensions and temperatures of diffusion tubes and containments necessary for the generation of a given m
42、ass flow rate of the calibration component. v (A) ln V Ap qD L pp = (1) BS EN ISO 6145-8:20084 where q V (A) is the volume flow rate of component A, in cubic metres per second; A is the cross-sectional area of the diffusion tube, in square metres; L is the length of the diffusion tube, in metres; D
43、is the diffusion coefficient, in square metres per second; p is the pressure in the diffusion cell, in pascals or kilopascals; p v is the partial pressure of the calibration component, in pascals or kilopascals. If no data for the diffusion coefficients exist, methods for their calculation are given
44、 in the literature. The method of Fuller, Schettler and Giddings 2is the most successful, but errors of up to 25 % can easily occur. Data for the atomic and structural volume increments applicable to calibration component and complementary gases and vapours are given in Reference 4. To achieve the b
45、est performance, diffusion tubes should remain within the following dimensional ranges: L 0,03 m; ratio of L to diameter of diffusion tube 3; diameter: 0,001 m to 0,02 m. NOTE Units which operate on the diffusion principle are commercially available and provide calibration gas mixtures containing hi
46、ghly adsorptive vapours. An example of one such unit for the preparation of reference standards of humidity in volume fractions of 109and its performance details are given in Annex A. 6 Procedure 6.1 Preliminary checks and operating conditions Before assembling or filling a diffusion cell, the purit
47、y of the substance to be used as calibration component is to be assessed using an appropriate analytical technique (e.g. Fourier-transform infrared spectrometry or gas chromatography) so as to quantify any likely major contaminants. Periodically check the diffusion mass flow at a known, fixed temper
48、ature and complementary gas flow rate as an indication of stability of the calibration compound in the reservoir. If the diffusion mass flow drifts by more than 1 % per month, this may be an indication of the presence of impurities. In that case, the contents of the diffusion cell should be replaced
49、. When first placing the diffusion cell in its containment, allow the system to equilibrate before performing the first weighing so as to ensure constancy of the diffusion mass flow. Generally, a period of 24 h is sufficient. To change the content of the calibration gas mixture, adjust the complementary gas flow rate. Alternatively, the calibration gas mixture can be further diluted, and its conten
