BS-4559-7-1981 ISO-6349-1979.pdf

BRITISH STANDARD BS 4559-7: 1981 ISO 6349:1979 Methods for Preparation of calibration gas mixtures Part 7: Permeation method ISO title: Gas analysis Preparation of calibration gas mixtures Permeation method UDC 543.27:54 13 185:53.089.62 Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:17:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-7:1981 This British Standard, having been prepared under the direction of the Environment and Pollution Standards Committee, was published under the authority of the Executive Board and comes into effect on 30 April 1981 © BSI 11-1999 First published as BS 4559 March 1970 First part revision as Part 7 April 1981 The following BSI references relate to the work on this standard: Committee reference EPC/46 Draft for comment 78/53755 DC ISBN 0 580 11851 7 Cooperating organizations The Environment and Pollution Standards Committee, under whose direction this British Standard was prepared, consists of representatives from the following: Association of County Councils British Gas Corporation* British Industrial Measuring and Control Apparatus Manufacturers Association* British Railways Board British Steel Industry Chemical Industries Association Chemical Society Committee for Environmental Conservation Confederation of British Industry Consumer Standards Advisory Committee of BSI Council of Engineering Institutions Department of Energy (Energy Technology) Department of Industry (Warren Spring Laboratory)* Department of the Environment (Water Engineering Division including Water Data Unit) Electricity Supply Industry in England and Wales* Engineering Equipment Users Association Greater London Council Health and Safety Executive Institute of Petroleum* Medical Research Council Ministry of Agriculture, Fisheries and Food National Coal Board* National Consumers Council National Water Council Scottish Office Society of Chemical Industry* Trades Union Congress The organizations marked with an asterisk in the above list, together with the following, were directly represented on the Technical Committee entrusted with the preparation of this British Standard: Cement Makers Federation Department of Energy (Gas Standards) Department of Industry Institution of Gas Engineers Scientific Instrument Manufacturers Association Society of Motor Manufacturers and Traders Limited Amendments issued since publication Amd. No.Date of issueComments Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:17:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-7:1981 © BSI 11-1999i Contents Page Cooperating organizationsInside front cover National forewordii 1Scope and field of application1 2Principle1 3Example of applying the method1 4Accuracy3 Bibliography6 Figure 1 Examples of permeation tubes4 Figure 2 Example of a permeation apparatus4 Figure 3 Further example of permeation apparatus5 Publication referred toInside back cover Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:17:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-7:1981 ii © BSI 11-1999 National foreword BS 4559 was first published in 1970 under the title “Methods for the preparation of gaseous mixtures”. The present revision is being issued in Parts to incorporate methods prepared by subcommittee 1, Preparation and definition of gas mixtures for calibration, of Technical Committee 158, Gas analysis, of the International Organization for Standardization (ISO) and to provide for revision where necessary of the methods included in the 1970 edition. This Part of this British Standard, which has been prepared under the direction of the Environment and Pollution Standards Committee, is identical with ISO 6349:1979 “Gas analysis Preparation of calibration gas mixtures Permeation method”. A permeation method is described briefly in 4.3.5 of BS 4559:1970 and is superseded by this Part in greater detail. Terminology and conventions. The text of the International Standard has been approved as suitable for publication as a British Standard without deviation. Some terminology and certain conventions are not identical with those used in British Standards; attention is especially drawn to the following. The comma has been used throughout as a decimal marker. In British Standards it is current practice to use a full point on the baseline as the decimal marker. Wherever the words “International Standard” appear, referring to this standard, they should be read as “British Standard”. Additional information. With reference to the footnote to 3.3.4.2, guidance on sampling may be obtained from BS 5309 “Methods for sampling chemical products” Part 2:1976 “Sampling of gases” pending the publication of the International Standard, at present in course of preparation. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 6, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:17:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-7:1981 © BSI 11-19991 1 Scope and field of application This International Standard describes the principles of the permeation method, which is a continuous dynamic method, for the preparation of calibration gas mixtures in which the components that it is desired to use generally have a concentration between 109 and 105 (by volume), with an achievable accuracy of 2 % of the concentration of the component. (In practice, concentrations may also be expressed in micrograms per cubic metre). In the concentration range considered, it is difficult to maintain the gas mixtures in a stable state; it is therefore necessary to prepare the calibration gas just prior to its use, and to transfer it by the shortest path possible to the place where it is to be used. 2 Principle The principle of the method relies on the permeation of the component which is to be used (SO2, NO2, NH3, etc.) through an appropriate membrane into a flow of carrier gas which constitutes the complementary gas of the mixture obtained. The constituent, in a pure state, is contained in a tube1) (see clause 3), which is totally or partially swept at a known and controlled flow rate by the complementary gas, which is completely clean, purified and, in particular, free from any trace of the component, and thus becomes charged with molecules having diffused. The complementary gas shall be chosen so as to avoid any interaction with the materials constituting the permeation tube. The diffusion rate of the substance through the membrane depends upon the substance itself, the nature, constitution and area of the membrane, and the temperature and differences between the partial pressure of the gas inside and outside the tube. These factors can be kept constant by proper operation of the tube. In the present state of knowledge, no formula is available to describe this phenomenon exactly. If the diffusion rate is maintained constant, it may be determined by simple weighings at convenient time intervals (methods other than weighing have been proposed in recent literature, mentioned in the bibliography, but are not dealt with in this International Standard). To allow this operation to be carried out, taking account of the various parameters affecting the diffusion rate, it is necessary that the following requirements be fulfilled