01-02-2013, 10:29 AM
LEVEL MEASUREMENT SYSTEMS
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INTRODUCTION
• Level measurements is an integral part of process control,
and may be used in a variety of industries
• Level measurement may be divided into two categories:
point level measurement
continuous level measurement
• Point level sensors are used to mark a single discrete
liquid height, a preset level condition (as a high alarm or a
low alarm condition)
• Continuous level sensors provide an analog output that
directly correlates to the level within the containing vessel.
This analog signal may be directly linked to a visual indicator
or to a process control loop, forming a level management
system.
Floats
• The basic float arm indicator comprises very simply a float
connected to a pivoted arm that drives pointer or a switch.
• The unit can be made for either side- or top- entry.
• Moving parts present a very definite disadvantage, since they are
situated in the liquid and are thus prone to corrosion and seizing
Switch Float
• Methods of the providing indication other then by linkage to a
pointer include the use of a potentiometer, or of magnetic or
inductive coupling
Vibrating level switches
Principle: A vibrating fork or blade
• An electronic circuit excites the blade of probe to its resonant frequency,
and when material comes into contact with the blade, vibration is
damped causing switching of the relay
• This device is suitable for control maximum levels of solids and liquids in
many types of applications (e.g. foods, grains, granules, pellets, cement,
powder)
Ultrasonic level measurement
The measuring equipment consists:
• A transmitter that periodically sends a sound pulse to the surface of
the liquid
• A receiver that amplifies the returning pulse
• A time interval counter that measures the time elapsing between the
transmission of a pulse and receipt of the corresponding pulse echo
Nuclear level systems
• Nuclear radiation from a selected source can be related to liquid or solids
levels in a vessel
• Cobalt-60, cesium-137 or radium-226 is used as the gamma radiation source
• The radioactive source is capable of transmitting through the container wall
• As a detector for converting nuclear gamma ray radiation into electrical
quantities related to level, some systems use Geiger counters