Measurement and Instrumentation Principles

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Contents
Preface xvii
Acknowledgements xx
Part 1: Principles of Measurement 1
1 INTRODUCTION TO MEASUREMENT 3
1.1 Measurement units 3
1.2 Measurement system applications 6
1.3 Elements of a measurement system 8
1.4 Choosing appropriate measuring instruments 9
2 INSTRUMENT TYPES AND PERFORMANCE
CHARACTERISTICS 12
2.1 Review of instrument types 12
2.1.1 Active and passive instruments 12
2.1.2 Null-type and deflection-type instruments 13
2.1.3 Analogue and digital instruments 14
2.1.4 Indicating instruments and instruments with a signal output 15
2.1.5 Smart and non-smart instruments 16
2.2 Static characteristics of instruments 16
2.2.1 Accuracy and inaccuracy (measurement uncertainty) 16
2.2.2 Precision/repeatability/reproducibility 17
2.2.3 Tolerance 17
2.2.4 Range or span 18
2.2.5 Linearity 19
2.2.6 Sensitivity of measurement 19
2.2.7 Threshold 20
2.2.8 Resolution 20
2.2.9 Sensitivity to disturbance 20
2.2.10 Hysteresis effects 22
2.2.11 Dead space 23
2.3 Dynamic characteristics of instruments 23
2.3.1 Zero order instrument 25
2.3.2 First order instrument 25
2.3.3 Second order instrument 28
2.4 Necessity for calibration 29
2.5 Self-test questions 30
3 ERRORS DURING THE MEASUREMENT PROCESS 32
3.1 Introduction 32
3.2 Sources of systematic error 33
3.2.1 System disturbance due to measurement 33
3.2.2 Errors due to environmental inputs 37
3.2.3 Wear in instrument components 38
3.2.4 Connecting leads 38
3.3 Reduction of systematic errors 39
3.3.1 Careful instrument design 39
3.3.2 Method of opposing inputs 39
3.3.3 High-gain feedback 39
3.3.4 Calibration 41
3.3.5 Manual correction of output reading 42
3.3.6 Intelligent instruments 42
3.4 Quantification of systematic errors 42
3.5 Random errors 42
3.5.1 Statistical analysis of measurements subject to
random errors 43
3.5.2 Graphical data analysis techniques – frequency
distributions 46
3.6 Aggregation of measurement system errors 56
3.6.1 Combined effect of systematic and random errors 56
3.6.2 Aggregation of errors from separate measurement
system components 56
3.6.3 Total error when combining multiple measurements 59
3.7 Self-test questions 60
References and further reading 63
4 CALIBRATION OF MEASURING SENSORS AND
INSTRUMENTS 64
4.1 Principles of calibration 64
4.2 Control of calibration environment 66
4.3 Calibration chain and traceability 67
4.4 Calibration records 71
References and further reading 72
5 MEASUREMENT NOISE AND SIGNAL PROCESSING 73
5.1 Sources of measurement noise 73
5.1.1 Inductive coupling 74
5.1.2 Capacitive (electrostatic) coupling 74
5.1.3 Noise due to multiple earths 74
5.1.4 Noise in the form of voltage transients 75
5.1.5 Thermoelectric potentials 75
5.1.6 Shot noise 76
5.1.7 Electrochemical potentials 76
5.2 Techniques for reducing measurement noise 76
5.2.1 Location and design of signal wires 76
5.2.2 Earthing 77
5.2.3 Shielding 77
5.2.4 Other techniques 77
5.3 Introduction to signal processing 78
5.4 Analogue signal filtering 78
5.4.1 Passive analogue filters 81
5.4.2 Active analogue filters 85
5.5 Other analogue signal processing operations 86
5.5.1 Signal amplification 87
5.5.2 Signal attenuation 88
5.5.3 Differential amplification 89
