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1、INTERNATIONAL STANDARD ISO 6145-7 First edition 2001-04-01 Reference number ISO 6145-7:2001(E) ISO 2001 Gas analysis Preparation of calibration gas mixtures using dynamic volumetric methods Part 7: Thermal mass-flow controllers Analyse des gaz Prparation des mlanges de gaz pour talonnage laide de mt
2、hodes volumtriques dynamiques Partie 7: Rgulateurs thermiques de dbit-masse Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/19/2007 21:55:08 MDTNo reproduction or networking permitted wi
3、thout license from IHS -,-,- ISO 6145-7:2001(E) ii ISO 2001 All rights reserved PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed to
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6、 found, please inform the Central Secretariat at the address given below. ISO 2001 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, elec- tronic or mechanical, including photocopying and microfilm, without permiss
7、ion in writing from either ISO at the address below or ISOs mem- ber body in the country of the requester. ISO copyright office Case postale 56 CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.ch Web www.iso.ch Printed in Switzerland Copyright International Organiza
8、tion for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/19/2007 21:55:08 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO 6145-7:2001(E) ISO 2001 All rights reserved iii ContentsPage 1Scope .1 2Nor
9、mative references .1 3Principles 1 4Preparation of gas mixtures 3 5Calculations 5 Annexes APre-mixed gases for preparation of mixtures of high dilution8 BPractical hints9 CUncertainty of volume fraction Mathematical derivation.10 Bibliography.12 Copyright International Organization for Standardizati
10、on Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/19/2007 21:55:08 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO 6145-7:2001(E) iv ISO 2001 All rights reserved Foreword ISO (the International Organization for S
11、tandardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the ri
12、ght to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. Internationa
13、l Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bo
14、dies casting a vote. Attention is drawn to the possibility that some of the elements of this part of ISO 6145 may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. International Standard ISO 6145-7 was prepared by Technical Committee IS
15、O/TC 158,Analysis of gases. ISO 6145 consists of the following parts, under the general titleGas analysis Preparation of calibration gas mixtures using dynamic volumetric methods: Part 1: Methods of calibration Part 2: Volumetric pumps Part 4: Continuous injection method Part 5: Capillary calibratio
16、n devices Part 6: Critical orifices Part 7: Thermal mass-flow controllers Part 9: Saturation method Part 10: Permeation method Diffusion will be the subject of a future part 8 to ISO 6145. Part 3 to ISO 6145, entitledPeriodic injections into a flowing stream, has been withdrawn. Annexes A, B and C o
17、f this part of ISO 6145 are for information only. Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/19/2007 21:55:08 MDTNo reproduction or networking permitted without license from IHS -,-
18、,- ISO 6145-7:2001(E) ISO 2001 All rights reserved v Introduction This part of ISO 6145 is one of a series of International Standards dealing with various dynamic volumetric methods used for the preparation of calibration gas mixtures. Copyright International Organization for Standardization Provide
19、d by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/19/2007 21:55:08 MDTNo reproduction or networking permitted without license from IHS -,-,- Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA T
20、echnical Standards 1/9972545001 Not for Resale, 04/19/2007 21:55:08 MDTNo reproduction or networking permitted without license from IHS -,-,- INTERNATIONAL STANDARDISO 6145-7:2001(E) ISO 2001 All rights reserved 1 Gas analysis Preparation of calibration gas mixtures using dynamic volumetric methods
21、Part 7: Thermal mass-flow controllers 1Scope This part of ISO 6145 specifies a method for the continuous production of calibration gas mixtures, containing two or more components, from pure gases or other gas mixtures by use of commercially available thermal mass-flow controllers. By adjustment of s
22、et-points on flow controllers to pre-determined values, it is possible to change the composition of the gas mixture rapidly and in a continuously variable manner. By selection of appropriate combinations of thermal mass-flow controllers and with use of pure gases, the volume fraction of the componen
23、t of interest in the complementary gas can be varied by a factor of. The relative expanded uncertainty of measurement,obtained by multiplying the relative combined standard uncertainty by a coverage factor, is not greater than. If pre-mixed gases are used instead of pure gases, mole fractions belowc
24、an be obtained. The measurement of mass flow is not absolute and the flow controller requires independent calibration. The merits of the method are that a large quantity of the gas mixture can be prepared on a continuous basis and that multi-component mixtures can be prepared as readily as binary mi
25、xtures if the appropriate number of thermal mass- flow controllers is utilized. Gas blending systems, based upon thermal mass-flow controllers, and some including the facility of computerization and automatic control, are commercially available. 2Normative references The following normative document
26、s contain provisions which, through reference in this text, constitute provisions of this part of ISO 6145. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this part of ISO 6145 are encouraged to investi
27、gate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of currently valid International Standards. ISO 6143,Gas analysis
