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1、BRITISH STANDARD BS 4559-5.6: 1987 ISO 6145-6: 1986 Methods for Preparation of calibration gas mixtures Part 5: Dynamic volumetric methods Section 5.6 Sonic orifices ISO title: Gas analysis Preparation of calibration gas mixtures Dynamic volumetric methods Part 6: Sonic orifices UDC 543.27:54 13 185
2、:53.089.62 Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:27:44 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-5.6:1987 This British Standard, having been prepared under the direction of the Environment and Pollution Standards Committee, was published under the authority of the Board
3、of BSI and comes into effect on 31 December 1987 BSI 12-1999 The following BSI references relate to the work on this standard: Committee reference EPC/46 Draft for comment 85/51116 DC ISBN 0 580 16257 5 Committees responsible for this British Standard The preparation of this British Standard was ent
4、rusted by the Environment and Pollution Standards Committee (EPC/-) to Technical Committee EPC/46, upon which the following bodies were represented: British Ceramic Research Association British Coal British Gas plc Cement Makers Federation Chemical Industries Association Department of Energy (Gas St
5、andards) Department of Trade and Industry (Electronics Application Division) Department of Trade and Industry (Laboratory of the Government Chemist) Department of Trade and Industry (Warren Spring Laboratory) Electricity Supply Industry in England and Wales GAMBICA (BEAMA Ltd.) Institute of Petroleu
6、m Institution of Chemical Engineers Institution of Gas Engineers Society of Chemical Industry Society of Glass Technology Society of Motor Manufacturers and Traders Limited Water-tube Boilermakers Association The following bodies were also represented in the drafting of the standard, through subcomm
7、ittees and panels: British Compressed Gases Association British Laboratory Ware Association Amendments issued since publication Amd. No.Date of issueComments Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:27:44 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-5.6:1987 BSI 12-1999i Conte
8、nts Page Committees responsibleInside front cover National forewordii 1Scope and field of application1 2References1 3Principle of the method1 4Practical examples3 5Operating conditions4 6Experimental determination of concentrations and examination of sources of error4 Figure 1 Gas mixer with sonic n
9、ozzles discharging into mixing chamber (schematic)3 Figure 2 Gas mixer with several sonic nozzles per component (schematic)3 Table Relative variations of C1)2 Publications referred toInside back cover 1) In preparation. Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:27:44 GMT+00:00 2006,
10、Uncontrolled Copy, (c) BSI BS 4559-5.6:1987 ii BSI 12-1999 National foreword This Section of BS 4559 has been prepared under the direction of the Environment and Pollution Standards Committee. It is identical with ISO 6145-6:1986 “Gas analysis Preparation of calibration gas mixtures Dynamic volumetr
11、ic methods Part 6: Sonic orifices”, published by the International Organization for Standardization (ISO). This Part of ISO 6145 was prepared as a result of discussions in Technical Committee 158, Gas analysis, in which the UK has participated. BS 4559 was first published in 1970 as a single standar
12、d under the title “Methods for the preparation of gaseous mixtures”. The present revision is being issued in Parts to implement methods prepared by ISO/TC 158 and to provide for revision, where necessary, of the methods included in the 1970 edition which was withdrawn in 1981. Part 5 of this standar
13、d covers the preparation of mixtures using dynamic volumetric methods. This Section covers sonic orifices. Other Sections in this Part of BS 4559 are as follows. Section 5.1: Methods of calibration; Section 5.2: Volumetric pumps method2); Section 5.3: Periodic injections into a flowing gas stream; S
14、ection 5.4: Continuous injection method; Section 5.5: Capillary method2); Section 5.7: Mass flow controllers method2); Section 5.8: Diffusion method2). Terminology and conventions. The text of the International Standard has been approved as suitable for publication as a British Standard without devi
15、ation. Some terminology and certain conventions are not identical with those used in British Standards; attention is drawn especially to the following. The comma has been used as a decimal marker. In British Standards it is current practice to use a full point on the baseline as the decimal marker.