5.5.4 Signal linearization 90
5.5.5 Bias (zero drift) removal 91
5.5.6 Signal integration 92
5.5.7 Voltage follower (pre-amplifier) 92
5.5.8 Voltage comparator 92
5.5.9 Phase-sensitive detector 93
5.5.10 Lock-in amplifier 94
5.5.11 Signal addition 94
5.5.12 Signal multiplication 95
5.6 Digital signal processing 95
5.6.1 Signal sampling 95
5.6.2 Sample and hold circuit 97
5.6.3 Analogue-to-digital converters 97
5.6.4 Digital-to-analogue (D/A) conversion 99
5.6.5 Digital filtering 100
5.6.6 Autocorrelation 100
5.6.7 Other digital signal processing operations 101
References and further reading 101
6 ELECTRICAL INDICATING AND TEST INSTRUMENTS 102
6.1 Digital meters 102
6.1.1 Voltage-to-time conversion digital voltmeter 103
6.1.2 Potentiometric digital voltmeter 103
6.1.3 Dual-slope integration digital voltmeter 103
6.1.4 Voltage-to-frequency conversion digital voltmeter 104
6.1.5 Digital multimeter 104
6.2 Analogue meters 104
6.2.1 Moving-coil meters 105
6.2.2 Moving-iron meter 106
6.2.3 Electrodynamic meters 107
6.2.4 Clamp-on meters 108
6.2.5 Analogue multimeter 108
6.2.6 Measuring high-frequency signals 109
6.2.7 Thermocouple meter 110
6.2.8 Electronic analogue voltmeters 111
6.2.9 Calculation of meter outputs for non-standard waveforms 112
6.3 Cathode ray oscilloscope 114
6.3.1 Cathode ray tube 115
6.3.2 Channel 116
6.3.3 Single-ended input 117
6.3.4 Differential input 117
6.3.5 Timebase circuit 117
6.3.6 Vertical sensitivity control 117
6.3.7 Display position control 118
6.4 Digital storage oscilloscopes 118
References and further reading 118
7 VARIABLE CONVERSION ELEMENTS 119
7.1 Bridge circuits 119
7.1.1 Null-type, d.c. bridge (Wheatstone bridge) 120
7.1.2 Deflection-type d.c. bridge 121
7.1.3 Error analysis 128
7.1.4 A.c. bridges 130
7.2 Resistance measurement 134
7.2.1 D.c. bridge circuit 135
7.2.2 Voltmeter–ammeter method 135
7.2.3 Resistance-substitution method 135
7.2.4 Use of the digital voltmeter to measure resistance 136
7.2.5 The ohmmeter 136
7.2.6 Codes for resistor values 137
7.3 Inductance measurement 138
7.4 Capacitance measurement 138
7.4.1 Alphanumeric codes for capacitor values 139
7.5 Current measurement 140
7.6 Frequency measurement 141
7.6.1 Digital counter-timers 142
7.6.2 Phase-locked loop 142
7.6.3 Cathode ray oscilloscope 143
7.6.4 The Wien bridge 144
7.7 Phase measurement 145
7.7.1 Electronic counter-timer 145
7.7.2 X–Y plotter 145
7.7.3 Oscilloscope 147
7.7.4 Phase-sensitive detector 147
7.8 Self-test questions 147
References and further reading 150
8 SIGNAL TRANSMISSION 151
8.1 Electrical transmission 151
8.1.1 Transmission as varying voltages 151
8.1.2 Current loop transmission 152
8.1.3 Transmission using an a.c. carrier 153
8.2 Pneumatic transmission 154
8.3 Fibre-optic transmission 155
8.3.1 Principles of fibre optics 156
8.3.2 Transmission characteristics 158
8.3.3 Multiplexing schemes 160
8.4 Optical wireless telemetry 160
8.5 Radio telemetry (radio wireless transmission) 161
8.6 Digital transmission protocols 163
References and further reading 164
9 DIGITAL COMPUTATION AND INTELLIGENT DEVICES 165
9.1 Principles of digital computation 165
9.1.1 Elements of a computer 165
9.1.2 Computer operation 168
9.1.3 Interfacing 174
9.1.4 Practical considerations in adding computers to