28、Comparison methods for determining and checking the composition of calibration gas mixtures. ISO 6145-1:1986,Gas analysis Preparation of calibration gas mixtures Dynamic volumetric methods Part 1: Methods of calibration. 3Principles 3.1Thermal mass-flow controller using a constant current supply To
29、prepare the gas mixture each gaseous component is passed at a known, controlled flowrate, and at a constant pressure, from a calibrated thermal mass-flow controller. 1 000 U,k =2 2 % 106 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Tec
30、hnical Standards 1/9972545001 Not for Resale, 04/19/2007 21:55:08 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO 6145-7:2001(E) 2 ISO 2001 All rights reserved A thermal mass-flow controller consists of a measuring unit for mass flow and a proportioning valve which is c
31、ontrolled by an electronic unit. The flowing gas is passed through a heater connected to a constant current supply and the temperature is measured upstream and downstream from the heater. The schema in Figure 1 shows the heater, temperature sensors and associated circuitry. The two temperature senso
32、rs, one upstream and one downstream from the heater form two resistors of a Wheatstone bridge circuit, which is balanced to give zero reading when there is no gas flow. When there is a gas flow through the system a temperature difference, is established between the two sensors such that the heat flu
33、x, is given by: (1) where is the heat capacity per unit mass, or molar heat capacity, of the gas at constant pressure; is the mass flowrate. The difference in temperature between sensors results in a potential difference across the Wheatstone bridge circuit and thus a signal. This signal is compared
34、 with an adjustable reference voltage in a differential amplifier. The resulting output signal is in turn used for operating a control valve to regulate the flow of gas. Key 1Temperature sensor 1 2Temperature sensor 2 3Heater 4Gas supply 5Current supply 6Wheatstone bridge 7Differential amplifier 8Si
35、gnal readout Figure 1 Thermal mass-flow controller with constant current supply T = CpTqm Cp qm Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/19/2007 21:55:08 MDTNo reproduction or net
36、working permitted without license from IHS -,-,- ISO 6145-7:2001(E) ISO 2001 All rights reserved 3 3.2Thermal mass-flow controller under constant temperature control In this system (Figure 2) the gas passes through three heaters in sequence and each of which is connected into an resistor of a self-r
37、egulating Wheatstone bridge. Instead of the difference in temperature being measured, the input to each heater is such that the temperature distribution along the flow path is maintained uniform. The Wheatstone bridge current is proportional to the heat loss and therefore proportional also to the ma
38、ss flow of the gas. The output signal is again used to operate a solenoid valve to control the mass flowrate. In the preparation of multicomponent mixtures, it is often necessary to use one mass-flow controller for each component. Dual-channel controllers are available and may be used for the prepar
39、ation of binary mixtures or, for example, for the preparation of mixtures of a given gas in air. 4Preparation of gas mixtures 4.1Description of the experimental procedure A schematic diagram of the arrangement for the preparation of binary mixtures is shown in Figure 3. Key 1Heater 1 2Heater 2 3Heat
40、er 3 4Gas supply 5Current supply 6Wheatstone bridge 7Differential amplifier 8Signal readout Figure 2 Thermal mass-flow controller under constant temperature control Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/997
41、2545001 Not for Resale, 04/19/2007 21:55:08 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO 6145-7:2001(E) 4 ISO 2001 All rights reserved Gas cylinders (1) and (7) containing the complementary gas and the calibration component, respectively, are connected to the thermal
42、 mass-flow controllers (5) and (11) through pressure regulators (2) and (8) and shut-off valves (3) and (9). The two in-line filters (4) and (10) provide protection against contamination. The gases from the flow controllers enter the mixing vessel (13). The recommended working range for the pressure
43、 regulators is 1) () to(). The pressure regulator for the “gaseous component” shall also be suitable for the particular component involved (for example the Key Complementary gas: 1Cylinder of pressurized gas 2Pressure regulator 3Shut-off valve 4Filter against contamination 5Thermal mass-flow control
44、ler 6Shut-off valve (optional) Calibration component: 7Cylinder of pressurized gas 8Pressure regulator 9Shut-off valve 10Filter against contamination 11Thermal mass-flow controller 12Shut-off valve 13Mixing vessel Figure 3 Mixing apparatus for production of binary gas mixtures by means of thermal ma
45、ss-flow controllers 1); 60 kPa0,6 bar600 kPa 6,0 bar 1 bar=100 kPa=0,1 MPa 1 Pa=1 N/m2 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/19/2007 21:55:08 MDTNo reproduction or networking p
46、ermitted without license from IHS -,-,- ISO 6145-7:2001(E) ISO 2001 All rights reserved 5 diaphragm shall be of stainless steel or other corrosion-resistant material). Similarly, the thermal mass-flow controllers shall be suitable for use with the gaseous components and for the requirements of the g
47、as mixture. Set the input pressures appropriate to the controllers using the pressure regulators and open the shut-off valves (3), (6) and (9). Purge the inlet path of the gaseous component through the shut-off valve (12), which shall be of a type which can be operated rapidly. Adjust the set points
48、 of the controllers so as to obtain the respective flowrates in the correct ratio for the desired composition of the binary gas mixture; meanwhile, continue the purging process of the input tube for the component gas by multiple opening and closing of valve (12), until a total volume of gas at least ten times the volume of the flow path has been vented. When