ABSTRACT
Conductivity-Type Level Switch 446 Pump Alternator Circuit 447 Advantages and Limitations 447
Field-Effect Level Switches 447 Bibliography 448
3.5 DIAPHRAGM LEVEL DETECTORS 449
Diaphragm Switches for Solids 450 Diaphragm Switches for Liquids 451 Diaphragm-Type Level Sensors and Repeaters 451 Electronic Diaphragm Level Sensors 452 Bibliography 453
3.6 DIFFERENTIAL PRESSURE LEVEL DETECTORS 454
Sensing Differential Pressure 455 Extended Diaphragms 455 Chemical Seals 456
Intelligent D/P Cells and Tank Expert Systems 456 Pressure Repeaters 457 Dry, Motion Balance Devices 457 Liquid Manometers 458 Level Applications of D/P Cells 458
Clean Liquids in Atmospheric Tanks 459 Clean Liquids in Pressurized Tanks 459 Hard-to-Handle Fluids in Atmospheric
Tanks 460 Hard-to-Handle Fluids in Pressurized
Tanks 460 Special Installations 461
Boiling Applications 461 Cryogenic Applications 461 Normal Ambient Temperature Bi-phase
Applications 462 Span, Elevation, and Depression 462 Interface Detection 463 Bibliography 464
3.7 DISPLACER LEVEL DEVICES 465
Introduction 465 Displacer Switch 466 Torque-Tube Displacers 466
Sizing of Displacers 467
Interface Measurement 468 Rag Layer 469 Features and Installation 469
Spring-Balance Displacer 470 Force-Balance Displacer 470 Flexible Disc Displacer 471 Flexible-Shaft Controllers 471 Conclusion 473 Bibliography 473
3.8 FLOAT LEVEL DEVICES 474
Introduction 475 Float Level Switches 475
Reed-Switch Designs 476 Float and Guide Tube Designs 477 Tilt Switches 478 Float-Operated Continuous Indicators 478 Pressurized Tank Applications 479 Magnetically Coupled Indicators 479
Density Measurement 481 Conclusion 481 Bibliography 481
3.9 LASER LEVEL SENSORS 482
Background 482 Pulsed Laser Sensors (Time of Flight) 482 Frequency-Modulated (Continuous-Wave)
Sensors 483 Triangulation Measurement Sensor 483
Pulsed-Laser Level Sensor 483 Installation 483 Vapor-Space Effects 483 Types of Targets and Angle of Repose 484 Laser Eye Safety 485 Laser Power and Ignition Safety 485
Summary 485 Bibliography 485
3.10 LEVEL GAUGES, INCLUDING MAGNETIC 486
Introduction 487 Tubular Glass Gauge 488 Circular Transparent Gauge 488 Transparent Gauge (Long Form) 488 Reflex Gauge 489
Armored Gauges 490 Gauge Glass Materials 490 Design Features 490
Gauging Inaccuracies 491 Accessories 491 Application-Specific Requirements 491
Installation 492 Magnetic Level Gauges 492
Magnetic Followers and Indicators 493 Magnetostrictive Transducers 494
Remote Reading Gauges 494 Differential Pressure 495 Conductivity 495 Circular Gauges 495 Magnetostrictive Transducers 495
Conclusion 496 References 496 Bibliography 496
3.11 MICROWAVE LEVEL SWITCHES 497
Reflection Switches 498 Beam-Breaker Switch 499 Coating Effects 499 Conclusion 499 References 499 Bibliography 499
3.12 OPTICAL LEVEL DEVICES 500
Light Refection 500 Light Transmission 501 Light Refraction 502 Conclusion 503 Reference 503 Bibliography 503
3.13 RADAR, NONCONTACTING LEVEL SENSORS 504
Principles of Operation 505 FMCW 506 Pulse 506 Accuracy and Resolution Factors 507 Application Considerations 507 References 507 Bibliography 507
3.14 RADAR, CONTACT LEVEL SENSORS (TDR, GWR, PDS) 508
Definition of Terms 509 Introduction 509 Theory of Operation 509
Guided Wave Radar 509 Phase Difference Sensors 511 Contact Radar Systems 511
Electronics 511 Probe (Waveguide) 511
Probe Selection and Application 512 Interface Measurement 512 Conclusion 513 References 513 Bibliography 513
3.15 RADIATION LEVEL SENSORS 514
Radiation Phenomenon 515 Source Materials 515 Units and Attenuation of Radiation 515
Source Sizing 516 Safety Considerations 517
Allowable Radiation Exposures 517 Nuclear Regulatory Commission 518
Detectors 518 Geiger-Mueller Tube 518 Gas Ionization Chamber 519 Scintillation 519
Level Switch Applications 519 Continuous Level Measurement 520
Narrow Vessels or Interface 521 Installation Notes 521
Calibration Considerations 522 Backscatter Designs 522
Traversing Designs and Density Measurement 522
Electronics 523 Conclusions and Trends 523 Bibliography 525
3.16 RESISTANCE TAPES 526
Actuation Depth 527 Pressure Effect 527 Temperature and Other Effects 528 Conclusion 529 Bibliography 529
3.17 ROTATING PADDLE SWITCHES 530
Introduction 530 Rotating Paddle Switches 531
Installations 531 Bibliography 532
3.18 TANK GAUGES INCLUDING FLOAT-TYPE TAPE GAUGES 533
History of Custody Transfer 534 Tank Gauge Designs 534 Accuracy 536 Traditional Tape Level Sensors 538
Wire-Guided Float Detectors 538 Encoding 539 Temperature Compensation 540
Inductively Coupled Tape Detector 540 Wire-Guided Thermal Sensor 541 Solids Level Detectors 541 Capacitance and Displacer Tape Devices 542 Multiple-Tank Systems 542 Conclusion 543 Reference 543 Bibliography 543
3.19 THERMAL LEVEL SENSORS 544
Thermal Level Switches 544 Thermal-Differential Level Transmitter 546 Using Thermometers as Level Sensors 546 Conclusion 546 Reference 547 Bibliography 547
3.20 ULTRASONIC LEVEL DETECTORS 548
The Nature of Ultrasound 549 Level Switches 550
Damped Vibration Type 550 Absorption Type 550 Interface Detector 551
Level Transmitters 551 Multi-Tank Packages 552 Recent Developments 553 Conclusion 554 Reference 554 Bibliography 554
3.21 VIBRATING LEVEL SWITCHES 556
Vibrating Level Switches 556 Tuning Fork 557 Vibrating Probes 558 Conclusion 558 Bibliography 558
INTRODUCTION
There are dozens of variations on the 22 technologies presented in this chapter. Each one has a slight advantage in terms of some of the infinite combinations of range, tank shape, process materials, available power, pressure and temperature, and accuracy requirements. The purpose of this section is to assist the reader in narrowing the choices and focusing on the most appropriate technologies for a particular application. In selecting the level instrument, we should determine which factors are desirable and which are not. In practice, this is seldom carried out, and, frankly, there is a great tendency to reach for a d/p transmitter, if not a displacer, and live with whatever performance it produces. This is the cliché solution and, like so many clichés, it is, if not the wrong answer, often not the best.