measurement systems 176
9.2 Intelligent devices 177
9.2.1 Intelligent instruments 177
9.2.2 Smart sensors 179
9.2.3 Smart transmitters 180
9.2.4 Communication with intelligent devices 183
9.2.5 Computation in intelligent devices 184
9.2.6 Future trends in intelligent devices 185
9.3 Self-test questions 185
References and further reading 186
10 INSTRUMENTATION/COMPUTER NETWORKS 187
10.1 Introduction 187
10.2 Serial communication lines 188
10.2.1 Asynchronous transmission 189
10.3 Parallel data bus 190
10.4 Local area networks (LANs) 192
10.4.1 Star networks 193
10.4.2 Ring and bus networks 194
10.5 Gateways 195
10.6 HART 195
10.7 Digital fieldbuses 196
10.8 Communication protocols for very large systems 198
10.8.1 Protocol standardization 198
10.9 Future development of networks 199
References and further reading 199
11 DISPLAY, RECORDING AND PRESENTATION OF
MEASUREMENT DATA 200
11.1 Display of measurement signals 200
11.1.1 Electronic output displays 200
11.1.2 Computer monitor displays 201
11.2 Recording of measurement data 202
11.2.1 Mechanical chart recorders 202
11.2.2 Ultra-violet recorders 208
11.2.3 Fibre-optic recorders (recording oscilloscopes) 209
11.2.4 Hybrid chart recorders 209
11.2.5 Magnetic tape recorders 209
11.2.6 Digital recorders 210
11.2.7 Storage oscilloscopes 211
11.3 Presentation of data 212
11.3.1 Tabular data presentation 212
11.3.2 Graphical presentation of data 213
11.4 Self-test questions 222
References and further reading 223
12 MEASUREMENT RELIABILITY AND SAFETY SYSTEMS 224
12.1 Reliability 224
12.1.1 Principles of reliability 224
12.1.2 Laws of reliability in complex systems 228
12.1.3 Improving measurement system reliability 229
12.1.4 Software reliability 232
12.2 Safety systems 236
12.2.1 Introduction to safety systems 236
12.2.2 Operation of safety systems 237
12.2.3 Design of a safety system 238
12.3 Self-test questions 241
References and further reading 242
Part 2: Measurement Sensors and Instruments 245
13 SENSOR TECHNOLOGIES 247
13.1 Capacitive and resistive sensors 247
13.2 Magnetic sensors 247
13.3 Hall-effect sensors 249
13.4 Piezoelectric transducers 250
13.5 Strain gauges 251
13.6 Piezoresistive sensors 252
13.7 Optical sensors (air path) 252
13.8 Optical sensors (fibre-optic) 253
13.8.1 Intrinsic sensors 254
13.8.2 Extrinsic sensors 258
13.8.3 Distributed sensors 259
13.9 Ultrasonic transducers 259
13.9.1 Transmission speed 260
13.9.2 Direction of travel of ultrasound waves 261
13.9.3 Directionality of ultrasound waves 261
13.9.4 Relationship between wavelength, frequency and directionality of ultrasound waves 262
13.9.5 Attenuation of ultrasound waves 262
13.9.6 Ultrasound as a range sensor 263
13.9.7 Use of ultrasound in tracking 3D object motion 264
13.9.8 Effect of noise in ultrasonic measurement systems 265
13.9.9 Exploiting Doppler shift in ultrasound transmission 265
13.9.10 Ultrasonic imaging 267
13.10 Nuclear sensors 267
13.11 Microsensors 268
References and further reading 270
14 TEMPERATURE MEASUREMENT 271
14.1 Principles of temperature measurement 271
14.2 Thermoelectric effect sensors (thermocouples) 272
14.2.1 Thermocouple tables 276
14.2.2 Non-zero reference junction temperature 277
14.2.3 Thermocouple types 279