16、Wherever the words “International Standard” and “part 6 of ISO 6145” appear, referring to this standard, they should be read as “British Standard” and “Section 5.6 of BS 4559” respectively. 2) In preparation. Cross-references International StandardCorresponding British Standard BS 4559 Methods for p
17、reparation of calibration gas mixtures Part 1 Weighing methods ISO 6142:1981Section 1.1:1983 Mixtures containing components fully vaporizable under ambient conditions (Identical) Part 5 Dynamic volumetric methods ISO 6145-1:1986Section 5.1:1987 Review of methods of calibration (Identical) Licensed C
18、opy: sheffieldun sheffieldun, na, Fri Dec 01 08:27:44 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-5.6:1987 BSI 12-1999iii 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. Compli
19、ance 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 to iv, pages 1 to 10, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have
20、 had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:27:44 GMT+00:00 2006, Uncontrolled Copy, (c) BSI iv blank Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:27:44 GMT+00:00 2006, U
21、ncontrolled Copy, (c) BSI BS 4559-5.6:1987 BSI 12-19991 1 Scope and field of application This International Standard constitutes part 6 of ISO 6145, dealing with the various dynamic volumetric techniques used for the preparation of calibration gas mixtures. It describes the method of preparation, us
22、ing orifices with sonic flow, of calibration gas mixtures with one or more components, with a volume ratio equal to or greater than 0,1 %, the repeatability of which on the concentration of each of the components is approximately 0,5 % in relative value. 2 References ISO 6142, Gas analysis Preparati
23、on of calibration gas mixtures Weighing methods. ISO 6145-1, Gas analysis Preparation of calibration gas mixtures Dynamic volumetric methods Part 1: Methods of calibration. 3 Principle of the method 3.1 General The method consists of adding, to a mixing chamber, constant mass flow rates of gas obtai
24、ned by sonic flow of gas at a constant pressure through appropriate channels. 3.2 Operating principle of an orifice with sonic or critical flow For a given upstream pressure p1, the mass flow rate of a pressure-difference device (orifice plate or nozzle) increases when the downstream pressure p2 dec
25、reases, to tend towards a limit value. If the ratio p1/p2 exceeds a value in the region of 2 (critical point), the flow rate remains constant; moreover, it is also necessary that the ratio d/D of the diameter of the pressure difference device d to the diameter of the upstream pipe D remains below 0,
26、2 so that the variations in dynamic pressure can be disregarded. 3.3 Expression of sonic mass flow rate The theoretical mass flow rate, qm, of a gas expanding isotropically through a sonic orifice can be expressed in the form where Acis the section area of the neck of the orifice; Cdis the contracti
27、on coefficient of the gas jet (also called discharge coefficient); p1is the absolute pressure upstream; T1is the temperature of the gas upstream; Mis the molar mass of the gas; is the ratio of the mass thermal capacities cp/cV of the gas (cp is the mass thermal capacity at constant pressure and cV i
28、s the mass thermal capacity at constant volume); Ris the molar gas constant. The term beneath the root sign is currently designated by C*, hence the simplified expression The product Ac Cd represents the area of the section of the sonic gas jet, the contraction coefficient Cd being generally between
29、 0,6 (circular orifice) and 1 (shaped nozzle). The mass flow rate for a given nozzle and gas depends solely on the pressure and upstream temperature, and is independent of the downstream pressure. . . . (1) Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:27:44 GMT+00:00 2006, Uncontrolled
30、Copy, (c) BSI BS 4559-5.6:1987 2 BSI 12-1999 3.4 Area of validity Pressure and temperature have a complicated influence on the flow rate of a sonic nozzle, as they act directly by p1 and and indirectly through the factor. The latter influence becomes more significant as the behaviour of the gas move
31、s further from that of a perfect gas. Examples of relative variations of C* for a temperature deviation %T1 of 10 K and a pressure deviation %p1 of 8 bar3) are given in the Table. Table Relative variations of C* It thus appears that, for the required accuracy, C* cannot be considered as constant whe
32、n the upstream conditions vary appreciably. Moreover, the contraction coefficient Cd depends basically on the geometry of the nozzle and, through the Reynolds number, on the nature of the gas. This means that, in practice, calibration of the nozzle under conditions close to those of its use is neces
33、sary. Knowledge of the theoretical variations of C*, which are themselves calculated from variations in , is subject to a certain inaccuracy. In addition, there is also a deviation due to the fact that the expansion is not completely isentropic, with the result that forecasts of C* prove difficult.
