The document provides an overview of various level sensing devices, categorizing them into direct and indirect sensing methods, as well as single point measurements. It discusses different types of float sensors, displacer switches, conductive and capacitive probes, ultrasonic and radar measurement techniques, highlighting their advantages and limitations. The information covers applications, operational principles, and considerations for selecting appropriate sensors for liquid level measurement.

1. FLOAT SENSOR

2. LEVEL SENSOR
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3. LEVEL MEASUREMENT
• Level sensing devices can be divided into four categories
• Direct sensing, in which the actual level is monitored
• Indirect sensing, in which a property of the liquid, such as pressure, is
sensed to determine the liquid level
• Single point measurement, in which it is only necessary to detect the
presence or absence of a liquid at a specific level
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4. SIMPLE SIGHT GLASSES
• A visual indication of the level can be obtained when part of the vessel is
constructed from transparent material or the liquid in a vessel is bypassed
through a transparent tube.
• The liquid level then can be observed directly in the sight glass Glass gauges are
cheap but easily broken, and should not be used with hazardous liquid.
• The advantage of using stop valves with the use of a bypass pipe, is the ease in
removal for cleaning.
•
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5. FLOAT SENSORS
• There are two types of floats shown: the angular
arm and the pulley
• The advantages of the float sensor are that they
are almost independent of the density of the liquid
or solid being monitored, are accurate and robust,
and have a linear output with level height.
• One of the significant types of float is a magnetrol
float level switch which consists of a plain float and
operates via a magnetic coupling action.
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6. FLOAT SENSORS
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7. DISPLACER SWITCHES
• In a typical displacer switch design, a spring is provided which is
burdened with weighted displacers.
• The displacers having weights greater than the process fluid gets
submerged in the liquid resulting in a buoyancy force change.
• This will cause a variation in the net force operating on the spring.
In general, the spring will compress with the raise in buoyancy
force.
• Just like the float level switches, a magnetic sleeve and a non-
magnetic barrier tube is also incorporated in displacer switches.
• The magnetic sleeve is attached to the spring and it moves
according to the spring movement resulting in activation of
switching mechanism
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8. CONDUCTIVE PROBES
• Conductive probes are used in single and multiple point measurements systems.
• Low voltages is applied to the electrodes as they are immersed in the liquids.
• The conductive liquid completes the electrical circuit of the control, which
activates a semiconductor switch.
• The advantage of conductive probes is their low cost and simple design.
• The disadvantage is that they are limited to point measurement and can be only
used with conductive liquids.
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9. CAPACITIVE PROBES
• They are used for continues measurements
• The value of the capacitance can be change by varying the dielectric.
• The probe and the metal wall of the tank form the two plates of a
capacitor, and the contents in the tank is the dielectric.
• When the tank is empty the dielectric is the air.
• As the level changes the dielectric constant changes and causes the
capacitance change.
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10. ULTRASONIC LEVEL SENSORS
• Ultrasonic or sonic devices can be used for single point or continuous level
measurement of a liquid or a solid.
• A pulse of sonic waves (approximately 10 kHz) or ultrasonic waves (more than 20
kHz) from the transmitter is reflected from the surface of the liquid to the
receiver, and the time for the echo to reach the receiver is measured.
• The time delay gives the distance from the transmitter and receiver to the surface
of the liquid, from which the liquid level can be calculated, knowing the velocity
of ultrasonic waves (approximately 340 m/s)
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11. RADAR MEASUREMENT
• Radar gauges differ from ultrasonic in that they use microwaves
instead of sound waves.
• Like ultrasonic devices they measure from the top of the vessel to
determine the product level.
• Two examples of radar gauges are the 5.8GHz and 24GHz systems.
• The higher transmission frequency can be used to detect dry, non-
conductive materials with very low bulk density.
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14. B.Varun
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