14.2.4 Thermocouple protection 280
14.2.5 Thermocouple manufacture 281
14.2.6 The thermopile 282
14.2.7 Digital thermometer 282
14.2.8 The continuous thermocouple 282
14.3 Varying resistance devices 283
14.3.1 Resistance thermometers (resistance temperature devices) 284
14.3.2 Thermistors 285
14.4 Semiconductor devices 286
14.5 Radiation thermometers 287
14.5.1 Optical pyrometers 289
14.5.2 Radiation pyrometers 290
14.6 Thermography (thermal imaging) 293
14.7 Thermal expansion methods 294
14.7.1 Liquid-in-glass thermometers 295
14.7.2 Bimetallic thermometer 296
14.7.3 Pressure thermometers 296
14.8 Quartz thermometers 297
14.9 Fibre-optic temperature sensors 297
14.10 Acoustic thermometers 298
14.11 Colour indicators 299
14.12 Change of state of materials 299
14.13 Intelligent temperature-measuring instruments 300
14.14 Choice between temperature transducers 300
14.15 Self-test questions 302
References and further reading 303
15 PRESSURE MEASUREMENT 304
15.1 Diaphragms 305
15.2 Capacitive pressure sensor 306
15.3 Fibre-optic pressure sensors 306
15.4 Bellows 307
15.5 Bourdon tube 308
15.6 Manometers 310
15.7 Resonant-wire devices 311
15.8 Dead-weight gauge 312
15.9 Special measurement devices for low pressures 312
15.10 High-pressure measurement (greater than 7000 bar) 315
15.11 Intelligent pressure transducers 316
15.12 Selection of pressure sensors 316
16 FLOW MEASUREMENT 319
16.1 Mass flow rate 319
16.1.1 Conveyor-based methods 319
16.1.2 Coriolis flowmeter 320
16.1.3 Thermal mass flow measurement 320
16.1.4 Joint measurement of volume flow rate and fluid density 321
16.2 Volume flow rate 321
16.2.1 Differential pressure (obstruction-type) meters 322
16.2.2 Variable area flowmeters (Rotameters) 327
16.2.3 Positive displacement flowmeters 328
16.2.4 Turbine meters 329
16.2.5 Electromagnetic flowmeters 330
16.2.6 Vortex-shedding flowmeters 332
16.2.7 Ultrasonic flowmeters 332
16.2.8 Other types of flowmeter for measuring volume
flow rate 336
16.3 Intelligent flowmeters 338
16.4 Choice between flowmeters for particular applications 338
References and further reading 339
17 LEVEL MEASUREMENT 340
17.1 Dipsticks 340
17.2 Float systems 340
17.3 Pressure-measuring devices (hydrostatic systems) 341
17.4 Capacitive devices 343
17.5 Ultrasonic level gauge 344
17.6 Radar (microwave) methods 346
17.7 Radiation methods 346
17.8 Other techniques 348
17.8.1 Vibrating level sensor 348
17.8.2 Hot-wire elements/carbon resistor elements 348
17.8.3 Laser methods 349
17.8.4 Fibre-optic level sensors 349
17.8.5 Thermography 349
17.9 Intelligent level-measuring instruments 351
17.10 Choice between different level sensors 351
References and further reading 351
18 MASS, FORCE AND TORQUE MEASUREMENT 352
18.1 Mass (weight) measurement 352
18.1.1 Electronic load cell (electronic balance) 352
18.1.2 Pneumatic/hydraulic load cells 354
18.1.3 Intelligent load cells 355
18.1.4 Mass-balance (weighing) instruments 356
18.1.5 Spring balance 359
18.2 Force measurement 359
18.2.1 Use of accelerometers 360
18.2.2 Vibrating wire sensor 360
18.3 Torque measurement 361
18.3.1 Reaction forces in shaft bearings 361
18.3.2 Prony brake 361
18.3.3 Measurement of induced strain 362
18.3.4 Optical torque measurement 364
19 TRANSLATIONAL MOTION TRANSDUCERS 365
