1、Sensor Performanceand Reliabilitysensors front of book.qxd 11/3/04 10:49 AM Page 1sensors front of book.qxd 11/3/04 10:49 AM Page 2Sensor Performanceand ReliabilityH. M. Hashemiansensors front of book.qxd 11/3/04 10:49 AM Page 3Notice The information presented in this publication is for the general
2、education of the reader. Because neitherthe author nor the publisher have any control over the use of the information by the reader, both theauthor and the publisher disclaim any and all liability of any kind arising out of such use. The reader isexpected to exercise sound professional judgment in u
3、sing any of the information presented in a particularapplication. Additionally, neither the author nor the publisher have investigated or considered the affect of any patentson the ability of the reader to use any of the information in a particular application. The reader is responsible for reviewin
4、g any possible patents that may affect any particular use of the infor-mation presented. Any references to commercial products in the work are cited as examples only. Neither the author nor thepublisher endorses any referenced commercial product. Any trademarks or trade names referenced belongto the
5、 respective owner of the mark or name. Neither the author nor the publisher makes any representa-tion regarding the availability of any referenced commercial product at any time. The manufacturersinstructions on use of any commercial product must be followed at all times, even if in conflict with th
6、einformation in this publication. Copyright 2005 ISA The Instrumentation, Systems, and Automation Society All rights reserved. Printed in the United States of America. 10 9 8 7 6 5 4 3 2ISBN 155617-897-2No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form
7、or byany means, electronic, mechanical, photocopying, recording or otherwise, without the prior written per-mission of the publisher. ISA 67 Alexander Drive P.O. Box 12277 Research Triangle Park, NC 27709 Library of Congress Cataloging-in-Publication DataHashemian, H. M.Sensor performance and reliab
8、ility / Hashem M. Hashemian.p. cm.ISBN 1-55617-897-2 (softbound)1. Detectors-Testing. I. Title. TK6565.D4H38 2005681.2-dc222004023221sensors front of book.qxd 11/3/04 10:49 AM Page 4DEDICATIONThis book is dedicated to my great family and the wonderful people of AMS.H.M. HashemianKnoxville, Tennessee
9、USAsensors front of book.qxd 11/3/04 10:49 AM Page 5sensors front of book.qxd 11/3/04 10:49 AM Page 6VIIABOUT THE AUTHORH. M. Hashemian “Hash” graduated with a B.S. degree in physics from theNational University of Iran and then came to the USA in 1974. He received hisM.S. degree in nuclear engineeri
10、ng from the University of Tennessee in 1977. He continued his studies toward a Ph.D. degree in nuclear engineering but leftin 1980 to attend full time to his company, Analysis and Measurement ServicesCorporation (AMS). He started AMS in 1977 to commercialize the product ofthe research and developmen
11、t work in the process instrumentation area that hehad done for his masters degree. His company has grown over the years and has a worldwide list of clients including major utilities in the USA and othercountries. AMS specializes in I others are filled withthe process fluid or oil. Fluid-sensing line
12、s can contribute to pressure sensor problems because they develop anomalies such as blockages and resonancescaused by voids and standing waves. Blockages reduce the dynamic response ofthe pressure-sensing system, while voids in fluid-sensing lines can lead to noisypressure signals, measurement error
13、s, and sluggish dynamic response. Both blockages and voids in fluid-sensing lines can be detected remotely whilethe plant is on line by using the noise analysis technique. The noise analysis technique is also used to test the response time of pressure sensors in-situ asinstalled in an operating proc
14、ess. The details are covered in Chapters 13 and 14. Table 1.1. RTDs and Thermocouples ComparedRTDsMore accurate than thermocouples, but not as effective in poor heat transfer media. Also, not as good for vibration environments, but betterthan thermocouples in noisy environments.ThermocouplesWider te
15、mperature range than RTDs, but less accurate and cannot be calibrated after use. Survive better than RTDs in vibration environ-ments, but not as good in noisy environments.Preferred SensorPerformance Indicator RTDs ThermocouplesAccuracy Air/Gas Temperature Measurement Vibration Environment Noisy Env
16、ironment High Temperature Range Post-Use Calibration INTRODUCTIONchap 1.qxd 11/1/04 2:46 PM Page 34In addition to sensors, process instrumentation problems can originate inthe cables that connect the sensors to indicating equipment. Both the conductorand the insulation or jacket material of cables a
17、re prone to degrade and fail. Thismay result in erratic signals, information loss, measurement error, and otherproblems. Fortunately, means are available to test for these problems and identifytheir location. These means are described in Chapter 17.1.2 Temperature Sensor ProblemsA typical problem in
18、 RTDs and thermocouples is the failure of the seal designed to keep moisture from entering the sensor. This will often causethe insulation material in the sensor to degrade or fail, which results in temperature indication errors, as well as noise, at the output of the sensor.Furthermore, the moistur
19、e in the sensor can accelerate the deterioration of thesensor material. For example, moisture can cause chemical interactionbetween an RTD sensing element and the sensors insulation material. Thisreduces the diameter of the sensing wire, which increases its resistance andcauses the RTD to show a hig
20、her-than-true temperature. To test for moisturein a temperature sensor, instrument and control engineers should measure itsinsulation resistance (IR) using a megohmeter set at 100 volts DC (VDC).The results should indicate IR values in tens of megohms (or higher) at roomtemperature for a sensor that
21、 is dry. If there is moisture in the sensor, the IRcould be as low as a few kilo-ohms. Moisture in the sensor will also make itdifficult to measure IR because the IR value would fluctuate so much that thereading would be hard to register.A sensor seal fails as a result of long-term exposure to heat,
22、 humidity, andother taxing environments in an industrial process. The heat can cause the sealto dry out and crack, allowing moisture to enter the sensor. Moisture can alsoenter sensors through microscopic cracks in the sheath, but this problem israre in high-quality sensors.Flow-induced vibration is
23、 another problem that causes temperature sensors to fail. RTDs and thermocouples or their thermowells can crack fromflow-induced vibration, causing moisture to enter the sensor. Vibration hasalso been responsible for ruptures and shearing of sensors or thermowells,resulting in catastrophic failures.
24、 RTDs that have failed are sometimes dissected to determine the rootcause of the failure. A review of some of these efforts has shown that RTDsensing elements usually become open at their weakest points, as one wouldexpect. The weak points are usually where the sensing element is welded toextension wires. Another weak point is where the sensing element is bent,especially when it is bent and passed through openings in the mandrel used toSENSOR PERFORMANCE AND RELIABILITYchap 1.qxd 11/1/04 2:46 PM Page 4