34、3.5 Principle of calculation The molar ratio of component A, XA, is defined by where qmA, qmB, qmC, .are the mass flow rates of components A, B, C, .; MA, MB, MC .are the respective molar masses. 3) 1 bar = 105 Pa Gas T1 = 30 0C, %p1 = 8 barp1 = 5 bar, %T1 = 10 K % He 0,02+ 1,6 H2 0,02+ 1,6 O2 0,4+
35、1,6 CH4 0,5+ 1,8 C2H6 1,7+ 1,9 CO2 1,3+ 1,8 1/ T1 C*2 bar()C*10 bar() C*2 bar() - - C*5 bar30 C()C*5 bar40 C() C*5 bar30 C() - Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:27:44 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-5.6:1987 BSI 12-19993 4 Practical examples A gas mixer wit
36、h sonic nozzles comes in the form of high-stability pressure regulator units and sonic nozzles discharging into a mixing chamber. A schematic example is shown in Figure 1. Another possibility consists of several sonic nozzles per component; the mixer thus gives a series of concentrations in a known
37、ratio, according to the combination of sonic nozzles in service. A schematic example is shown in Figure 2. Figure 1 Gas mixer with sonic nozzles discharging into mixing chamber (schematic) Figure 2 Gas mixer with several sonic nozzles per component (schematic) Licensed Copy: sheffieldun sheffieldun,
38、 na, Fri Dec 01 08:27:44 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-5.6:1987 4 BSI 12-1999 5 Operating conditions 5.1 Pressures The absolute upstream pressure p1 of the sonic nozzles is between 3 and 6 bar. This pressure range enables a variation in the downstream back-pressures of the sonic
39、 nozzles of 250 mbar to be achieved, without affecting the value of the concentrations of the mixture. Only the supply pressure p1 of the sonic nozzles shall satisfy precise conditions in order to attain levels of repeatability for the concentration proportions stipulated in clause 1. The stability
40、of this pressure p1 shall be greater than 5 104 in relative value. 5.2 Flow rates In practice, the minimum flow rates are 5 cm3/min at ambient pressure and temperature. 5.3 Filtration Filtration of the components and the complementary gas is applied in relation to the section of the passage in the m
41、ixer components. 5.4 Temperatures A simultaneous variation of the absolute temperatures of the components and of the complementary gas does not introduce any variation in the concentration of the mixture. In the case of a variation in temperature of one component in relation to another component of
42、the mixture, the concentration is modified and this modification can be calculated. 6 Experimental determination of concentrations and examination of sources of error 6.1 General Any of the calibration methods described in clause 3 of ISO 6145-1 can be used. The example of calibration given below in
43、volves the gravimetric method (sub-clause 3.2.3.2 of ISO 6145-1), the calculation of the effect of variation of temperature and pressure controllers. 6.2 Presentation of the method The coefficient Ac Cd C* is obtained from equation (3) where The sources of error of the method arise from the followin
44、g: a) weighing, refer to ISO 6142 (4.2.1) for the procedure and error calculation; b) variations in pressure and temperature linked to the quality of the regulators used, and, where applicable, uncertainties in the measurement of these parameters; c) gas losses arising from compression and decompres
45、sion in the transfer lines. . (2) . (3) is the mean temperature upstream of the nozzle; tis the time; is the mean pressure upstream of the nozzle. qm m t -= T 1 p 1 Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08:27:44 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 4559-5.6:1987 BSI 12-1999
46、5 By differentiating equation (3): An evaluation is obtained of the uncertainty on the determination of Ac Cd C* by When the nozzle which has been calibrated in this way is used, with the service pressure and temperature p1 and T1 being as close as possible to the values used during calibration, the
47、 mass flow rate is given by equation (1): If the values of p1 and T1 are subject to measurable fluctuations, mean values should then be taken and qm becomes a mean flow rate. By differentiating equation (1): The uncertainty in the determination of qm is evaluated using the equation The calibration e
48、rror, %Cal, is calculated by The precision error, %f, depending upon the random variations of the parameters pressure and temperature both during one operation and from one operation to another, is given by 6.3 Numerical example Determination of the molar ratio of a binary mixture CO2 and N2 In this
49、 example of a calculation, it will be assumed that the pressures and temperatures were measured with the same instruments during calibration and use, and that fluctuations in these parameters were negligible during the period of time t. . . . (4) . . . (5) Licensed Copy: sheffieldun sheffieldun, na, Fri Dec 01 08: