Level Measurement

Table of Contents

Chapter 1: Principles of Level Measurement

Topics: Surface-sensing and storage tank gauges; Sight glasses; Magnetic and displacer gauges; Buoyancy; Level, mercury, and magnetic reed switches; Switches with multiple displacers

Learning Objectives:

• Define datum point, and contrast direct and indirect level measurement.
• Describe the main kinds of surface-sensing gauges.
• Define buoyant force and explain how it is used in displacer gauges to measure liquid level.
• Describe maintenance procedures for float devices, displacer gauges, and sight glasses.
• Compare the use of sight glasses, mercury level switches, and magnetic reed switches.

Chapter 2: Electrical Instruments

Topics: Conductivity and liquid level; Capacitance probes; Zero and span adjustments; Ultrasonic, resistance, and photoelectric level detectors

Learning Objectives:

• Differentiate between continuous and point level measurements, and between direct and indirect level measurement.
• Describe the operation of a conductance probe in a conducting liquid.
• Describe the operation of a capacitance probe in a dielectric liquid.
• Explain the operation of ultrasonic, resistance, and photoelectric level sensors.
• Describe conductance point level probes, capacitance point level probes, and ultrasonic point level detectors.

Chapter 3: Pressure Head Instruments

Topics: Hydrostatic pressure; Relative density; Pressurized fluids; Air bellows; Air/liquid purge systems; Force-balance diaphragm system

Learning Objectives:

• Define hydrostatic pressure and explain how it is calculated by means of the relative density (specific gravity) of a liquid in a tank.
• Discuss the relationship between pressure head and the location of the pressure (level) indicator.
• Compare the air bellows and air purge systems and discuss advantages for each.
• Explain how a force-balance diaphragm system works.
• Describe the operation of a differential pressure transmitter and explain how it is used to measure level and density.

Chapter 4: Solid Level Measurement

Topics: Weight method; Ultrasonic, microwave, and radiation level detectors; Capacitance and resistance probes; Bob-and-cable tension method

Learning Objectives:

• List the data needed to compute the level of a bulk solid in a bin.
• Describe and compare the operation of wire strain gauges and semiconductor strain gauges.
• Compare the advantages and disadvantages of ultrasonic and microwave level measuring methods.
• Discuss the operation of capacitance probes, resistance probes, and bob-and-cable units in measuring bulk solids.
• Describe how diaphragm switches and tilt switches are used for point level detection in automatic bin fillers.
• Discuss the use of rotating paddle detectors in controlling level within a band.

Chapter 5: Other Level Measurement Instruments

Topics: Radiation level detectors; Ionization, semiconductor, and photoelectric radiation sensors; Infrared detectors; Interface levels

Learning Objectives:

• Explain how radiation level detectors are used for both continuous and point level measurement.
• Describe the operation of ionization radiation sensors, semiconductor radiation sensors, and scintillation counters.
• Discuss the operation of an infrared point level detector.
• Describe several methods of measuring interface levels.
• Explain how range suppression and range elevation are used.
• Discuss the important considerations in equipment selection.

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