19.1 Displacement 365
19.1.1 The resistive potentiometer 365
19.1.2 Linear variable differential transformer (LVDT) 368
19.1.3 Variable capacitance transducers 370
19.1.4 Variable inductance transducers 371
19.1.5 Strain gauges 371
19.1.6 Piezoelectric transducers 373
19.1.7 Nozzle flapper 373
19.1.8 Other methods of measuring small displacements 374
19.1.9 Measurement of large displacements (range sensors) 378
19.1.10 Proximity sensors 381
19.1.11 Selection of translational measurement transducers 382
19.2 Velocity 382
19.2.1 Differentiation of displacement measurements 382
19.2.2 Integration of the output of an accelerometer 383
19.2.3 Conversion to rotational velocity 383
19.3 Acceleration 383
19.3.1 Selection of accelerometers 385
19.4 Vibration 386
19.4.1 Nature of vibration 386
19.4.2 Vibration measurement 386
19.5 Shock 388
20 ROTATIONAL MOTION TRANSDUCERS 390
20.1 Rotational displacement 390
20.1.1 Circular and helical potentiometers 390
20.1.2 Rotational differential transformer 391
20.1.3 Incremental shaft encoders 392
20.1.4 Coded-disc shaft encoders 394
20.1.5 The resolver 398
20.1.6 The synchro 399
20.1.7 The induction potentiometer 402
20.1.8 The rotary inductosyn 402
20.1.9 Gyroscopes 402
20.1.10 Choice between rotational displacement transducers 406
20.2 Rotational velocity 407
20.2.1 Digital tachometers 407
20.2.2 Stroboscopic methods 410
20.2.3 Analogue tachometers 411
20.2.4 Mechanical flyball 413
20.2.5 The rate gyroscope 415
20.2.6 Fibre-optic gyroscope 416
20.2.7 Differentiation of angular displacement measurements 417
20.2.8 Integration of the output from an accelerometer 417
20.2.9 Choice between rotational velocity transducers 417
20.3 Measurement of rotational acceleration 417
References and further reading 418
21 SUMMARY OF OTHER MEASUREMENTS 419
21.1 Dimension measurement 419
21.1.1 Rules and tapes 419
21.1.2 Callipers 421
21.1.3 Micrometers 422
21.1.4 Gauge blocks (slip gauges) and length bars 423
21.1.5 Height and depth measurement 425
21.2 Angle measurement 426
21.3 Flatness measurement 428
21.4 Volume measurement 428
21.5 Viscosity measurement 429
21.5.1 Capillary and tube viscometers 430
21.5.2 Falling body viscometer 431
21.5.3 Rotational viscometers 431
21.6 Moisture measurement 432
21.6.1 Industrial moisture measurement techniques 432
21.6.2 Laboratory techniques for moisture measurement 434
21.6.3 Humidity measurement 435
21.7 Sound measurement 436
21.8 pH measurement 437
21.8.1 The glass electrode 438
21.8.2 Other methods of pH measurement 439
21.9 Gas sensing and analysis 439
21.9.1 Catalytic (calorimetric) sensors 440
21.9.2 Paper tape sensors 441
21.9.3 Liquid electrolyte electrochemical cells 441
21.9.4 Solid-state electrochemical cells (zirconia sensor) 442
21.9.5 Catalytic gate FETs 442
21.9.6 Semiconductor (metal oxide) sensors 442
21.9.7 Organic sensors 442
21.9.8 Piezoelectric devices 443
21.9.9 Infra-red absorption 443
21.9.10 Mass spectrometers 443
21.9.11 Gas chromatography 443
References and further reading 444
APPENDIX 1 Imperial–metric–SI conversion tables 445
APPENDIX 2 Thevenin’s theorem 452 ´
APPENDIX 3 Thermocouple tables 458
APPENDIX 4 Solutions to self-test questions 464
INDEX 469

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