for a given tube containing a known substance: the tube shall be maintained at a temperature as constant as possible between two weighings; the outside of the tube shall be permanently swept by the carrier gas in order to maintain the partial pressure of the constituent under consideration outside the tube at a negligible value; the gas pressure inside the tube shall be maintained constant between two weighings, i.e. either the substance shall be partly in the liquid phase or the quantity of the substance in the tube shall be large compared to the mass lost by permeation. The concentration C of the calibration gas mixture so prepared is a function of the diffusion rate of the tube and the flow rate of the complementary gas. It is given by the formula 3 Example of applying the method 3.1 Examples of permeation tubes (see Figure 1) The tube contains a liquid phase and a gaseous phase; the membrane through which the diffusion takes place may be in contact with the liquid phase only, or with the gaseous phase only, or with both. However, unless it has been ascertained that the phase in contact has no effect on the permeation rate, the tube shall be used with the phase in contact identical to that used for the determination of the permeation rate. 1) The word “tube” is used whatever the shape of the vessel. It is also understood (except in 3.3.4) to include the tube contents. where qmis the permeation rate (mass flow rate) of the calibration component under given conditions; qVis the complementary gas flow rate (in appropriate units). C qm qV - -= Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:17:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-7:1981 2 © BSI 11-1999 3.2 Examples of permeation apparatus (see Figure 2 and Figure 3) It is recommended that, before use, the permeation tube be kept in an anhydrous atmosphere, in an airtight container in a cold place (at approximately 5 °C, for example in the bottom of a refrigerator) in order to reduce the diffusion rate, and hence the substance loss, to a minimum and to avoid any condensation on the tube. During use, the tube is placed in a flow system which enables the exterior of the permeation membrane to be swept by the carrier gas (for example pure nitrogen from a cylinder). The whole device is placed in a temperature controlled bath (gaseous or, preferably, liquid). The constant flow rate of the carrier gas may be achieved by a controlling system and monitored by a flowmeter. The value of the flow rate is determined by means of a gas meter. The existence of an outlet for surplus gas enables the analyser to take the gas flow rate necessary for its calibration or control. For special applications, it is possible to place in the same temperature-controlled bath, several permeation tubes respectively filled with different substances, provided that any interaction risk is avoided. 3.3 Operating precautions 3.3.1 Use of the permeation tube When first used, it is necessary to allow the system to reach a state of equilibrium over approximately 48 h before carrying out the first weighing, in order to ensure that the permeation rate is well stabilized. It is essential to control the temperature of the bath to within 0,1 K because the tube diffusion rate may for example double for an increase in temperature of approximately 7 K. NOTEUnder certain circumstances, in which the diffusing gas is highly soluble in the membrane polymer, an increase in temperature may reduce the permeation rate. The time interval between two weighings (of the order of several days) depends upon the required accuracy. It is recommended that the tube be weighed regularly (especially towards the end of its life) in order to ensure that its diffusion rate is constant. Weighing intervals shall be chosen so that the measured mass loss is normally at least 10 mg. The weighing should be made preferably under the same environmental conditions in which the tube is operated. Thermal shocks and adsorption of water during the weighing procedure must be avoided. During the period of use, it is preferable to leave the tube always at the same temperature, principally to avoid the delay, sometimes very lengthy, which is necessary to restore equilibrium. Thermal shocks should be avoided as far as possible. 3.3.2 Sweeping of the tube by the carrier gas According to the permeation tubes used, it is sometimes desirable to use perfectly pure and anhydrous carrier gases. Before it reaches the tube, it is essential that the temperature of the carrier gas be controlled at the chosen value; any system which enables the carrier gas to remain in the temperature-controlled bath for a sufficient period of time is satisfactory (beads, baffles, etc.) (see Figure 2). The dilution system may have one stage only (see Figure 2) or two stages; in this latter case, the tube can always be swept at a constant flow rate. In order to alter the concentration of the mixture obtained, it is recommended that this be achieved by adjustment of the carrier gas flow rate (avoiding any change of the tube diffusion rate by changing the temperature); in this case, equilibrium is rapidly obtained. 3.3.3 Choice of temperature The choice of temperature depends on the tube characteristics and the permeation rate required; in practice, temperature control should be carried out as follows: a) If control is effected only by heating, the temperature shall be sufficiently high for the ambient temperature variations not to affect it, for example 40 °C. b) If control is also effected by cooling, a temperature closer to or even below ambient temperature may be suitable. The choice of a temperature close to the ambient temperature has two advantages: 1) significant variations in the tube temperature are avoided during weighings, 2) the temperature of the carrier gas may be more easily controlled. 3.3.4 Choice of materials 3.3.4.1 PERMEATION TUBE A number of polymers having sufficient chemical and mechanical resistance may be used for the permeation membrane. In general, polytetrafluoroethylene (PTFE), polyethylene, polypropylene or a copolymer of tetrafluoroethylene and hexafluoropropylene (FEP) is used. Account should be taken of the variations of the material characteristics with change of temperature. Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:17:51 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-7:1981 © BSI 11-19993 3.3.4.2 PERMEATION APPARATUS The materials used for the permeation apparatus shall be chosen so as to avoid any alteration of the concentration of the calibration component by sorption (chemical or physical). The smaller the desired final concentration, the greater is the effect of the adsorption phenomena until establishment of a constant concentration. If possible, glass shall be used. Flexible tube materials and metals shall be chosen in relation to the constituent for calibration, especially with regard to the transfer of the gas between the permeation apparatus and the analyser2). 3.3.5 Choice of permeating substances The permeating substances shall be of the highest purity in order to avoid any effect of impurities on the permeation rate; if this is not possible, the nature and quantities o