TOTAL AND PARTIAL PRESSURE MEASUREMENT IN VACUUM SYSTEMS

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1 TOTAL AND PARTIAL PRESSURE MEASUREMENT IN VACUUM SYSTEMS J.H. LEeK Emeritus Professor Department of Electrical Engineering and Electronics University of Liverpool Blackie Glasgow and London

2 B1ackie & Son Limited Bishopbriggs, Glasgow G64 2NZ 7 Leicester Place London WC2H 7BP 1989 Blackie & Son Ltd. First published 1989 Softcover reprint of the hardcover 1st edition 1989 All rights reserved No part of this publication may be reproduced. stored in a retrieval system. or transmitted. in any form or by any means. electronic. mechanical. recording or otherwise. without prior permission of the Publishers British Library Cataloguing in Publication Data Leek, J:H. Total and partial pressure measurement in vacuum systems I. Vacuum technology I. Title 621.5'5 ISBN 13: DOl: / e ISBN 13: To Esma Phototypesetting by Thomson Press (india) Limited, New Delhi

3 Preface This book deals with the underlying theory and practical aspects of pressure gauges that are at present in general use. Because of the ever-increasing demands to provide a wider range of sophisticated and reliable vacuum equipment a good understanding of these instruments is of vital importance to all workers in the research and industrial sectors. Of the gauges considered only the mechanical types are absolute, in the sense that they measure pressure directly as a force upon a liquid column or a solid surface. Under ideal conditions it is possible to calculate their sensitivities, which are the same for all gases and vapours. The recent developments in the viscous or molecular damping gauges indicate that these may also be considered absolute. Other gauges are indirect in that they involve the measurement of some secondary phenomenon which is pressure-dependent and therefore these gauges can only be used for measurement after calibration against an absolute standard. The radiometer or Knudsen type gauge has been excluded from the text since these are now only of historic interest. Also no mention is made of the integration techniques involving surface changes (such as work function) although these could have application under very special circumstances. The McLeod gauge is dealt with in some detail, for even though this gauge has few practical applications, it is the most sensitive absolute gauge available and has value as a reference standard. Throughout, emphasis is on the precision and reproducibility of measurement. A short but important chapter is devoted to the subject of calibration, as this is important in many laboratories where the reproducibility of measurement must be guaranteed over long periods. Two chapters are devoted to the subject of the mass spectrographic analysis of gases at low pressure, and, in particular, to the theory, design and operation of the residual gas analyser. One chapter deals with the quadrupole mass filter, which is universally used as a monitor of gas composition in all types of vacuum system. This is a good example of an instrument which, provided its characteristics are understood and it is operated correctly, can be an almost invaluable tool to the vacuum engineer. At the present time the units for pressure measurement are not standardized. Because of the move towards SI units,. the traditional "torr" has been replaced in many laboratories by either the Pascal (Pa) or the millibar (mbar). However, the millibar has been used throughout the book because it is presently favoured by the majority of equipment manufacturers and system operators. In sections where published results are quoted, original units have been retained. iii

4 iv PREFACE The author gratefully acknowledges the help and encouragement he has received both from his colleagues in the Department of Engineering & Electronics at the University of Liverpool and from other laboratories. Most of all his thanks are to his wife Esma without whose assistance in typing the whole of the manuscript and correcting spelling and grammatical errors, the book would not have been completed. JHL Acknowledgments Permission to reproduce the following material is gratefully acknowledged: Figs. 1.8, 1.9 (Berman); 3.4 (Bartmess and Georgiadis), 3.22, 3.23 (Poulter and Sutton); 3.25, 3.26 (Kudzia and St6wko); 3.27 (Kuo); 5.4, 5.6 (Peggs); 6.8 (Craig and Harden); 7.16 (Reid and James); 7.18,7.19, 7.20(Mao et a/., Mao and Leek) from Vacuum with the permission of the authors and of Pergamon Journals Ltd, Oxford. Figs. l.l3 (Sullivan); l.l4 (Hyland and Tilford); l.l5 (Ono et a/.); l.l7 (Fremerey); 3.9, 3.10 (Watanabe); 3.11 (Blechschmidt); 3.19 (Pittaway); 3.21 (Poulter et a/.); 5.7 (McCulloh et a/.); 7.13 (Blanchard et a/.); 7.22, 7.23 (Reagan et a!.) from J. Vac. Sci. Techno!. with the permission of the authors and of the American Institute of Physics, New York. Figs. l.l6 (Fremerey and Boden); 2.14, 2.15 (English et a/.); 2.17, 2.l8 (Steckelmacher and Fletcher); 5.1 (Poulter); 7.9 (Holme et a/.) from J. Phys. D. J. Phys. E. J. Sci. Instrum. with the permission of the authors and of the Institute of Physics, Bristol. Fig (Pittaway) from Phillips Research Rept. 29, with the permission of Phillips Int. BY, Eindhoven. Fig (Holme et a!.) from Int. J. Mass Spectrom. Ion Phys. with the permission of the authors and Elsevier Scientific Publications BY, Amsterdam. U nits of Pressure I bar I millibar (mbar) "" 760 torr I torr I millibar (mbar) I torr 10 5 Pascal (Pa) 100 Pascal (Pa) I standard atmosphere 1 mm of mercury to within I part in 7 x torr 0.75 torr to within I part ii millibar (mbar) 1.33 millibar (mbar) to within 0.25%

5 ContentS 1 Mechanical manometers 1.1 Liquid manometers 1.2 The McLeod gauge 1.3 The diaphragm manometer 1.4 Viscous or friction-type gauges References 1 I Thermal conductivity gauges Basic principles Measurement of thermal conductivity Sensitivity End losses Accommodation coefficient and relative sensitivity Alternative methods of bridge control Useful range of the constant-voltage bridge The lower limit to the useful pressure range The importance of bridge-voltage and temperature fluctuations at high pressure Compensation for temperature and voltage fluctuations Physical changes in the gauge wire (ageing effects) Extension of working range to atmospheric pressure Commercial gauges for laboratory and industrial use The thermocouple gauge 61 Appendix 64 References 66 3 Thermionic cathode ionization gauges Positive ion production in a gas The principle of the thermionic cathode ionization gauge The relative sensitivity for different gases The measurement of low pressures Extension of the range of the BA gauge to very low pressures The precision to which measurements can be made with the hot cathode gauge Gauges specially designed to operate at high pressure Chemical and physical reactions in the hot cathode ionization gauge 107 References Cold-cathode ionization gauges , The development of cold-cathode (crossed-field) gauges Commercial gauges for high- and ultra-high vacuum applications 120 References Gauge calibration Basic considerations Calibration against the transfer gauge Comparison with absolute gauges Series expansion techniques 127 v

6 vi CONTENTS 5.5 Dynamic flow techniques The measurement of gas throughput 132 References Gas analysis in vacuum systems: magnetic, crossed-field and time-offlight analysers Introduction The magnetic deflection mass spectrometer The trochoidal (or cyc1oidal) mass spectrometer The omegatron Time-of-flight (TOF) mass spectrometer Interpretation of mass spectra 148 References Gas analysis in vacuum systems: quadrupole mass analysers Introduction Principles of the quadrupole mass filter Design of small residual gas analysers (RGAs) The operating characteristics of the RGAs designed for general laboratory and industrial use The use of electron multipliers for signal detection Non-conventional methods of quadrupole operation The monopole mass spectrometer The three-dimensional quadrupole ion trap 188 References 192 Index 193

The Experts in Vacuum Solutions

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