Level Measurement
LEVEL MEASUREMENT
by N. Asyiddin
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TABLE OF CONTENTS
LEVEL MEASUREMENT ...... 1
DEFINITION OF LEVEL ...... 1 UNITS OF LEVEL ...... 2 Conversions between Units ...... 2
METHODS OF LEVEL MEASUREMENT ...... 3
A. Mechanical or Direct Method...... 3 Dip Sticks and Lead Lines...... 3 Sight Glass...... 4 Chain or Float Gauge...... 5
B. Inferential or Indirect Methods ...... 7 Buoyancy ...... 8 Hydrostatic Head or Differential Pressure ...... 20 Open tank head type measurement ...... 21 Closed tank level measurement...... 23 Air Bubblers...... 27 Sonar or Ultrasonic Type Detector...... 29 Ultrasonic detectors with single sensor ...... 30 Ultrasonic detectors with two sensors ...... 30 Microwave (Radar)...... 32 Conductance...... 35 Capacitance ...... 36 Radiation...... 37 Weight...... 41 Resistance ...... 42 Micro-impulse...... 44 Tuning fork ...... 45
ADVANTAGES/DISADVANTAGES OF LEVEL MEASURING SYSTEMS ...... 46
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LEVEL MEASUREMENT
Liquid level was probably the first of the process variable to be measured and controlled. History records early examples of level control in dams used for the storage and orderly release of water for agriculture use.
This section describes the operating principles and mechanical details of the different types of level measurement instruments currently used in petrochemical industries.
REASONS FOR LEVEL MEASUREMENT